Merge branch 'feat/parse_invitation' into 'development'

Feat/parse invitation

See merge request GeneralProtocols/xo/utils!22

.cspell.json

@@ -1,6 +1,25 @@
 {
     "version": "0.1",
     "import": ["@generalprotocols/cspell-dictionary/cspell.json"],
-    "words": ["nonfungible", "lockscript", "cashassembly", "checksigverify", "inputindex", "utxovalue"],
+    "words": [
+        "nonfungible",
+        "lockscript",
+        "cashassembly",
+        "checksigverify",
+        "inputindex",
+        "utxovalue",
+        "bivariance",
+        "bytecodes",
+        "outputbytecode",
+        "utxobytecode",
+        "outputtokencategory",
+        "utxotokencategory",
+        "utxotokenamount",
+        "outputtokenamount",
+        "outputvalue",
+        "numequal",
+        "Prefault",
+        "cashtoken"
+    ],
     "ignorePaths": ["source/template/xo-template.schema.json"]
 }

eslint.config.mjs

@@ -2,7 +2,7 @@ import baseConfig from '@xo-cash/eslint-config';
 
 export default [
     {
-        ignores: [ 'scripts/**', 'docs/**' ],
+        ignores: [ 'scripts/**' ],
     },
     ...baseConfig,
 ];

package.json

@@ -1,14 +1,17 @@
 {
     "name": "@xo-cash/utils",
-    "version": "0.0.3",
+    "version": "0.0.10",
     "description": "XO Cash utilities",
     "type": "module",
     "types": "./dist/index.d.mts",
     "source": "./source/index.ts",
     "module": "./dist/index.mjs",
     "exports": {
-        "types": "./dist/index.d.mts",
-        "default": "./dist/index.mjs"
+        ".": {
+            "types": "./dist/index.d.mts",
+            "import": "./dist/index.mjs",
+            "default": "./dist/index.mjs"
+        }
     },
     "publishConfig": {
         "access": "public"
@@ -45,17 +48,18 @@
     "dependencies": {
         "@bitauth/libauth": "^3.1.0-next.8",
         "@xo-cash/primitives": "0.0.2",
-        "@xo-cash/types": "0.0.4",
+        "@xo-cash/types": "0.0.8",
         "zod": "^4.3.6"
     },
     "overrides": {
         "echarts": "6.1.0",
-        "minimatch": "10.2.6"
+        "minimatch": "10.2.6",
+        "qs": "6.16.0"
     },
     "devDependencies": {
         "@generalprotocols/cspell-dictionary": "^1.0.1",
         "@types/node": "^25.5.0",
-        "@vitest/coverage-v8": "^4.0.17",
+        "@vitest/coverage-v8": "^4.1.11",
         "@viz-kit/esbuild-analyzer": "^1.0.0",
         "@xo-cash/eslint-config": "1.0.2",
         "@xo-cash/templates": "0.0.3",
@@ -66,6 +70,6 @@
         "typedoc": "^0.28.16",
         "typedoc-plugin-coverage": "^4.0.2",
         "typescript": "^5.3.2",
-        "vitest": "^4.0.17"
+        "vitest": "^4.1.11"
     }
 }

sandbox/sandbox-llm.ts

@@ -0,0 +1,93 @@
+import { SSESession } from '../source/sse-session/index.ts';
+
+// Because this is in a library, and we don't want to add the node types to this as it is intended to be used in a browser
+// we will just declare the process object here locally so we don't get type errors from this script
+declare const process: {
+    env: Record<string, string>;
+    stdout: {
+        write: (data: string) => void;
+    };
+};
+
+// Recommended URL: https://openrouter.ai/api/v1/chat/completions
+// Recommended Model: nvidia/nemotron-3-nano-omni-30b-a3b-reasoning:free
+// Example Command: API_KEY="your-api-key" URL="https://openrouter.ai/api/v1/chat/completions" MODEL="ibm-granite/granite-4.1-8b" PROMPT="Hello, Tell me a joke about robots?" npx tsx ./sandbox/sandbox-llm.ts
+
+// Read the Environment Variables for url, model, prompt and api key
+const url = process.env.URL ?? 'https://openrouter.ai/api/v1/chat/completions';
+const model = process.env.MODEL ?? 'nvidia/nemotron-3-nano-omni-30b-a3b-reasoning:free';
+const prompt = process.env.PROMPT ?? 'Hello, Tell me a joke about robots?';
+
+const apiKey = process.env.API_KEY ?? '';
+
+// Throw an error if the api key is not set
+if (!apiKey) {
+    throw new Error('API key is required');
+}
+
+// Create a function to get the auth header
+const getAuthHeader = (): string => {
+    return `Bearer ${apiKey}`;
+};
+
+// Create our sse session
+const sseSession = new SSESession(url, {
+    // LLMs use requests
+    method: 'POST',
+
+    // Create the body of the request
+    body: JSON.stringify({
+        model: model,
+        messages: [
+            {
+                role: 'user',
+                content: prompt,
+            },
+        ],
+        stream: true,
+    }),
+
+    // Create a custom request handler to set the headers
+    onRequest: async (requestInit): Promise<RequestInit> => {
+        requestInit.headers ??= {} as HeadersInit;
+
+        // Handle typescript annoyances
+        const headers = requestInit.headers as Record<string, string>;
+
+        // Set our headers - We could also do this using the `headers` property in the SSESession constructor
+        // Doing it here to demonstrate dynamic headers, for example a signed timestamp could be used to authenticate the request.
+        headers.Authorization = getAuthHeader();
+        headers['Content-Type'] = 'application/json';
+
+        return requestInit;
+    },
+});
+
+// Connect to the SSESession
+await sseSession.connect();
+
+// Loop over the message chunks using `for await`
+for await (const message of sseSession.messages) {
+    // Handle `[DONE]` (this may be specific to OpenRouter)
+    if (message.data === '[DONE]') {
+        continue;
+    }
+
+    // First, we will parse the event to JSON
+    const responseJson = JSON.parse(message.data);
+
+    // Then we will grab the relevant part of the response (we want to first grab the choices array)
+    const choices = responseJson.choices;
+
+    // Then we will grab the first choice
+    const firstChoice = choices[0];
+
+    // Then we will grab the next chunk of text
+    const messageContent = firstChoice.delta?.content;
+
+    // Then we will append the text to the console
+    process.stdout.write(messageContent || '');
+}
+
+// Just a terminal/node thing. If we dont put a new line, the console will overwrite the text with the `cwd` or next command input
+process.stdout.write('\n');

sandbox/sandbox-price-oracle.ts

@@ -0,0 +1,17 @@
+import { SSESession, type SSEvent } from '../source/sse-session/index.ts';
+
+// Command: npx tsx ./sandbox/sandbox-price-oracle.ts
+
+// Use the GP Price Oracle API
+const url = 'https://oracles.generalprotocols.com/sse/v1/messages';
+
+// Create our sse session
+const sseSession = await SSESession.create(url);
+
+// Create a message event handler
+sseSession.on('message', (message: SSEvent) => {
+    console.log(message);
+});
+
+// Connect to the SSESession
+await sseSession.connect();

source/cash-assembly/bytes.ts

@@ -6,9 +6,9 @@ import { CashAssemblyNumberNotSafeIntegerError, CashAssemblyUnsupportedValueType
  *
  * @param {unknown} value - Value to encode, should be one of: Uint8Array, bigint, boolean, string, or a safe integer number.
  * @param {string} valueIdentifier - Identifier used in error messages.
- * @returns {Uint8Array} Bytes representation of the value.
- * @throws {@link CashAssemblyNumberNotSafeIntegerError} When a number is not a safe integer.
- * @throws {@link CashAssemblyUnsupportedValueTypeError} When the value type cannot be resolved.
+ * @returns {Uint8Array} - Bytes representation of the value.
+ * @throws {@link CashAssemblyNumberNotSafeIntegerError} - When a number is not a safe integer.
+ * @throws {@link CashAssemblyUnsupportedValueTypeError} - When the value type cannot be resolved.
  */
 export const convertValueToBytes = (value: unknown, valueIdentifier: string): Uint8Array => {
     if (value instanceof Uint8Array) {

source/cash-assembly/collect-evaluations.ts

@@ -0,0 +1,257 @@
+import { describeExpectedInput, parseScript } from '@bitauth/libauth';
+import type { CashAssemblyScriptSegment } from '@bitauth/libauth';
+import { CashAssemblyCompilationFailedError } from './errors.ts';
+import type { CashAssemblyTemplateScripts } from './types.ts';
+
+/**
+ * Shared state while scanning a parse tree and visiting nested template scripts.
+ */
+type CollectFromParseTreeContext = {
+
+    /**
+     * Script ids present in the template scripts map.
+     */
+    knownScriptIdentifiers: ReadonlySet<string>;
+
+    /**
+     * Script ids whose sources were already visited.
+     */
+    visitedScriptIdentifiers: ReadonlySet<string>;
+
+    /**
+     * Pending script ids queued to visit.
+     */
+    scriptIdentifiersToVisit: string[];
+
+    /**
+     * WalletData variable names.
+     */
+    variableNames: Set<string>;
+};
+
+/**
+ * Parameters for scanning one parsed Script node.
+ */
+type CollectFromParsedScriptParameters = CollectFromParseTreeContext & {
+
+    /**
+     * Parsed Script whose children are Push, Evaluation, Identifier, or literals.
+     */
+    scriptSegment: CashAssemblyScriptSegment;
+
+    /**
+     * True when this Script is the value of a Push.
+     * Direct Identifier children are WalletData unless they are template script ids.
+     */
+    isDirectPushContent: boolean;
+};
+
+/**
+ * Parameters for handling one Identifier node.
+ */
+type CollectFromParsedIdentifierParameters = CollectFromParseTreeContext & {
+
+    /**
+     * Identifier text from parseScript, which may include `.` and `_`.
+     */
+    identifier: string;
+
+    /**
+     * True when this Identifier is a direct child of a Push Script.
+     */
+    isDirectPushContent: boolean;
+};
+
+/**
+ * Enqueues a template script id so its source can be visited when it exists in the scripts map.
+ *
+ * @param {string} scriptIdentifier - Script id from an evaluation or nested script source.
+ * @param {ReadonlySet<string>} knownScriptIdentifiers - Script ids present in the template scripts map.
+ * @param {ReadonlySet<string>} visitedScriptIdentifiers - Script ids whose sources were already visited.
+ * @param {string[]} scriptIdentifiersToVisit - Pending script ids queued to visit.
+ */
+const enqueueReachableTemplateScriptIdentifier = (
+    scriptIdentifier: string,
+    knownScriptIdentifiers: ReadonlySet<string>,
+    visitedScriptIdentifiers: ReadonlySet<string>,
+    scriptIdentifiersToVisit: string[],
+): void => {
+    // Return when this id is not a template script.
+    if (knownScriptIdentifiers.has(scriptIdentifier) === false) {
+        return;
+    }
+
+    // Skip ids whose sources were already visited.
+    if (visitedScriptIdentifiers.has(scriptIdentifier) === true) {
+        return;
+    }
+
+    // Avoid duplicate queue entries when the same id is referenced more than once.
+    if (scriptIdentifiersToVisit.includes(scriptIdentifier) === true) {
+        return;
+    }
+
+    // Queue this id so its source is parsed after the current scan finishes.
+    scriptIdentifiersToVisit.push(scriptIdentifier);
+};
+
+/**
+ * Parses CashAssembly source with parseScript and throws when the parse fails.
+ *
+ * @param {string} cashAssemblyText - Evaluation or script source to parse.
+ * @returns {CashAssemblyScriptSegment} - Parsed Script root.
+ * @throws {@link CashAssemblyCompilationFailedError} - When parseScript rejects the source.
+ */
+const parseCashAssemblySource = (cashAssemblyText: string): CashAssemblyScriptSegment => {
+    // Parse so Push and Identifier nodes are available for collection.
+    const parseResult = parseScript(cashAssemblyText);
+
+    // parseScript returns a union. status false is the failure arm and must not be read as a Script.
+    if (parseResult.status === false) {
+        // describeExpectedInput turns the expected input list into readable text for the typed error.
+        const expectedInputDescription = describeExpectedInput(parseResult.expected);
+
+        throw new CashAssemblyCompilationFailedError(`${expectedInputDescription} Line ${String(parseResult.index.line)}, column ${String(parseResult.index.column)}.`);
+    }
+
+    return parseResult.value;
+};
+
+/**
+ * Identifier visitor for the parseScript tree.
+ * Records WalletData or queues a nested script from one Identifier node.
+ *
+ * @param {CollectFromParsedIdentifierParameters} parameters - Identifier text and the parse tree context.
+ */
+const collectFromParsedIdentifier = (parameters: CollectFromParsedIdentifierParameters): void => {
+    const { identifier, isDirectPushContent, knownScriptIdentifiers, visitedScriptIdentifiers, scriptIdentifiersToVisit, variableNames } = parameters;
+
+    // Script ids are visited whether they appear in a Push or beside opcodes.
+    if (knownScriptIdentifiers.has(identifier) === true) {
+        enqueueReachableTemplateScriptIdentifier(identifier, knownScriptIdentifiers, visitedScriptIdentifiers, scriptIdentifiersToVisit);
+
+        return;
+    }
+
+    // Direct Push names that are not script ids are WalletData, including dotted names.
+    if (isDirectPushContent === true) {
+        variableNames.add(identifier);
+    }
+};
+
+/**
+ * Visitor for one Script node in the parseScript tree.
+ * Recurses into Push and Evaluation children and dispatches Identifier nodes.
+ *
+ * @param {CollectFromParsedScriptParameters} parameters - Parsed Script, Push depth flag, and parse tree context.
+ */
+const collectFromParsedScript = (parameters: CollectFromParsedScriptParameters): void => {
+    const { scriptSegment, isDirectPushContent, knownScriptIdentifiers, visitedScriptIdentifiers, scriptIdentifiersToVisit, variableNames } =
+        parameters;
+
+    for (const child of scriptSegment.value) {
+        if (child.name === 'Push') {
+            // Inner Script of a Push is where `<name>` WalletData lives, including nested angle brackets.
+            collectFromParsedScript({
+                scriptSegment: child.value,
+                isDirectPushContent: true,
+                knownScriptIdentifiers,
+                visitedScriptIdentifiers,
+                scriptIdentifiersToVisit,
+                variableNames,
+            });
+
+            continue;
+        }
+
+        if (child.name === 'Evaluation') {
+            // Evaluation contents are opcodes and nested Pushes, not WalletData by themselves.
+            collectFromParsedScript({
+                scriptSegment: child.value,
+                isDirectPushContent: false,
+                knownScriptIdentifiers,
+                visitedScriptIdentifiers,
+                scriptIdentifiersToVisit,
+                variableNames,
+            });
+
+            continue;
+        }
+
+        if (child.name === 'Identifier') {
+            collectFromParsedIdentifier({
+                identifier: child.value,
+                isDirectPushContent,
+                knownScriptIdentifiers,
+                visitedScriptIdentifiers,
+                scriptIdentifiersToVisit,
+                variableNames,
+            });
+        }
+    }
+};
+
+/**
+ * Collects WalletData names from the parseScript tree using the visitor pattern.
+ *
+ * parseScript returns a tree of Script, Push, Evaluation, and Identifier nodes. Nested Push and
+ * Evaluation nodes are entered immediately. A template script id is queued whether it appears inside a Push or next to
+ * opcodes. Queued script sources are visited after the current tree finishes, in the order they were
+ * first seen.
+ * A visited id is skipped on later encounters. Opcodes and literals are ignored. Unrelated scripts
+ * in the map are not visited. An Identifier inside a nested Push is collected from that inner Push,
+ * for example `<$(<someKey.public_key> OP_HASH160)>` collects `someKey.public_key`.
+ *
+ * @param {string} evaluation - CashAssembly evaluation or script id text to scan.
+ * @param {CashAssemblyTemplateScripts} [templateScripts] - Optional template scripts map used to visit nested script ids.
+ * @returns {string[]} - WalletData names from reachable Pushes. Script ids are omitted when
+ * `templateScripts` is provided.
+ * @throws {@link CashAssemblyCompilationFailedError} - When parseScript rejects the starting text or a visited script source.
+ */
+export const collectVariablesUsingParseScript = (evaluation: string, templateScripts: CashAssemblyTemplateScripts = {}): string[] => {
+    // Create a set of the script identifiers from the template scripts map.
+    const knownScriptIdentifiers = new Set(Object.keys(templateScripts));
+
+    const visitedScriptIdentifiers = new Set<string>();
+    const scriptIdentifiersToVisit: string[] = [];
+    const variableNames = new Set<string>();
+
+    // Share by reference context.
+    const parseTreeContext: CollectFromParseTreeContext = {
+        knownScriptIdentifiers,
+        visitedScriptIdentifiers,
+        scriptIdentifiersToVisit,
+        variableNames,
+    };
+
+    // Parse the starting text so nested Pushes are visible before visiting nested script sources.
+    const startingScriptSegment = parseCashAssemblySource(evaluation);
+
+    // Scan the starting parse tree before visiting nested script sources.
+    collectFromParsedScript({
+        ...parseTreeContext,
+        scriptSegment: startingScriptSegment,
+        isDirectPushContent: false,
+    });
+
+    for (const scriptIdentifier of scriptIdentifiersToVisit) {
+        if (visitedScriptIdentifiers.has(scriptIdentifier) === true) {
+            continue;
+        }
+
+        visitedScriptIdentifiers.add(scriptIdentifier);
+
+        // Get the script definition from the template scripts map.
+        const scriptDefinition = templateScripts[scriptIdentifier];
+
+        const nestedScriptSegment = parseCashAssemblySource(scriptDefinition);
+
+        collectFromParsedScript({
+            ...parseTreeContext,
+            scriptSegment: nestedScriptSegment,
+            isDirectPushContent: false,
+        });
+    }
+
+    return [ ...variableNames ];
+};

source/cash-assembly/defaults.ts

@@ -1,61 +1,41 @@
 /**
- * Detects whether a string is a pure CashAssembly expression.
- *
- * CashAssembly expressions look like `$(<variable>)` or `$(<a> <b>)`. This pattern checks
- * that the entire string is one such expression and nothing else. It will not match if
- * there is other text surrounding the expression.
- *
- * For example:
- *   `$(<fee>)`          matches (a full expression)
- *   `OP_DUP $(<fee>)`   does not match (extra text before it)
- *   `$()`               does not match (empty expression)
- */
-export const CASHASSEMBLY_EXPRESSION_PATTERN = /^\$\([^)]+\)$/;
-
-/**
- * Finds all CashAssembly evaluations embedded in a larger string.
- *
- * An evaluation looks like `$(...)`, for example `$(<fee>)` or `$(<a> <b>)`. This pattern
- * locates every occurrence in the input and returns them all (global flag `g`).
- * Empty evaluations `$()` are intentionally excluded because they reference no variables.
- *
- * For example, scanning `"OP_DUP <$(<pubkeyHash>)> OP_HASH160 $(<fee>)"` would return
- * `['$(<pubkeyHash>)', '$(<fee>)']`.
- */
-export const CASHASSEMBLY_EVALUATION_PATTERN = /\$\([^)]+\)/g;
-
-/**
- * Extracts variable names from angle-bracket references inside a CashAssembly evaluation.
- *
- * Inside an evaluation like `$(<pubkeyHash> <fee>)`, variables are referenced as `<name>`.
- * This pattern captures the name between the brackets. The global flag `g` allows iterating
- * over every variable reference in a single evaluation string.
- *
- * For example, running this against `$(<pubkeyHash> <fee>)` would return the variable names
- * `["pubkeyHash", "fee"]`.
- */
-export const CASHASSEMBLY_VARIABLE_PATTERN = /<([^>]+)>/g;
-
-/**
- * Identifies CashAssembly literal tokens that appear inside angle-bracket push statements.
- *
- * Inside an evaluation, not everything between `<` and `>` is a variable name. Literals are
- * also valid push contents: numeric literals (e.g. `<0>`, `<32>`), hex literals (`<0x02>`),
- * binary literals (`<0b1010>`), and string literals (`<"minting">`, `<'hello'>`). This pattern
- * matches any captured token that starts with a digit or a quote character.
- */
-export const CASHASSEMBLY_LITERAL_TOKEN_PATTERN = /^[0-9"']/;
-
-/**
- * Matches a single dot variable method reference inside an angle-bracket identifier.
+ * Matches a single dot variable method reference inside an angle bracket identifier.
  *
  * Used to detect primitive method references such as `expiry.toIso8601`.
  *
- * For example:
- *   `expiry.toIso8601`                         matches (base `expiry`, method `toIso8601`)
- *   `requestedSatoshis`                        does not match (no method)
- *   `key.schnorr_signature.all_outputs`        does not match (more than one dot)
- *   `key.public_key`                           matches the pattern shape but it is only resolved
- *                                              when `hint` maps to a primitive
+ * For example `expiry.toIso8601` matches with base expiry and method toIso8601.
+ * `requestedSatoshis` does not match because it has no method.
+ * `key.schnorr_signature.all_outputs` does not match because it has more than one dot.
  */
 export const CASHASSEMBLY_VARIABLE_METHOD_REFERENCE_PATTERN = /^([^.]+)\.([^.]+)$/;
+
+/**
+ * Character count of the `$(` evaluation opener, which is two characters long.
+ *
+ * Adding this length moves the read position to the first character after that sequence so scanning
+ * starts on the evaluation contents rather than on the opening parenthesis.
+ */
+export const CASHASSEMBLY_EVALUATION_START_LENGTH = 2;
+
+/**
+ * Character count of the `//` and `/*` comment openers, which are both two characters long.
+ *
+ * Adding this length moves the read position to the first character after that sequence so the '*'
+ * of a block comment opener cannot be reused as the '*' of its closer.
+ */
+export const CASHASSEMBLY_COMMENT_START_LENGTH = 2;
+
+/**
+ * Character count of the block comment closer, which is two characters long.
+ *
+ * Adding this length moves the read position to the first character after that sequence so the
+ * closing slash cannot pair with the next character and open another comment.
+ */
+export const CASHASSEMBLY_BLOCK_COMMENT_END_LENGTH = 2;
+
+/**
+ * `$()` with nothing between the parentheses.
+ *
+ * Callers use this to detect an evaluation that has no contents to compile.
+ */
+export const EMPTY_CASHASSEMBLY_EVALUATION = '$()';

source/cash-assembly/errors.ts

@@ -1,16 +1,24 @@
 /* eslint-disable max-classes-per-file */
+import type { IdentifierResolutionType } from '@bitauth/libauth';
 
 /**
  * Error thrown when a required variable is missing.
  */
 export class CashAssemblyRequiredVariableMissingError extends Error {
-    constructor(variableNames?: string[]) {
+    /**
+     * Variable names that were required but absent from the variables map.
+     */
+    readonly variableNames: string[];
+
+    constructor(variableNames: string[] = []) {
         const defaultMessage = 'Missing required variable';
-        if (variableNames !== undefined && variableNames.length > 0) {
+        if (variableNames.length > 0) {
             super(`${defaultMessage}: variableNames [${variableNames.join(', ')}]`);
         } else {
             super(defaultMessage);
         }
+
+        this.variableNames = variableNames;
     }
 }
 
@@ -24,6 +32,104 @@ export class CashAssemblyCompilationFailedError extends Error {
     }
 }
 
+/**
+ * Error thrown when a quoted string inside `$()` never closes.
+ */
+export class CashAssemblyQuotedLiteralUnclosedError extends Error {
+    /**
+     * Index of the opening quote that never found a closer.
+     */
+    readonly openingQuoteIndex: number;
+
+    /**
+     * Quote character that opened the literal.
+     */
+    readonly quoteCharacter: string;
+
+    /**
+     * Text from the opening quote through the end of the scanned string.
+     */
+    readonly unclosedText: string;
+
+    constructor(openingQuoteIndex: number, quoteCharacter: string, cashAssemblyText: string) {
+        const unclosedText = cashAssemblyText.slice(openingQuoteIndex);
+        const defaultMessage = 'Quoted literal in a CashAssembly evaluation is unclosed';
+        const details = [
+            `quoteCharacter ${JSON.stringify(quoteCharacter)}`,
+            `openingQuoteIndex ${String(openingQuoteIndex)}`,
+            `unclosedText ${JSON.stringify(unclosedText)}`,
+        ].join(', ');
+
+        super(`${defaultMessage}: ${details}`);
+
+        this.openingQuoteIndex = openingQuoteIndex;
+        this.quoteCharacter = quoteCharacter;
+        this.unclosedText = unclosedText;
+    }
+}
+
+/**
+ * Error thrown when a block comment inside `$()` never closes.
+ */
+export class CashAssemblyBlockCommentUnclosedError extends Error {
+    /**
+     * Index of the first slash of the block comment opener.
+     */
+    readonly commentStartIndex: number;
+
+    /**
+     * Text from the block comment opener through the end of the scanned string.
+     */
+    readonly unclosedText: string;
+
+    constructor(commentStartIndex: number, cashAssemblyText: string) {
+        const unclosedText = cashAssemblyText.slice(commentStartIndex);
+        const defaultMessage = 'Block comment in a CashAssembly evaluation is unclosed';
+        const details = [ `commentStartIndex ${String(commentStartIndex)}`, `unclosedText ${JSON.stringify(unclosedText)}` ].join(', ');
+
+        super(`${defaultMessage}: ${details}`);
+
+        this.commentStartIndex = commentStartIndex;
+        this.unclosedText = unclosedText;
+    }
+}
+
+/**
+ * Error thrown when a `$()` evaluation never finds its matching closer.
+ */
+export class CashAssemblyEvaluationUnclosedError extends Error {
+    /**
+     * Index of the `$` that opened the evaluation.
+     */
+    readonly evaluationStartIndex: number;
+
+    /**
+     * How many `$()` remain open at the end of the string, including nested evaluations.
+     */
+    readonly remainingOpenEvaluations: number;
+
+    /**
+     * Text from the opening `$` through the end of the scanned string.
+     */
+    readonly unclosedText: string;
+
+    constructor(evaluationStartIndex: number, remainingOpenEvaluations: number, cashAssemblyText: string) {
+        const unclosedText = cashAssemblyText.slice(evaluationStartIndex);
+        const defaultMessage = 'CashAssembly evaluation is unclosed';
+        const details = [
+            `evaluationStartIndex ${String(evaluationStartIndex)}`,
+            `remainingOpenEvaluations ${String(remainingOpenEvaluations)}`,
+            `unclosedText ${JSON.stringify(unclosedText)}`,
+        ].join(', ');
+
+        super(`${defaultMessage}: ${details}`);
+
+        this.evaluationStartIndex = evaluationStartIndex;
+        this.remainingOpenEvaluations = remainingOpenEvaluations;
+        this.unclosedText = unclosedText;
+    }
+}
+
 /**
  * Error thrown when a variable's runtime type does not match the type required for compilation.
  */
@@ -35,12 +141,12 @@ export class CashAssemblyVariableTypeMismatchError extends Error {
 }
 
 /**
- * Error thrown when a supported primitive hint does not expose the requested method.
+ * Error thrown when a method resolvable primitive type does not expose the requested method.
  */
 export class CashAssemblyPrimitiveMethodMissingError extends Error {
-    constructor(identifier: string, methodName: string, hint: string) {
+    constructor(identifier: string, methodName: string, type: string) {
         const defaultMessage = 'CashAssembly primitive method does not exist';
-        super(`${defaultMessage}: identifier "${identifier}", methodName "${methodName}", hint "${hint}"`);
+        super(`${defaultMessage}: identifier "${identifier}", methodName "${methodName}", type "${type}"`);
     }
 }
 
@@ -65,7 +171,7 @@ export class CashAssemblyNumberNotSafeIntegerError extends Error {
 }
 
 /**
- * Error thrown when a supported primitive is selected but its value is missing when provided in the variables map.
+ * Error thrown when a method resolvable primitive is selected but its value is missing from the variables map.
  */
 export class CashAssemblyPrimitiveVariableMissingError extends Error {
     constructor(identifier: string, variableName: string) {
@@ -74,6 +180,30 @@ export class CashAssemblyPrimitiveVariableMissingError extends Error {
     }
 }
 
+/**
+ * Error thrown when one identifier would resolve as more than one {@link IdentifierResolutionType}.
+ */
+export class CashAssemblyIdentifierCollisionError extends Error {
+    /**
+     * Identifier that more than one resolution type would match.
+     */
+    readonly identifier: string;
+
+    /**
+     * The list of resolution types that the identifier matches.
+     */
+    readonly resolutionTypes: IdentifierResolutionType[];
+
+    constructor(identifier: string, resolutionTypes: IdentifierResolutionType[]) {
+        const defaultMessage = 'CashAssembly identifier exists for more than one resolution type';
+
+        super(`${defaultMessage}: identifier "${identifier}", resolutionTypes [${resolutionTypes.join(', ')}]`);
+
+        this.identifier = identifier;
+        this.resolutionTypes = resolutionTypes;
+    }
+}
+
 /**
  * Error thrown when compiled evaluation bytes cannot be decoded as a VM number.
  */

source/cash-assembly/evaluations.ts

@@ -7,26 +7,26 @@
  * ## Syntax (CashAssembly)
  *
  * `<expression>` is a push statement. Compiles the contents and pushes the result onto the VM stack.
- * `<someKey.public_key>` pushes the 33 byte compressed public key. `<1>` pushes the integer 1.
+ * For example `<someKey.public_key>` pushes the 33 byte compressed public key and `<1>` pushes the integer 1.
  *
  * `$(<expression>)` is an evaluation. Runs the inner script in the VM and inserts the top stack
  * item as VM bytecode.
- * `$(<someKey.public_key> OP_HASH160)` inserts the HASH160 of the public key.
+ * For example `$(<someKey.public_key> OP_HASH160)` inserts the HASH160 of the public key.
  *
  * `<$(<expression>)>` is a push of an evaluation result. It evaluates first then pushes.
- * For a P2PKH locking script example see
+ * For example a P2PKH locking script looks like
  * `OP_DUP OP_HASH160 <$(<someKey.public_key> OP_HASH160)> OP_EQUALVERIFY OP_CHECKSIG`.
  *
- * `variableId.operation` is a variable with a compiler resolved operation. `someKey.public_key`
- * produces the public key bytes. `someKey.schnorr_signature.all_outputs` produces a Schnorr
- * signature.
+ * `variableId.operation` is a variable with a compiler resolved operation.
+ * For example `someKey.public_key` produces the public key bytes and
+ * `someKey.schnorr_signature.all_outputs` produces a Schnorr signature.
  *
  * Opcodes (`OP_DUP`, `OP_HASH160`, and similar) are inserted as their bytecode equivalent directly.
  *
  * ## Name resolution priority (CashAssembly)
  *
  * When the compiler encounters an identifier it resolves it in this order.
- * 1. Opcode always wins. Naming a variable or script `OP_ADD` will not shadow it.
+ * 1. Opcode always wins. For example naming a variable or script `OP_ADD` will not shadow it.
  * 2. Variable shadows scripts of the same name.
  * 3. Script is the script's bytecode.
  *
@@ -34,80 +34,40 @@
  *
  * Supported `<base.method>` pushes are resolved to bytes before CashAssembly compiles.
  * Inside CashAssembly the order is Opcode then Variable then Script.
+ *
+ * ## This compile path
+ *
+ * This file is a layer on CashAssembly. Text outside `$()` is kept. Only `$()` is compiled.
+ * For example `Paid $(scriptA) of $(scriptB)` keeps the words Paid and of.
+ *
+ * `$()` is found by scanning left to right with parenthesis matching by depth count. Quoted strings
+ * and comments are skipped so a `)` there cannot end `$()`. Nested `$()` stay inside the outer
+ * evaluation. That text is never passed to parseScript.
+ *
+ * Empty `$()` still uses this mixed path. When the string contains no `$()` at all, the entire
+ * string is compiled as CashAssembly so push opcodes remain.
  */
 import type { CompilerBch } from '@bitauth/libauth';
 import { binToHex, binToUtf8, createCompilerBch, vmNumberToBigInt } from '@bitauth/libauth';
-import type { XOInvitationVariableValue, XOTemplate } from '@xo-cash/types';
+import { convertValueToBytes } from './bytes.ts';
 import {
     CashAssemblyCompilationFailedError,
-    CashAssemblyVariableTypeMismatchError,
     CashAssemblyRequiredVariableMissingError,
+    CashAssemblyVariableTypeMismatchError,
     CashAssemblyVmNumberDecodeError,
 } from './errors.ts';
-import {
-    CASHASSEMBLY_EVALUATION_PATTERN,
-    CASHASSEMBLY_EXPRESSION_PATTERN,
-    CASHASSEMBLY_LITERAL_TOKEN_PATTERN,
-    CASHASSEMBLY_VARIABLE_PATTERN,
-} from './defaults.ts';
-import { convertValueToBytes } from './bytes.ts';
 import { resolvePrimitiveMethodBytes } from './primitive-evaluations.ts';
-
-/**
- * Supported decode modes for compiled CashAssembly evaluation bytes.
- */
-export type CompiledCashAssemblyDecodeMode = 'utf8' | 'hex' | 'boolean' | 'bigint' | 'uint8array';
-
-/**
- * Parameters for compiling CashAssembly string.
- */
-export type CompileCashAssemblyStringParameters = {
-
-    /**
-     * Text that may embed CashAssembly evaluations such as `$(<fee>)` or `$(<expiry.toIso8601>)`.
-     */
-    cashAssemblyText: string;
-
-    /**
-     * Used for both primitive method evaluations and normal CashAssembly compilation.
-     */
-    variables: Record<string, XOInvitationVariableValue | Uint8Array>;
-
-    /**
-     * The mode to decode compiled evaluation bytes into a string.
-     */
-    evaluationDecodeMode?: CompiledCashAssemblyDecodeMode;
-
-    /**
-     * Optional template variable definitions. When provided, each `<name.method>` push whose `hint`
-     * maps to a supported primitive class is resolved to bytes before CashAssembly compilation.
-     */
-    templateVariables?: XOTemplate['variables'];
-};
-
-/**
- * Checks if the expression is a CashAssembly expression.
- *
- * @param {unknown} expression - The expression to check.
- * @returns {boolean} True if the expression is a CashAssembly expression, false otherwise.
- */
-export const isCashAssemblyExpression = (expression: unknown): boolean => {
-    return typeof expression === 'string' && CASHASSEMBLY_EXPRESSION_PATTERN.test(expression);
-};
-
-/**
- * Extracts all CashAssembly evaluations (i.e., substrings like $(...)) from the input text.
- *
- * @param {string} text - The input string to scan for CashAssembly evaluations.
- * @returns {string[]} An array of evaluation strings found in the input.
- *
- * @example
- *   extractCashAssemblyEvaluations("OP_DUP <$(<foo>)> OP_HASH160 $(<bar>)");
- *   // returns ['$(<foo>)', '$(<bar>)']
- */
-export const extractCashAssemblyEvaluations = (text: string): string[] => {
-    return text.match(CASHASSEMBLY_EVALUATION_PATTERN) ?? [];
-};
+import { assertNoIdentifierCollisions } from './identifier-collisions.ts';
+import { collectVariablesUsingParseScript } from './collect-evaluations.ts';
+import { EMPTY_CASHASSEMBLY_EVALUATION } from './defaults.ts';
+import { scanCashAssemblyEvaluations } from './scan-evaluations.ts';
+import type {
+    CompileCashAssemblyEvaluationsParameters,
+    CompileCashAssemblySourceToBytesParameters,
+    CompileCashAssemblySourceToDecodedTextParameters,
+    CompileCashAssemblyStringParameters,
+    CompiledCashAssemblyDecodeMode,
+} from './types.ts';
 
 /**
  * Returns the segment of `identifier` before the first `.`.
@@ -116,7 +76,7 @@ export const extractCashAssemblyEvaluations = (text: string): string[] => {
  * Multi segment identifiers such as `foo.bar.baz` resolve to `foo`.
  *
  * @param {string} identifier - Identifier that may contain a dot.
- * @returns {string} The base name before the first `.`.
+ * @returns {string} - The base name before the first `.`.
  */
 const resolveIdentifierBaseName = (identifier: string): string => {
     const firstDotIndex = identifier.indexOf('.');
@@ -128,31 +88,6 @@ const resolveIdentifierBaseName = (identifier: string): string => {
     return identifier.slice(0, firstDotIndex);
 };
 
-/**
- * Extracts unique variable identifiers enclosed in angle brackets from each evaluation string.
- *
- * CashAssembly literal tokens such as hex bytes, numbers, and quoted strings are excluded via
- * {@link CASHASSEMBLY_LITERAL_TOKEN_PATTERN}. For example, `<0x02>` and `<"minting">` are not returned.
- *
- * @param {string[]} evaluations - An array of evaluation strings from which to extract variable names.
- * @returns {string[]} An array of variable names.
- */
-export const extractVariablesFromEvaluations = (evaluations: string[]): string[] => {
-    const uniqueVariables = new Set<string>();
-
-    for (const evaluation of evaluations) {
-        for (const [ , extractedIdentifier ] of evaluation.matchAll(CASHASSEMBLY_VARIABLE_PATTERN)) {
-            if (CASHASSEMBLY_LITERAL_TOKEN_PATTERN.test(extractedIdentifier)) {
-                continue;
-            }
-
-            uniqueVariables.add(extractedIdentifier);
-        }
-    }
-
-    return [ ...uniqueVariables ];
-};
-
 /**
  * Decodes compiled CashAssembly evaluation bytes into a string representation.
  *
@@ -161,10 +96,9 @@ export const extractVariablesFromEvaluations = (evaluations: string[]): string[]
  * such as hashes, and `boolean` to represent boolean values.
  *
  * @param {Uint8Array} compiledResult - The compiled evaluation bytecode.
- * @param {CompiledCashAssemblyDecodeMode} [evaluationDecodeMode='utf8'] - The decode mode used to convert
- * bytes to text.
- * @returns {string} The decoded value as a string suitable for inline replacement.
- * @throws {@link CashAssemblyVmNumberDecodeError} When `evaluationDecodeMode` is `bigint` and the bytes are not a VM number.
+ * @param {CompiledCashAssemblyDecodeMode} evaluationDecodeMode - The decode mode used to convert bytes to text.
+ * @returns {string} - The decoded value as a string suitable for inline replacement.
+ * @throws {@link CashAssemblyVmNumberDecodeError} - When `evaluationDecodeMode` is `bigint` and the bytes are not a VM number.
  */
 export const decodeCompiledCashAssemblyEvaluation = (
     compiledResult: Uint8Array,
@@ -206,43 +140,52 @@ export const decodeCompiledCashAssemblyEvaluation = (
  * @param {CompilerBch} compiler - The libauth compiler from {@link compileCashAssemblyEvaluations}.
  * @param {string} evaluation - The specific evaluation string to compile.
  * @param {Record<string, Uint8Array>} variables - A record mapping variable names to their values.
- * @returns {Uint8Array} The compiled bytecode.
- * @throws {@link CashAssemblyRequiredVariableMissingError} If a required variable is not present.
- * @throws {@link CashAssemblyVariableTypeMismatchError} If a variable value is not a Uint8Array.
- * @throws {@link CashAssemblyCompilationFailedError} If libauth compilation fails.
+ * @param {string[]} requiredVariableNames - WalletData names from {@link collectVariablesUsingParseScript} that must be present.
+ * @returns {Uint8Array} - The compiled bytecode.
+ * @throws {@link CashAssemblyRequiredVariableMissingError} - If a required variable is not present.
+ * @throws {@link CashAssemblyVariableTypeMismatchError} - If a variable value is not a Uint8Array.
+ * @throws {@link CashAssemblyCompilationFailedError} - If libauth compilation fails.
  */
-export const generateCashAssemblyBytecode = (compiler: CompilerBch, evaluation: string, variables: Record<string, Uint8Array>): Uint8Array => {
-    const variableNames = extractVariablesFromEvaluations([ evaluation ]);
+export const generateCashAssemblyBytecode = (
+    compiler: CompilerBch,
+    evaluation: string,
+    variables: Record<string, Uint8Array>,
+    requiredVariableNames: string[],
+): Uint8Array => {
+    // Missing WalletData must fail here. Libauth would otherwise compile as if the value were empty bytes.
+    const missingVariables = requiredVariableNames.filter((name: string) => Object.hasOwn(variables, name) === false);
 
-    // Validate that all required variables are provided
-    const missingVariables = variableNames.filter((name: string) => !Object.hasOwn(variables, name));
+    // Refuse invented defaults so a missing value cannot compile into incorrect bytecode.
     if (missingVariables.length > 0) {
         throw new CashAssemblyRequiredVariableMissingError(missingVariables);
     }
 
-    // Construct the bytecode object using the keys in variableNames, mapping to the values in variables
     const bytecode: Record<string, Uint8Array> = {};
-    for (const variableName of variableNames) {
+
+    // Pass only required names so extra keys in `variables` are not sent to Libauth.
+    for (const variableName of requiredVariableNames) {
         const value = variables[variableName];
 
-        // By the time execution reaches this point, the value should be a Uint8Array.
-        if (!(value instanceof Uint8Array)) {
+        // Libauth WalletData is always bytes. A number would compile incorrectly if we trusted TypeScript alone.
+        if (value instanceof Uint8Array === false) {
             throw new CashAssemblyVariableTypeMismatchError(variableName, 'Uint8Array', typeof value);
         }
 
         bytecode[variableName] = value;
     }
 
+    // `scriptId` is the source string registered in compileCashAssemblyEvaluations.
     const compiledBytecode = compiler.generateBytecode({
         data: { bytecode },
         scriptId: evaluation,
     });
 
-    if (!compiledBytecode.success) {
-        // Collapse libauth's full errors list into one string because
-        // CashAssemblyCompilationFailedError only carries a single message.
+    // A failed compile must not return partial bytecode.
+    if (compiledBytecode.success === false) {
+        // CashAssemblyCompilationFailedError carries one message. Libauth may report several errors.
         let compilationFailureMessage = 'unknown compilation failure';
 
+        // Prefer Libauth's own error text when the failure object includes an errors list.
         if ('errors' in compiledBytecode && compiledBytecode.errors.length > 0) {
             compilationFailureMessage = compiledBytecode.errors.map((compilationError) => compilationError.error).join('; ');
         }
@@ -254,28 +197,50 @@ export const generateCashAssemblyBytecode = (compiler: CompilerBch, evaluation:
 };
 
 /**
- * Prepares a compiler for the provided CashAssembly evaluations, setting required variables as 'WalletData'.
+ * Prepares a compiler for the provided CashAssembly source strings, setting required variables as 'WalletData'.
  *
- * @param {string[]} evaluations - Array of evaluation strings (e.g., ['$(<var1>)', '$(<var2> <var3>)']).
- * @returns {CompilerBch} A Libauth compiler instance for use with these evaluations.
+ * Each evaluation string is registered as a script whose id and source are that string. Template
+ * scripts are then copied onto the same table. If a source is a template id such as `scriptA`, that
+ * id must compile the template source. Leaving the id as its own source would compile the text
+ * `scriptA` and include itself.
+ *
+ * @param {CompileCashAssemblyEvaluationsParameters} parameters - Source strings, optional template scripts, and already collected WalletData names.
+ * @returns {CompilerBch} - A Libauth compiler instance for use with these source strings.
  */
-export const compileCashAssemblyEvaluations = (evaluations: string[]): CompilerBch => {
-    // Create a scripts object where each key is the evaluation and the value is also the evaluation
+export const compileCashAssemblyEvaluations = (parameters: CompileCashAssemblyEvaluationsParameters): CompilerBch => {
+    const { evaluations, templateScripts, variableNames } = parameters;
     const scripts: Record<string, string> = {};
+
+    // Libauth looks up scripts by id. Using the source string as the id lets generateBytecode request that exact source.
     for (const evaluation of evaluations) {
         scripts[evaluation] = evaluation;
     }
 
-    // Get the variable names from the evaluations.
-    const variableNames = extractVariablesFromEvaluations(evaluations);
+    const knownScriptIdentifiers = new Set<string>();
+
+    if (templateScripts !== undefined) {
+        for (const [ scriptIdentifier, scriptDefinition ] of Object.entries(templateScripts)) {
+            // Compiling `scriptA` must run the template source. Otherwise the id compiles as the text `scriptA` and includes itself.
+            scripts[scriptIdentifier] = scriptDefinition;
+
+            // Template ids compile as scripts. They must not also be registered as WalletData.
+            knownScriptIdentifiers.add(scriptIdentifier);
+        }
+    }
 
-    // Register each base name once. Dotted pushes share one WalletData entry under the base name.
     const variables: Record<string, { type: 'WalletData' }> = {};
+
+    // Register caller collected names. This function does not parse or collect again.
     for (const variableName of variableNames) {
+        // A name that is a template script id is already a compiler script, not WalletData.
+        if (knownScriptIdentifiers.has(variableName) === true) {
+            continue;
+        }
+
+        // Libauth WalletData is the name before the first `.`. `someKey.public_key` registers as `someKey`.
         variables[resolveIdentifierBaseName(variableName)] = { type: 'WalletData' as const };
     }
 
-    // Create the libauth compiler.
     const compiler = createCompilerBch({
         scripts,
         variables,
@@ -285,69 +250,188 @@ export const compileCashAssemblyEvaluations = (evaluations: string[]): CompilerB
 };
 
 /**
- * Compiles all CashAssembly evaluations in a text string and replaces each evaluation
- * with a decoded string representation.
+ * Compiles one CashAssembly source string to bytecode.
+ *
+ * The source may be an evaluation such as `$(scriptA)` or CashAssembly such as
+ * `scriptA`. WalletData names come from {@link collectVariablesUsingParseScript}.
+ *
+ * @param {CompileCashAssemblySourceToBytesParameters} parameters - Source text and compilation context.
+ * @returns {Uint8Array} - Compiled bytecode for that source.
+ * @throws {@link CashAssemblyRequiredVariableMissingError} - When a required variable is not present in the variables map.
+ * @throws {@link CashAssemblyPrimitiveMethodMissingError} - When a supported primitive hint has an unknown method.
+ * @throws {@link CashAssemblyPrimitiveVariableMissingError} - When a supported primitive method is missing its runtime value.
+ * @throws {@link CashAssemblyUnsupportedValueTypeError} - When a primitive method return type cannot be embedded as bytes.
+ * @throws {@link CashAssemblyNumberNotSafeIntegerError} - When a number variable is not a safe integer.
+ * @throws {@link CashAssemblyIdentifierCollisionError} - When one identifier matches two resolution types.
+ * @throws {@link CashAssemblyCompilationFailedError} - When Libauth compilation fails.
+ */
+export const compileCashAssemblySourceToBytes = (parameters: CompileCashAssemblySourceToBytesParameters): Uint8Array => {
+    const { cashAssemblySource, variables, templateVariables, templateScripts } = parameters;
+
+    // Throw if there is a identifier collision between opcodes, variable, or template script.
+    assertNoIdentifierCollisions({ variables, templateScripts });
+
+    // Collect WalletData names from the source and every reachable nested script source.
+    const variableNames = collectVariablesUsingParseScript(cashAssemblySource, templateScripts);
+
+    // Resolve supported primitive methods to bytes before treating remaining names as WalletData.
+    const primitiveMethodBytes = resolvePrimitiveMethodBytes({
+        variableNames,
+        templateVariables,
+        variables,
+    });
+
+    // Collect names that still need a caller supplied value after primitive resolution.
+    const missingVariables: string[] = [];
+    for (const variableName of variableNames) {
+        // Primitive methods already produced bytes under the full name.
+        if (Object.hasOwn(primitiveMethodBytes, variableName) === true) {
+            continue;
+        }
+
+        // A present runtime value will be converted to bytes below.
+        if (Object.hasOwn(variables, variableName) === true) {
+            continue;
+        }
+
+        missingVariables.push(variableName);
+    }
+
+    // Refuse invented defaults so a missing value cannot compile into incorrect bytecode.
+    if (missingVariables.length > 0) {
+        throw new CashAssemblyRequiredVariableMissingError(missingVariables);
+    }
+
+    // Convert each variable to its bytes before compilation.
+    const variableBytes: Record<string, Uint8Array> = {};
+    for (const variableName of variableNames) {
+        // Prefer the primitive method result so CashAssembly sees bytes under `base.method`.
+        if (Object.hasOwn(primitiveMethodBytes, variableName) === true) {
+            variableBytes[variableName] = primitiveMethodBytes[variableName];
+            continue;
+        }
+
+        // Remaining names are caller WalletData that still needs a byte encoding.
+        variableBytes[variableName] = convertValueToBytes(variables[variableName], variableName);
+    }
+
+    // Reuse the names collected above so this compiler is not built from a second parse.
+    const compiler = compileCashAssemblyEvaluations({
+        evaluations: [ cashAssemblySource ],
+        templateScripts,
+        variableNames,
+    });
+
+    return generateCashAssemblyBytecode(compiler, cashAssemblySource, variableBytes, variableNames);
+};
+
+/**
+ * Compiles one CashAssembly source to bytes and decodes those bytes to text.
+ *
+ * @param {CompileCashAssemblySourceToDecodedTextParameters} parameters - Source text, WalletData, optional template context, and decode mode.
+ * @returns {string} - Decoded compilation result for that source.
+ * @throws {@link CashAssemblyRequiredVariableMissingError} - When a required variable is not present in the variables map.
+ * @throws {@link CashAssemblyPrimitiveMethodMissingError} - When a supported primitive hint has an unknown method.
+ * @throws {@link CashAssemblyPrimitiveVariableMissingError} - When a supported primitive method is missing its runtime value.
+ * @throws {@link CashAssemblyUnsupportedValueTypeError} - When a primitive method return type cannot be embedded as bytes.
+ * @throws {@link CashAssemblyNumberNotSafeIntegerError} - When a number variable is not a safe integer.
+ * @throws {@link CashAssemblyVmNumberDecodeError} - When `evaluationDecodeMode` is `bigint` and compiled bytes are not a VM number.
+ * @throws {@link CashAssemblyIdentifierCollisionError} - When one identifier matches two resolution types.
+ * @throws {@link CashAssemblyCompilationFailedError} - When Libauth compilation fails.
+ */
+const compileCashAssemblySourceToDecodedText = (parameters: CompileCashAssemblySourceToDecodedTextParameters): string => {
+    const { cashAssemblySource, variables, templateVariables, templateScripts, evaluationDecodeMode } = parameters;
+
+    const compilationResult = compileCashAssemblySourceToBytes({
+        cashAssemblySource,
+        variables,
+        templateVariables,
+        templateScripts,
+    });
+
+    return decodeCompiledCashAssemblyEvaluation(compilationResult, evaluationDecodeMode);
+};
+
+/**
+ * Compiles CashAssembly text. WalletData names come from parseScript. `$()` evaluations in text
+ * are found by scanning the string and matching parentheses.
+ *
+ * When `cashAssemblyText` contains `$()`, each non empty evaluation is compiled and
+ * replaced in place. Empty `$()` is copied through. Surrounding text is kept. Evaluation drops
+ * push opcodes and keeps the stack payload.
+ *
+ * When `cashAssemblyText` contains no `$()` at all, the entire string is compiled as CashAssembly.
+ * Push opcodes from `<...>` remain. Pass a template script id such as `scriptA` or
+ * concatenated ids such as `scriptA scriptB`.
+ *
+ * Required WalletData names come from {@link collectVariablesUsingParseScript}.
  *
  * @param {CompileCashAssemblyStringParameters} parameters - Parameters for compiling the CashAssembly string.
- * @param {string} parameters.cashAssemblyText - The string with CashAssembly evaluations.
- * @param {Record<string, XOInvitationVariableValue | Uint8Array>} parameters.variables - Object mapping
- * variable names to values for compilation.
- * @param {CompiledCashAssemblyDecodeMode} [parameters.evaluationDecodeMode='utf8'] - The decode mode used
- * after each evaluation is compiled. See {@link decodeCompiledCashAssemblyEvaluation}.
- * @param {XOTemplate['variables']} [parameters.templateVariables] - Optional template variable definitions
- * used to resolve supported `<name.method>` pushes via each variable's `hint`.
- * @returns {string} Compiled text with all evaluations replaced by decoded string values.
- * @throws {@link CashAssemblyRequiredVariableMissingError} When a required variable is not present in the variables map.
- * @throws {@link CashAssemblyPrimitiveMethodMissingError} When a supported primitive hint has an unknown method.
- * @throws {@link CashAssemblyPrimitiveVariableMissingError} When a supported primitive method is missing its runtime value.
- * @throws {@link CashAssemblyUnsupportedValueTypeError} When a primitive method return type cannot be embedded as bytes.
- * @throws {@link CashAssemblyNumberNotSafeIntegerError} When a number variable is not a safe integer.
- * @throws {@link CashAssemblyVmNumberDecodeError} When `evaluationDecodeMode` is `bigint` and an evaluation is not a VM number.
+ * @returns {string} - Compiled text with evaluations replaced, or decoded bytecode when the whole string is compiled as CashAssembly.
+ * @throws {@link CashAssemblyRequiredVariableMissingError} - When a required variable is not present in the variables map.
+ * @throws {@link CashAssemblyPrimitiveMethodMissingError} - When a supported primitive hint has an unknown method.
+ * @throws {@link CashAssemblyPrimitiveVariableMissingError} - When a supported primitive method is missing its runtime value.
+ * @throws {@link CashAssemblyUnsupportedValueTypeError} - When a primitive method return type cannot be embedded as bytes.
+ * @throws {@link CashAssemblyNumberNotSafeIntegerError} - When a number variable is not a safe integer.
+ * @throws {@link CashAssemblyVmNumberDecodeError} - When `evaluationDecodeMode` is `bigint` and compiled bytes are not a VM number.
+ * @throws {@link CashAssemblyIdentifierCollisionError} - When one identifier matches two resolution types.
+ * @throws {@link CashAssemblyCompilationFailedError} - When Libauth compilation fails.
+ * @throws {@link CashAssemblyEvaluationUnclosedError} - When an evaluation never closes.
+ * @throws {@link CashAssemblyQuotedLiteralUnclosedError} - When a quoted literal never closes.
+ * @throws {@link CashAssemblyBlockCommentUnclosedError} - When a block comment never closes.
  */
 export const compileCashAssemblyString = (parameters: CompileCashAssemblyStringParameters): string => {
-    const { cashAssemblyText, variables, evaluationDecodeMode = 'utf8', templateVariables } = parameters;
+    const { cashAssemblyText, variables, evaluationDecodeMode = 'utf8', templateVariables, templateScripts } = parameters;
 
-    return cashAssemblyText.replace(CASHASSEMBLY_EVALUATION_PATTERN, (evaluation) => {
-        // Extract variable identifiers required by the current evaluation.
-        const variableNames = extractVariablesFromEvaluations([ evaluation ]);
+    // Scan once so empty `$()` still copies text around `$()` instead of compiling as CashAssembly.
+    const scannedEvaluations = scanCashAssemblyEvaluations(cashAssemblyText);
 
-        const primitiveMethodBytes = resolvePrimitiveMethodBytes({
-            identifiers: variableNames,
-            templateVariables,
+    if (scannedEvaluations.length === 0) {
+        return compileCashAssemblySourceToDecodedText({
+            cashAssemblySource: cashAssemblyText,
             variables,
+            templateVariables,
+            templateScripts,
+            evaluationDecodeMode,
         });
+    }
 
-        // Prefer resolved method bytes. Fall back to converting the raw variable value.
-        const missingVariables = variableNames.filter((variableName) => {
-            if (Object.hasOwn(primitiveMethodBytes, variableName) === true) {
-                return false;
-            }
+    // Copy text outside `$()` and compile each `$()`.
+    let textWithCompiledEvaluations = '';
 
-            return Object.hasOwn(variables, variableName) === false;
-        });
+    // Start of the next original text slice in cashAssemblyText. Gaps between evaluations must stay intact.
+    let nextTextStartIndex = 0;
 
-        if (missingVariables.length > 0) {
-            throw new CashAssemblyRequiredVariableMissingError(missingVariables);
+    // When every `$()` is empty, return cashAssemblyText so the original text is unchanged.
+    let hasCompiledNonEmptyEvaluation = false;
+
+    for (const scannedEvaluation of scannedEvaluations) {
+        textWithCompiledEvaluations += cashAssemblyText.slice(nextTextStartIndex, scannedEvaluation.startIndex);
+
+        // Empty `$()` has no contents to compile.
+        if (scannedEvaluation.evaluationText === EMPTY_CASHASSEMBLY_EVALUATION) {
+            textWithCompiledEvaluations += scannedEvaluation.evaluationText;
+        } else {
+            hasCompiledNonEmptyEvaluation = true;
+            textWithCompiledEvaluations += compileCashAssemblySourceToDecodedText({
+                cashAssemblySource: scannedEvaluation.evaluationText,
+                variables,
+                templateVariables,
+                templateScripts,
+                evaluationDecodeMode,
+            });
         }
 
-        // Convert each variable to its bytes before compilation.
-        const variableBytes: Record<string, Uint8Array> = {};
-        for (const variableName of variableNames) {
-            if (Object.hasOwn(primitiveMethodBytes, variableName) === true) {
-                variableBytes[variableName] = primitiveMethodBytes[variableName];
-                continue;
-            }
+        // Advance past this `$()` so nested evaluations inside it are not copied again.
+        nextTextStartIndex = scannedEvaluation.closeIndex + 1;
+    }
 
-            variableBytes[variableName] = convertValueToBytes(variables[variableName], variableName);
-        }
+    // Copy text after the last `$()` so the suffix is kept.
+    textWithCompiledEvaluations += cashAssemblyText.slice(nextTextStartIndex);
 
-        // Compile the evaluation in isolation.
-        const compiler = compileCashAssemblyEvaluations([ evaluation ]);
+    if (hasCompiledNonEmptyEvaluation === false) {
+        return cashAssemblyText;
+    }
 
-        // Generate the bytes for the evaluation.
-        const compilationResult: Uint8Array = generateCashAssemblyBytecode(compiler, evaluation, variableBytes);
-
-        // Decode the bytes into a string as per the decode mode.
-        return decodeCompiledCashAssemblyEvaluation(compilationResult, evaluationDecodeMode);
-    });
+    return textWithCompiledEvaluations;
 };

source/cash-assembly/identifier-collisions.ts

@@ -0,0 +1,59 @@
+import { generateBytecodeMap, IdentifierResolutionType, OpcodesBchSpec } from '@bitauth/libauth';
+import { CashAssemblyIdentifierCollisionError } from './errors.ts';
+import type { CompileCashAssemblyContext } from './types.ts';
+
+/**
+ * Set of opcode names that the compiler recognizes.
+ */
+const COMPILER_OPCODE_NAMES: ReadonlySet<string> = new Set(Object.keys(generateBytecodeMap(OpcodesBchSpec)));
+
+/**
+ * Parameters for {@link assertNoIdentifierCollisions} function.
+ */
+export type AssertNoIdentifierCollisionsParameters = Pick<CompileCashAssemblyContext, 'variables' | 'templateScripts'>;
+
+/**
+ * Libauth resolves opcodes, then variables, then scripts, and the first match silently hides the
+ * rest, leading to unexpected compilation results and no errors. This function throws when one identifier matches more than one resolution type.
+ *
+ * @param {AssertNoIdentifierCollisionsParameters} parameters - Provided variables and optional template scripts.
+ * @throws {@link CashAssemblyIdentifierCollisionError} - When one identifier matches more than one resolution type.
+ */
+export const assertNoIdentifierCollisions = (parameters: AssertNoIdentifierCollisionsParameters): void => {
+    const { variables, templateScripts } = parameters;
+
+    // Create a set of the script identifiers from the template scripts map.
+    const scriptIdentifiers = new Set(Object.keys(templateScripts ?? {}));
+
+    const variableNames = new Set(Object.keys(variables));
+
+    // Combine both, the variableNames and the scriptIdentifiers, to get a set of all identifiers.
+    const declaredIdentifiers = new Set([ ...variableNames, ...scriptIdentifiers ]);
+
+    // This loop will check for more than one occurrence of an identifier in OP_CODES, variables, and scripts
+    for (const identifier of declaredIdentifiers) {
+        // Resolution types is a native construct from libauth.
+        const resolutionTypes: IdentifierResolutionType[] = [];
+
+        // If the identifier is an opcode, then push it to the resolution types array, this will also be
+        // detected by either or both of the variableNames and scriptIdentifiers checks.
+        if (COMPILER_OPCODE_NAMES.has(identifier) === true) {
+            resolutionTypes.push(IdentifierResolutionType.opcode);
+        }
+
+        // If the identifier is in variables, then push it to the resolution types array.
+        if (variableNames.has(identifier) === true) {
+            resolutionTypes.push(IdentifierResolutionType.variable);
+        }
+
+        // If the identifier is a script identifier, then push it to the resolution types array.
+        if (scriptIdentifiers.has(identifier) === true) {
+            resolutionTypes.push(IdentifierResolutionType.script);
+        }
+
+        // If there are more than one resolution types, then throw an error.
+        if (resolutionTypes.length > 1) {
+            throw new CashAssemblyIdentifierCollisionError(identifier, resolutionTypes);
+        }
+    }
+};

source/cash-assembly/index.ts

@@ -1,5 +1,5 @@
-export * from './evaluations.ts';
-export * from './primitive-evaluations.ts';
-export * from './bytes.ts';
-export * from './defaults.ts';
+export { collectVariablesUsingParseScript } from './collect-evaluations.ts';
 export * from './errors.ts';
+export { compileCashAssemblyString } from './evaluations.ts';
+export { extractCashAssemblyEvaluations, isCashAssemblyExpression } from './scan-evaluations.ts';
+export type { CompiledCashAssemblyDecodeMode, CompileCashAssemblyStringParameters } from './types.ts';

source/cash-assembly/primitive-evaluations.ts

@@ -16,10 +16,10 @@ import { CASHASSEMBLY_VARIABLE_METHOD_REFERENCE_PATTERN } from './defaults.ts';
 import { convertValueToBytes } from './bytes.ts';
 
 /**
- * Template hint values mapped to a primitive class for resolving primitive method evaluations.
- * Keys are values from XOTemplatePrimitiveTypes.
+ * Maps method resolvable template types to `@xo-cash/primitives` classes for `base.method` resolution.
+ * Keys are a subset of {@link XOTemplatePrimitiveTypes}.
  */
-const PRIMITIVE_BY_TEMPLATE_HINT = {
+const RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE = {
     [XOTemplatePrimitiveTypes.FUNGIBLE_TOKEN_AMOUNT]: FungibleTokenAmount,
     [XOTemplatePrimitiveTypes.NFT_COMMITMENT]: NFTCommitment,
     [XOTemplatePrimitiveTypes.PUBLIC_KEY]: PublicKey,
@@ -31,7 +31,11 @@ const PRIMITIVE_BY_TEMPLATE_HINT = {
     [XOTemplatePrimitiveTypes.TRANSACTION_HASH]: TransactionHash,
 } as const;
 
-type SupportedPrimitiveHint = keyof typeof PRIMITIVE_BY_TEMPLATE_HINT;
+/**
+ * Template primitive types that map to an `@xo-cash/primitives` class for `base.method` resolution.
+ * This is a subset of {@link XOTemplatePrimitiveType}, not every declared template type.
+ */
+type ResolvablePrimitiveType = keyof typeof RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE;
 
 /**
  * Inputs needed to call a primitive method for a `name.method` push.
@@ -54,21 +58,21 @@ type CallPrimitiveMethodParameters = {
     value: unknown;
 
     /**
-     * Supported template hint that selects the primitive class.
+     * Method resolvable template type that selects the primitive class.
      */
-    hint: SupportedPrimitiveHint;
+    type: ResolvablePrimitiveType;
 };
 
 /**
- * Inputs needed to resolve primitive method pushes from extracted CashAssembly identifiers.
+ * Inputs needed to resolve primitive method pushes from collected CashAssembly variable names.
  */
 export type ResolvePrimitiveMethodBytesParameters = {
 
     /**
-     * Variable identifiers from {@link extractVariablesFromEvaluations}, for example
+     * Variable names from {@link collectVariablesUsingParseScript}, for example
      * `['amount.toSatoshis', 'fee.toSatoshis']`.
      */
-    identifiers: string[];
+    variableNames: string[];
 
     /**
      * Variable names and values object.
@@ -77,30 +81,30 @@ export type ResolvePrimitiveMethodBytesParameters = {
 
     /**
      * Template variable definitions. When omitted, no primitive methods are resolved.
-     * The `hint` on each entry selects the primitive class.
+     * The `type` on each entry selects the primitive class.
      */
     templateVariables?: XOTemplate['variables'];
 };
 
 /**
- * Returns true when `hint` maps to a supported primitive class in `PRIMITIVE_BY_TEMPLATE_HINT`.
+ * Returns true when `type` maps to a primitive class in `RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE` for method resolution.
  *
- * @param {XOTemplatePrimitiveType | undefined} hint - Template variable hint.
- * @returns {boolean} True when the hint selects a supported primitive class.
+ * @param {XOTemplatePrimitiveType | undefined} type - Template variable type.
+ * @returns {boolean} True when the type can resolve `base.method` through a primitive class.
  */
-const isSupportedPrimitiveHint = (hint: XOTemplatePrimitiveType | undefined): hint is SupportedPrimitiveHint => {
-    return hint !== undefined && Object.hasOwn(PRIMITIVE_BY_TEMPLATE_HINT, hint) === true;
+const isResolvablePrimitiveType = (type: XOTemplatePrimitiveType | undefined): type is ResolvablePrimitiveType => {
+    return type !== undefined && Object.hasOwn(RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE, type) === true;
 };
 
 /**
- * Returns true when `methodName` is an own function on the primitive class for `hint`.
+ * Returns true when `methodName` is an own function on the primitive class for `type`.
  *
- * @param {SupportedPrimitiveHint} hint - Supported template hint.
+ * @param {ResolvablePrimitiveType} type - Method resolvable template type.
  * @param {string} methodName - Method name from the evaluation text, for example `toSatoshis`.
- * @returns {boolean} True when that class exposes the named method.
+ * @returns {boolean} - True when that class exposes the named method.
  */
-const canResolvePrimitiveMethod = (hint: SupportedPrimitiveHint, methodName: string): boolean => {
-    const PrimitiveClass = PRIMITIVE_BY_TEMPLATE_HINT[hint];
+const canResolvePrimitiveMethod = (type: ResolvablePrimitiveType, methodName: string): boolean => {
+    const PrimitiveClass = RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE[type];
 
     // Check own properties only so inherited Object.prototype names are rejected without needing a value.
     if (Object.hasOwn(PrimitiveClass.prototype, methodName) === false) {
@@ -113,17 +117,17 @@ const canResolvePrimitiveMethod = (hint: SupportedPrimitiveHint, methodName: str
 /**
  * Constructs a primitive from a raw value and calls one instance method on it.
  *
- * Call only after `canResolvePrimitiveMethod` is true for the same hint and method.
+ * Call only after `canResolvePrimitiveMethod` is true for the same type and method.
  *
- * @param {CallPrimitiveMethodParameters} parameters - Identifier, method, value, and supported hint.
+ * @param {CallPrimitiveMethodParameters} parameters - Identifier, method, value, and resolvable type.
  * @returns {unknown} Method return value, later encoded as CashAssembly push bytes.
  * @throws {@link CashAssemblyPrimitiveMethodMissingError} When the prototype member is not a function.
  * @throws When the primitive constructor rejects the raw value (validation errors from `@xo-cash/primitives`).
  */
 const callPrimitiveMethod = (parameters: CallPrimitiveMethodParameters): unknown => {
-    const { identifier, methodName, value, hint } = parameters;
+    const { identifier, methodName, value, type } = parameters;
 
-    const PrimitiveClass = PRIMITIVE_BY_TEMPLATE_HINT[hint];
+    const PrimitiveClass = RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE[type];
 
     // Constructing runs each primitive's own input validation (range checks, hex length, etc).
     // `as never` satisfies TypeScript across constructors that accept different input shapes.
@@ -133,45 +137,45 @@ const callPrimitiveMethod = (parameters: CallPrimitiveMethodParameters): unknown
     const primitiveMethod = Reflect.get(PrimitiveClass.prototype, methodName);
 
     if (typeof primitiveMethod !== 'function') {
-        throw new CashAssemblyPrimitiveMethodMissingError(identifier, methodName, hint);
+        throw new CashAssemblyPrimitiveMethodMissingError(identifier, methodName, type);
     }
 
     return primitiveMethod.call(primitiveInstance);
 };
 
 /**
- * Resolves supported `base.method` identifiers to CashAssembly variable bytes.
+ * Resolves method resolvable `base.method` identifiers to CashAssembly variable bytes.
  *
- * Each single dot identifier whose `hint` maps to a supported primitive is resolved and stored under
- * the full identifier (`base.method`). Unsupported or multi dot identifiers are left for CashAssembly.
+ * Each single dot identifier whose `type` is in `RESOLVABLE_PRIMITIVE_CLASS_BY_TYPE` is resolved and stored under
+ * the full identifier (`base.method`). Other template types or multi dot identifiers are left for CashAssembly.
  *
  * When `templateVariables` is omitted, returns an empty map.
  *
  * @param {ResolvePrimitiveMethodBytesParameters} parameters - Identifiers, values, and optional template metadata.
  * @returns {Record<string, Uint8Array>} Resolved method identifiers mapped to bytes for CashAssembly pushes.
- * @throws {@link CashAssemblyPrimitiveMethodMissingError} When the hint is a supported primitive but the method is missing.
+ * @throws {@link CashAssemblyPrimitiveMethodMissingError} When the type is method resolvable but the method is missing.
  * @throws {@link CashAssemblyPrimitiveVariableMissingError} When the runtime value is missing from the variables map.
  * @throws {@link CashAssemblyUnsupportedValueTypeError} When the method return type cannot be embedded.
  * @throws {@link CashAssemblyNumberNotSafeIntegerError} When a method return value is a number that is not a safe integer.
  */
 export const resolvePrimitiveMethodBytes = (parameters: ResolvePrimitiveMethodBytesParameters): Record<string, Uint8Array> => {
-    const { identifiers, templateVariables, variables } = parameters;
+    const { variableNames, templateVariables, variables } = parameters;
 
-    // Without template metadata there is no hint to select a primitive class.
+    // Without template metadata there is no type to select a primitive class.
     if (templateVariables === undefined) {
         return {};
     }
 
     const resolvedBytes: Record<string, Uint8Array> = {};
 
-    for (const identifier of identifiers) {
-        // The same identifier can appear more than once. Resolve it only once.
-        if (Object.hasOwn(resolvedBytes, identifier) === true) {
+    for (const variableName of variableNames) {
+        // The same name can appear more than once. Resolve it only once.
+        if (Object.hasOwn(resolvedBytes, variableName) === true) {
             continue;
         }
 
-        // Find the primitive method reference in the identifier.
-        const methodReferenceMatch = identifier.match(CASHASSEMBLY_VARIABLE_METHOD_REFERENCE_PATTERN);
+        // Find the primitive method reference in the name.
+        const methodReferenceMatch = variableName.match(CASHASSEMBLY_VARIABLE_METHOD_REFERENCE_PATTERN);
 
         if (methodReferenceMatch === null) {
             continue;
@@ -179,37 +183,38 @@ export const resolvePrimitiveMethodBytes = (parameters: ResolvePrimitiveMethodBy
 
         const [ , baseName, methodName ] = methodReferenceMatch;
 
-        // Unknown identifiers and CashAssembly native operations such as someKey.schnorr_signature.all_outputs
+        // Unknown names and CashAssembly native operations such as someKey.schnorr_signature.all_outputs
         // must be left for CashAssembly rather than treated as primitive failures.
         if (Object.hasOwn(templateVariables, baseName) === false) {
             continue;
         }
 
-        const hint = templateVariables[baseName].hint;
+        const type = templateVariables[baseName].type;
 
-        if (isSupportedPrimitiveHint(hint) === false) {
+        // Template types without a mapped primitive class are left for CashAssembly.
+        if (isResolvablePrimitiveType(type) === false) {
             continue;
         }
 
-        // Supported hint with an unknown method should be thrown as an error.
-        if (canResolvePrimitiveMethod(hint, methodName) === false) {
-            throw new CashAssemblyPrimitiveMethodMissingError(identifier, methodName, hint);
+        // Method resolvable type with an unknown method should throw.
+        if (canResolvePrimitiveMethod(type, methodName) === false) {
+            throw new CashAssemblyPrimitiveMethodMissingError(variableName, methodName, type);
         }
 
         // If the method is known but the runtime value is missing, throw an error.
         if (Object.hasOwn(variables, baseName) === false) {
-            throw new CashAssemblyPrimitiveVariableMissingError(identifier, baseName);
+            throw new CashAssemblyPrimitiveVariableMissingError(variableName, baseName);
         }
 
         const methodResult = callPrimitiveMethod({
-            identifier,
+            identifier: variableName,
             methodName,
             value: variables[baseName],
-            hint,
+            type,
         });
 
-        // Store the result under identifier.
-        resolvedBytes[identifier] = convertValueToBytes(methodResult, identifier);
+        // CashAssembly looks up the full `base.method` name as WalletData bytes.
+        resolvedBytes[variableName] = convertValueToBytes(methodResult, variableName);
     }
 
     return resolvedBytes;

source/cash-assembly/scan-evaluations.ts

@@ -0,0 +1,272 @@
+import {
+    CASHASSEMBLY_BLOCK_COMMENT_END_LENGTH,
+    CASHASSEMBLY_COMMENT_START_LENGTH,
+    CASHASSEMBLY_EVALUATION_START_LENGTH,
+    EMPTY_CASHASSEMBLY_EVALUATION,
+} from './defaults.ts';
+import { CashAssemblyBlockCommentUnclosedError, CashAssemblyEvaluationUnclosedError, CashAssemblyQuotedLiteralUnclosedError } from './errors.ts';
+
+/**
+ * Skips a quoted CashAssembly string so a `)` inside it cannot end `$()`.
+ *
+ * CashAssembly UTF8 literals use `"..."` or `'...'`. Everything between the quotes is payload,
+ * including parentheses.
+ * For example `$(<"hello)world">)` must close after the literal, not at the `)` inside the quotes.
+ *
+ * @param {string} cashAssemblyText - Text that contains the quoted literal.
+ * @param {number} openingQuoteIndex - Index of the opening `"` or `'`.
+ * @returns {number} - Index of the character after the closing quote.
+ * @throws {@link CashAssemblyQuotedLiteralUnclosedError} - When the quoted literal never closes.
+ */
+const skipQuotedCashAssemblyLiteral = (cashAssemblyText: string, openingQuoteIndex: number): number => {
+    const quoteCharacter = cashAssemblyText[openingQuoteIndex];
+    let currentIndex = openingQuoteIndex + 1;
+
+    // CashAssembly puts payload `)` inside the quotes. Stop only at the matching quote.
+    while (currentIndex < cashAssemblyText.length) {
+        if (cashAssemblyText[currentIndex] === quoteCharacter) {
+            return currentIndex + 1;
+        }
+
+        currentIndex += 1;
+    }
+
+    throw new CashAssemblyQuotedLiteralUnclosedError(openingQuoteIndex, quoteCharacter, cashAssemblyText);
+};
+
+/**
+ * This function skips a `//` comment so a `)` written in the comment cannot end `$()`.
+ * A line comment runs from `//` to the newline, or to the end of the string when there is no newline.
+ * A `)` in that span is comment text, not the closer of `$()`.
+ *
+ * @param {string} cashAssemblyText - Text that contains the line comment.
+ * @param {number} commentStartIndex - Index of the first `/` of `//`.
+ * @returns {number} - Index after the newline that ends the comment, or the end of the string if there is no newline.
+ */
+const skipSingleLineCashAssemblyComment = (cashAssemblyText: string, commentStartIndex: number): number => {
+    let currentIndex = commentStartIndex + CASHASSEMBLY_COMMENT_START_LENGTH;
+
+    // CashAssembly line comments hide `)` until the newline. End of string also ends them.
+    while (currentIndex < cashAssemblyText.length) {
+        if (cashAssemblyText[currentIndex] === '\n') {
+            return currentIndex + 1;
+        }
+
+        currentIndex += 1;
+    }
+
+    return currentIndex;
+};
+
+/**
+ * This function skips a block comment so a `)` written in the comment cannot end `$()`.
+ * A `)` inside a block comment is comment text, not the closer of `$()`.
+ *
+ * @param {string} cashAssemblyText - Text that contains the block comment.
+ * @param {number} commentStartIndex - Index of the first slash of the block comment opener.
+ * @returns {number} - Index of the character after the comment closer.
+ * @throws {@link CashAssemblyBlockCommentUnclosedError} - When the block comment never closes.
+ */
+const skipBlockCashAssemblyComment = (cashAssemblyText: string, commentStartIndex: number): number => {
+    let currentIndex = commentStartIndex + CASHASSEMBLY_COMMENT_START_LENGTH;
+
+    // CashAssembly block comments hide `)` until the closer is found.
+    while (currentIndex + 1 < cashAssemblyText.length) {
+        if (cashAssemblyText[currentIndex] === '*' && cashAssemblyText[currentIndex + 1] === '/') {
+            return currentIndex + CASHASSEMBLY_BLOCK_COMMENT_END_LENGTH;
+        }
+
+        currentIndex += 1;
+    }
+
+    throw new CashAssemblyBlockCommentUnclosedError(commentStartIndex, cashAssemblyText);
+};
+
+/**
+ * Parenthesis matching with a depth count.
+ *
+ * Returns the index of the `)` that closes the `$()` starting at `evaluationStartIndex`.
+ * Nested `$()` raise the depth so an inner closer cannot finish the outer evaluation.
+ *
+ * @param {string} cashAssemblyText - Text that contains the evaluation.
+ * @param {number} evaluationStartIndex - Index of the `$` that opens this evaluation.
+ * @returns {number} - Index of the matching closing parenthesis.
+ * @throws {@link CashAssemblyEvaluationUnclosedError} - When the evaluation never closes.
+ * @throws {@link CashAssemblyQuotedLiteralUnclosedError} - When a quoted literal inside the evaluation never closes.
+ * @throws {@link CashAssemblyBlockCommentUnclosedError} - When a block comment inside the evaluation never closes.
+ */
+const findCashAssemblyEvaluationCloseIndex = (cashAssemblyText: string, evaluationStartIndex: number): number => {
+    let currentIndex = evaluationStartIndex + CASHASSEMBLY_EVALUATION_START_LENGTH;
+    let remainingOpenEvaluations = 1;
+
+    // Scan the evaluation contents until the matching closer at depth zero.
+    while (currentIndex < cashAssemblyText.length) {
+        const currentCharacter = cashAssemblyText[currentIndex];
+        const nextCharacter = cashAssemblyText[currentIndex + 1];
+
+        // CashAssembly allows `)` inside `"..."` and `'...'`. Skip the literal so that closer is not structural.
+        if (currentCharacter === '"' || currentCharacter === "'") {
+            currentIndex = skipQuotedCashAssemblyLiteral(cashAssemblyText, currentIndex);
+
+            continue;
+        }
+
+        // CashAssembly `//` comments may mention `)`. Skip them so that `)` is not treated as a closer.
+        if (currentCharacter === '/' && nextCharacter === '/') {
+            currentIndex = skipSingleLineCashAssemblyComment(cashAssemblyText, currentIndex);
+
+            continue;
+        }
+
+        // CashAssembly block comments may mention `)`. Skip them so that `)` is not treated as a closer.
+        if (currentCharacter === '/' && nextCharacter === '*') {
+            currentIndex = skipBlockCashAssemblyComment(cashAssemblyText, currentIndex);
+
+            continue;
+        }
+
+        // Nested `$()` must close before this evaluation can close.
+        if (currentCharacter === '$' && nextCharacter === '(') {
+            // Since all previous checks have passed then it means this is start of another evaluation, it we must
+            // increment the remainingOpenEvaluations counter and move the currentIndex to the next character.
+            remainingOpenEvaluations += 1;
+            currentIndex += CASHASSEMBLY_EVALUATION_START_LENGTH;
+
+            continue;
+        }
+
+        // A closer at depth one ends this evaluation. Inner closers only reduce nested depth.
+        if (currentCharacter === ')') {
+            // Since all previous checks have passed then it means this is a closer for the current evaluation, we must
+            // decrement the remainingOpenEvaluations counter and check if it is now zero. If it is then we have found the
+            // matching closer and we can return the currentIndex.
+            remainingOpenEvaluations -= 1;
+
+            if (remainingOpenEvaluations === 0) {
+                return currentIndex;
+            }
+        }
+
+        // Move the currentIndex to the next character.
+        currentIndex += 1;
+    }
+
+    // The matching closer was never found.
+    throw new CashAssemblyEvaluationUnclosedError(evaluationStartIndex, remainingOpenEvaluations, cashAssemblyText);
+};
+
+/**
+ * One `$()` found in text that is not nested inside another `$()`.
+ */
+type CashAssemblyEvaluationScanMatch = {
+
+    /**
+     * Complete `$()` text including the opening `$` and the matching closer.
+     */
+    evaluationText: string;
+
+    /**
+     * Index of the `$` that opens this evaluation.
+     */
+    startIndex: number;
+
+    /**
+     * Index of the matching closing parenthesis.
+     */
+    closeIndex: number;
+};
+
+/**
+ * Scans text and returns every `$()` that starts in this string, including empty `$()`.
+ *
+ * Nested `$()` stay inside the outer evaluation.
+ *
+ * @param {string} cashAssemblyText - Text that may contain `$()`.
+ * @returns {CashAssemblyEvaluationScanMatch[]} - Evaluations in left to right order.
+ * @throws {@link CashAssemblyEvaluationUnclosedError} - When an evaluation never closes.
+ * @throws {@link CashAssemblyQuotedLiteralUnclosedError} - When a quoted literal never closes.
+ * @throws {@link CashAssemblyBlockCommentUnclosedError} - When a block comment never closes.
+ */
+export const scanCashAssemblyEvaluations = (cashAssemblyText: string): CashAssemblyEvaluationScanMatch[] => {
+    const scannedEvaluations: CashAssemblyEvaluationScanMatch[] = [];
+    let currentIndex = 0;
+
+    // Find `$()` that starts in this string. Nested evaluations stay inside the outer evaluation.
+    while (currentIndex < cashAssemblyText.length) {
+        const currentCharacter = cashAssemblyText[currentIndex];
+        const nextCharacter = cashAssemblyText[currentIndex + 1];
+
+        // A `$` that is followed by `(`.
+        if (currentCharacter === '$' && nextCharacter === '(') {
+            // Count parentheses so quotes, comments, and nested `$()` cannot close this evaluation early.
+            const closeIndex = findCashAssemblyEvaluationCloseIndex(cashAssemblyText, currentIndex);
+            // Slice the complete `$()` so later steps do not re-scan these characters.
+            const evaluationText = cashAssemblyText.slice(currentIndex, closeIndex + 1);
+
+            // Store the outer `$()`. Nested evaluations are already inside evaluationText.
+            scannedEvaluations.push({
+                closeIndex,
+                evaluationText,
+                startIndex: currentIndex,
+            });
+
+            // Skip to after this `$()` so nested evaluations are not scanned as their own matches.
+            currentIndex = closeIndex + 1;
+
+            continue;
+        }
+
+        currentIndex += 1;
+    }
+
+    return scannedEvaluations;
+};
+
+/**
+ * This function returns each `$()` that starts in this string.
+ * Empty `$()` is omitted.
+ *
+ * @param {string} cashAssemblyText - Text that may contain `$()`.
+ * @returns {string[]} - Complete non empty evaluation strings in left to right order.
+ * @throws {@link CashAssemblyEvaluationUnclosedError} - When an evaluation never closes.
+ * @throws {@link CashAssemblyQuotedLiteralUnclosedError} - When a quoted literal never closes.
+ * @throws {@link CashAssemblyBlockCommentUnclosedError} - When a block comment never closes.
+ */
+export const extractCashAssemblyEvaluations = (cashAssemblyText: string): string[] => {
+    const scannedEvaluations = scanCashAssemblyEvaluations(cashAssemblyText);
+    const evaluations: string[] = [];
+
+    // Omit empty `$()` so callers that compile extracted text do not treat it as an evaluation.
+    for (const scannedEvaluation of scannedEvaluations) {
+        if (scannedEvaluation.evaluationText !== EMPTY_CASHASSEMBLY_EVALUATION) {
+            evaluations.push(scannedEvaluation.evaluationText);
+        }
+    }
+
+    return evaluations;
+};
+
+/**
+ * This function returns true when the value is exactly one CashAssembly `$()`.
+ * Text with characters outside `$()`, empty `$()`, or a failed scan returns false.
+ *
+ * @param {unknown} expression - Value to test.
+ * @returns {boolean} - True when the value is a string that is one complete evaluation and nothing else.
+ */
+export const isCashAssemblyExpression = (expression: unknown): boolean => {
+    // Numbers and other types are never CashAssembly evaluations.
+    if (typeof expression !== 'string') {
+        return false;
+    }
+
+    try {
+        const evaluations = extractCashAssemblyEvaluations(expression);
+
+        // If there is only one evaluation extracted and the returned value matches to that of the expression passed in the parameter,
+        // then return true.
+        return evaluations.length === 1 && evaluations[0] === expression;
+    } catch {
+        // A failed scan is not a complete evaluation.
+        return false;
+    }
+};

source/cash-assembly/types.ts

@@ -0,0 +1,108 @@
+import type { XOInvitationVariableValue, XOTemplate } from '@xo-cash/types';
+
+/**
+ * Supported decode modes for compiled CashAssembly evaluation bytes.
+ */
+export type CompiledCashAssemblyDecodeMode = 'utf8' | 'hex' | 'boolean' | 'bigint' | 'uint8array';
+
+/**
+ * Template script map passed into CashAssembly compilation so evaluations can resolve bare script ids.
+ */
+export type CashAssemblyTemplateScripts = NonNullable<XOTemplate['scripts']>;
+
+/**
+ * WalletData and template context shared by CashAssembly compilation.
+ */
+export type CompileCashAssemblyContext = {
+
+    /**
+     * Runtime WalletData keyed by variable name.
+     * {@link compileCashAssemblySourceToBytes} collects required names with
+     * {@link collectVariablesUsingParseScript} then looks them up here for
+     * primitive method resolution and CashAssembly compilation. A collected name that is absent throws
+     * {@link CashAssemblyRequiredVariableMissingError}. Missing keys are never replaced with defaults.
+     */
+    variables: Record<string, XOInvitationVariableValue | Uint8Array>;
+
+    /**
+     * Template variable definitions. When provided, each `<name.method>` push whose `hint` maps to
+     * a supported primitive class is resolved to bytes before CashAssembly compilation.
+     */
+    templateVariables?: XOTemplate['variables'] | undefined;
+
+    /**
+     * Named CashAssembly scripts from {@link XOTemplate.scripts}.
+     * Each key is a script id such as `scriptA`. Each value is that script's source.
+     * Pass this when compilation refers to those ids, for example `$(scriptA)`, `scriptA`, or
+     * `scriptA scriptB OP_ADD`. The compiler looks up those ids here. The visitor collects
+     * WalletData from those sources and from nested ids.
+     * When omitted, named scripts are not given to the compiler and only WalletData pushes in the
+     * starting text are collected.
+     */
+    templateScripts?: CashAssemblyTemplateScripts | undefined;
+};
+
+/**
+ * Parameters for compiling CashAssembly text.
+ */
+export type CompileCashAssemblyStringParameters = CompileCashAssemblyContext & {
+
+    /**
+     * Text to compile, which may contain `$()`.
+     *
+     * When this text contains `$()`, each CashAssembly evaluation is compiled and replaced with its
+     * decoded payload. Surrounding text is kept.
+     * When this text contains no `$()`, the entire string is compiled as CashAssembly so push
+     * opcodes remain.
+     */
+    cashAssemblyText: string;
+
+    /**
+     * The mode to decode compiled bytecode into a string.
+     */
+    evaluationDecodeMode?: CompiledCashAssemblyDecodeMode;
+};
+
+/**
+ * Parameters for compiling one CashAssembly source string to bytecode.
+ */
+export type CompileCashAssemblySourceToBytesParameters = CompileCashAssemblyContext & {
+
+    /**
+     * Evaluation or CashAssembly source passed to the compiler as a script id.
+     */
+    cashAssemblySource: string;
+};
+
+/**
+ * Parameters for compiling one CashAssembly source string to decoded text.
+ */
+export type CompileCashAssemblySourceToDecodedTextParameters = CompileCashAssemblySourceToBytesParameters & {
+
+    /**
+     * The mode to decode compiled bytecode into a string.
+     */
+    evaluationDecodeMode: CompiledCashAssemblyDecodeMode;
+};
+
+/**
+ * Parameters for preparing a Libauth compiler for CashAssembly source strings.
+ */
+export type CompileCashAssemblyEvaluationsParameters = {
+
+    /**
+     * CashAssembly source strings such as `$(<var1>)` or `scriptA`.
+     */
+    evaluations: string[];
+
+    /**
+     * Optional template scripts so nested ids compile to the template sources.
+     */
+    templateScripts?: CashAssemblyTemplateScripts | undefined;
+
+    /**
+     * WalletData names already collected with {@link collectVariablesUsingParseScript}.
+     * This function does not collect again.
+     */
+    variableNames: string[];
+};

source/errors.ts

@@ -0,0 +1,41 @@
+import type { $ZodIssue } from 'zod/v4/core';
+
+/**
+ * Formats the Zod validation failures into a single string with one line each: "- <field>: <message>" and top level failures
+ * with no field path show as "(root)" for better readability.
+ *
+ * @param issues The Zod validation failures to format.
+ * @returns A human readable error string for better debugging.
+ */
+export const buildErrorDescription = (issues: $ZodIssue[]): string => {
+    // Initialize an empty array to store the formatted lines.
+    const lines: string[] = [];
+
+    // Iterate over the issues and format them into a string.
+    for (const issue of issues) {
+        // Get the issue path.
+        const issuePath = issue.path.length > 0 ? issue.path.join('.') : '(root)';
+
+        // The prefix that Zod adds to messages.
+        const messagePrefix = 'Invalid input: ';
+
+        // Remove the prefix for better readability.
+        const issueMessage = issue.message.startsWith(messagePrefix) ? issue.message.slice(messagePrefix.length) : issue.message;
+
+        // Add the formatted line to the array.
+        lines.push(`- ${issuePath}: ${issueMessage}`);
+    }
+
+    // Return the formatted string.
+    return `\n${lines.join('\n')}`;
+};
+
+/**
+ * Error thrown when a waitFor timeout is reached
+ */
+export class WaitForTimeoutError extends Error {
+    constructor(type: string) {
+        super(`Timeout waiting for event "${type}"`);
+        this.name = 'WaitForTimeoutError';
+    }
+}

source/event-emitter.ts

@@ -0,0 +1,278 @@
+import type { DeeplyReadonly } from './types.ts';
+import type { deepFreeze } from './misc.ts';
+
+import { WaitForTimeoutError } from './errors.ts';
+
+export type EventMap = Record<string, unknown>;
+
+type Listener<T> = (detail: T) => void;
+
+/**
+ * Internally permits listeners for individual event payloads to be stored
+ * in a collection typed with the union of all event payloads.
+ */
+type StoredListener<T> = {
+    bivarianceHack(detail: T): void;
+}['bivarianceHack'];
+
+/**
+ * A listener entry.
+ * @template T - The event payload type.
+ */
+interface ListenerEntry<T> {
+    listener: StoredListener<T>;
+    wrappedListener: StoredListener<T>;
+    cancel: () => void;
+}
+
+/**
+ * Callback returned by {@link on} and {@link once} for removing a listener.
+ */
+export type OffCallback = () => void;
+
+/**
+ * A simple event emitter implementation.
+ * @template T - The event map type.
+ */
+export class EventEmitter<T extends EventMap> {
+    /**
+     * The listeners map.
+     * @private
+     */
+    #listeners: Map<keyof T, Set<ListenerEntry<T[keyof T]>>> = new Map();
+
+    /**
+     * Add a listener for an event.
+     * @param type - The event type.
+     * @param listener - The listener function.
+     * @param debounceMilliseconds - The debounce time in milliseconds.
+     * @returns An off callback that can be called to stop listening for events.
+     */
+    on<K extends keyof T>(type: K, listener: Listener<T[K]>, debounceMilliseconds: number = 0): OffCallback {
+        const { cancel, listener: cancellableListener } = this.#cancellable(listener);
+
+        // Create a wrapped listener so that the debounce can be applied.
+        const wrappedListener = debounceMilliseconds > 0 ? this.#debounce(cancellableListener, debounceMilliseconds) : cancellableListener;
+
+        // If the listeners map does not have the event type, create a new set.
+        if (!this.#listeners.has(type)) {
+            this.#listeners.set(type, new Set());
+        }
+
+        // Create a listener entry.
+        const listenerEntry: ListenerEntry<T[K]> = {
+            listener,
+            wrappedListener,
+            cancel,
+        };
+
+        // Add the listener entry to the listeners map.
+        this.#listeners.get(type)?.add(listenerEntry);
+
+        // Return an "off" callback that can be called to stop listening for events.
+        return () => this.off(type, listener);
+    }
+
+    /**
+     * Add a one-time listener for an event.
+     * @param type - The event type.
+     * @param listener - The listener function.
+     * @param debounceMilliseconds - The debounce time in milliseconds.
+     * @returns An off callback that can be called to stop listening for events.
+     */
+    once<K extends keyof T>(type: K, listener: Listener<T[K]>, debounceMilliseconds: number = 0): OffCallback {
+        const wrappedListener: Listener<T[K]> = (detail: T[K]) => {
+            this.off(type, listener);
+            listener(detail);
+        };
+
+        // Create a cancellable listener.
+        const { cancel, listener: cancellableListener } = this.#cancellable(wrappedListener);
+
+        // Create a debounced listener.
+        const debouncedListener = debounceMilliseconds > 0 ? this.#debounce(cancellableListener, debounceMilliseconds) : cancellableListener;
+
+        // If the listeners map does not have the event type, create a new set.
+        if (!this.#listeners.has(type)) {
+            this.#listeners.set(type, new Set());
+        }
+
+        // Create a listener entry.
+        const listenerEntry: ListenerEntry<T[K]> = {
+            listener,
+            wrappedListener: debouncedListener,
+            cancel,
+        };
+
+        // Add the listener entry to the listeners map.
+        this.#listeners.get(type)?.add(listenerEntry);
+
+        // Return an "off" callback that can be called to stop listening for events.
+        return () => this.off(type, listener);
+    }
+
+    /**
+     * Remove a listener for an event.
+     * @param type - The event type.
+     * @param listener - The listener function.
+     */
+    off<K extends keyof T>(type: K, listener?: Listener<T[K]>): void {
+        // Get the listeners for the event type.
+        const listeners = this.#listeners.get(type);
+        if (!listeners) return;
+
+        // Find the listener entries (If a listener was provided, only 1 entry will be returned. Otherwise, all entries will be returned).
+        const listenerEntries = Array.from(listeners).filter((entry) => !listener || entry.listener === listener || entry.wrappedListener === listener);
+
+        // Remove the listener entries from the listeners set.
+        listenerEntries.forEach((entry) => {
+            // Set the wrapped listener to a no-op function to prevent it from being called by debounced events after it's been removed.
+            entry.cancel();
+
+            // Remove the listener entry from the listeners set.
+            listeners.delete(entry);
+        });
+
+        // If no listener was provided and no listeners are left for the event type, remove the listeners set from the listeners map.
+        if (!listener || this.#listeners.get(type)?.size === 0) {
+            this.#listeners.delete(type);
+        }
+    }
+
+    /**
+     * Emit an event.
+     *
+     * @remarks The caller is responsible for ensuring the payload suits the intended mutability requirements.
+     *          By default, the payload will be mutable, so listeners may mutate the payload, effecting both
+     *          the original object and the other listeners.
+     *          To prevent this, the caller can use the {@link Object.freeze} or {@link deepFreeze} function to freeze the payload.
+     *          This will need to be defined in the EventMap using the built-in {@link Readonly} type or the provided {@link DeeplyReadonly} type.
+     *
+     * @param type - The event type.
+     * @param payload - The event payload.
+     * @returns True if there are listeners for the event, false otherwise.
+     */
+    emit<K extends keyof T>(type: K, payload: T[K]): boolean {
+        // Get the listeners for the event type.
+        const listeners = this.#listeners.get(type);
+        if (!listeners) return false;
+
+        // Emit the event to all listeners.
+        listeners.forEach((entry) => {
+            try {
+                entry.wrappedListener(payload);
+            } catch (error) {
+                console.error(error);
+            }
+        });
+
+        // Return true if there are listeners for the event, false otherwise.
+        return listeners.size > 0;
+    }
+
+    /**
+     * Remove all listeners.
+     */
+    removeAllListeners(): void {
+        for (const [ type, listeners ] of this.#listeners.entries()) {
+            listeners.forEach((entry) => {
+                this.off(type, entry.listener);
+            });
+        }
+    }
+
+    /**
+     * Wait for an event to be emitted that matches the provided predicate function's criteria.
+     * @param type - The event type.
+     * @param predicate - Predicate function to filter for whether the event payload matches the criteria.
+     * @param timeoutMs - The timeout in milliseconds.
+     * @returns The event payload.
+     */
+    async waitFor<K extends keyof T>(type: K, predicate: (payload: T[K]) => boolean, timeoutMs?: number): Promise<T[K]> {
+        // Create a promise to wait for the event to be emitted.
+        return new Promise((resolve, reject) => {
+            let timeoutId: ReturnType<typeof setTimeout> | undefined;
+
+            // Create a cleanup function to remove the listener and clear the timeout if it is still pending.
+            const cleanup = (listener: Listener<T[K]>): void => {
+                // Remove the listener from the listeners map.
+                this.off(type, listener);
+
+                // Clear the timeout if it is still pending.
+                if (timeoutId !== undefined) {
+                    clearTimeout(timeoutId);
+                }
+            };
+
+            // Create a listener function.
+            const listener = (payload: T[K]): void => {
+                try {
+                    // If the event payload does not match the predicate condition, return.
+                    if (!predicate(payload)) {
+                        return;
+                    }
+
+                    cleanup(listener);
+                    resolve(payload);
+                } catch (error) {
+                    cleanup(listener);
+                    reject(error);
+                }
+            };
+
+            // Set up timeout if specified
+            if (timeoutMs !== undefined) {
+                timeoutId = setTimeout(() => {
+                    this.off(type, listener);
+                    reject(new WaitForTimeoutError(String(type)));
+                }, timeoutMs);
+            }
+
+            // Add the listener to the listeners map.
+            this.on(type, listener);
+        });
+    }
+
+    /**
+     * Debounce a function.
+     *
+     * @remarks If {@link off} is called on a listener while it is debounced, the timeout is not cleared with clearTimeout.
+     *          Instead, the function is no-oped.
+     *
+     * @param func - The function to debounce.
+     * @param wait - The wait time in milliseconds.
+     * @returns The debounced function.
+     */
+    #debounce<K extends keyof T>(func: Listener<T[K]>, wait: number): Listener<T[K]> {
+        // Create a timeout variable.
+        let timeout: ReturnType<typeof setTimeout>;
+
+        return (detail: T[K]) => {
+            // If a debounce timer is already pending, clear it before scheduling the next one.
+            if (timeout !== undefined) {
+                clearTimeout(timeout);
+            }
+
+            timeout = setTimeout(() => {
+                func(detail);
+            }, wait);
+        };
+    }
+
+    /**
+     * Make a function cancellable.
+     * @param func - The function to make cancelable.
+     * @returns The cancellable function with a cancel method.
+     */
+    #cancellable<K extends keyof T>(func: Listener<T[K]>): { cancel: () => void; listener: Listener<T[K]> } {
+        let cancelled = false;
+
+        return {
+            cancel: (): boolean => (cancelled = true),
+            listener: (detail: T[K]): void => {
+                if (cancelled) return;
+                func(detail);
+            },
+        };
+    }
+}

source/exponential-backoff/errors.ts

@@ -0,0 +1,92 @@
+/* eslint-disable max-classes-per-file */
+
+/**
+ * Error thrown when the maximum number of retries is hit in an exponential backoff
+ */
+export class ExponentialBackoffMaxRetriesHitError extends Error {
+    constructor(errors: Array<Error>) {
+        super('Exponential backoff: Max retries hit', { cause: errors });
+        this.name = 'ExponentialBackoffMaxRetriesHitError';
+    }
+}
+
+/**
+ * Error thrown when the exponential backoff retries are stopped
+ */
+export class ExponentialBackoffStoppedRetriesError extends Error {
+    constructor(reason: unknown) {
+        // Convert the reason to an error if it is not an error
+        const reasonError = reason instanceof Error ? reason : new Error(`${reason}`);
+
+        super(`Exponential backoff was aborted: "${reasonError.message}"`, { cause: reasonError });
+        this.name = 'ExponentialBackoffStoppedRetriesError';
+    }
+}
+
+/**
+ * Error thrown when an exponential backoff option is too small
+ */
+export class ExponentialBackoffNumberTooSmallError extends Error {
+    constructor(option: string, value: number, min: number) {
+        super(`Exponential backoff option "${option}" is too small. Must be at least ${min}. Received value: ${value}`);
+        this.name = 'ExponentialBackoffNumberTooSmallError';
+    }
+}
+
+/**
+ * Error thrown when an exponential backoff option is out of bounds
+ */
+export class ExponentialBackoffNumberOutOfBoundsError extends Error {
+    constructor(option: string, value: number, min: number, max: number) {
+        super(`Exponential backoff option "${option}" is out of bounds. Must be between ${min} and ${max}. Received value: ${value}`);
+        this.name = 'ExponentialBackoffNumberOutOfBoundsError';
+    }
+}
+
+/**
+ * Error thrown when an exponential backoff option is an invalid infinite integer
+ */
+export class ExponentialBackoffNumberNotFiniteError extends Error {
+    constructor(option: string, value: number) {
+        super(`Exponential backoff option "${option}" is invalid. Must be a finite number. Received value: ${value}`);
+        this.name = 'ExponentialBackoffNumberNotFiniteError';
+    }
+}
+
+/**
+ * Error thrown when an exponential backoff option is not an integer
+ */
+export class ExponentialBackoffNonIntegerError extends Error {
+    constructor(option: string, value: number) {
+        super(`Exponential backoff option "${option}" is invalid. Must be an integer. Received value: ${value}`);
+        this.name = 'ExponentialBackoffNonIntegerError';
+    }
+}
+
+/**
+ * Error thrown when an externally aborted exponential backoff is aborted
+ * due to the external abort signal that was passed in to the constructor being aborted by an upstream consumer
+ */
+export class ExternallyAbortedExponentialBackoffExternalSignalAbortedError extends Error {
+    constructor(reason: unknown) {
+        // Convert the reason to an error if it is not an error
+        const reasonError = reason instanceof Error ? reason : new Error(`${reason}`);
+
+        super(`Externally aborted exponential backoff: "${reasonError.message}"`, { cause: reasonError });
+        this.name = 'ExternallyAbortedExponentialBackoffExternalSignalAbortedError';
+    }
+}
+
+/**
+ * Error thrown when an externally aborted exponential backoff is aborted
+ * due to the internal abort signal being aborted using the .abort() method
+ */
+export class ExternallyAbortedExponentialBackoffInternalSignalAbortedError extends Error {
+    constructor(reason: unknown) {
+        // Convert the reason to an error if it is not an error
+        const reasonError = reason instanceof Error ? reason : new Error(`${reason}`);
+
+        super(`Externally aborted exponential backoff: "${reasonError.message}"`, { cause: reasonError });
+        this.name = 'ExternallyAbortedExponentialBackoffInternalSignalAbortedError';
+    }
+}

source/exponential-backoff/exponential-backoff-externally-aborted.ts

@@ -0,0 +1,96 @@
+import {
+    ExternallyAbortedExponentialBackoffExternalSignalAbortedError,
+    ExternallyAbortedExponentialBackoffInternalSignalAbortedError,
+} from './errors.ts';
+import { ExponentialBackoff } from './exponential-backoff.ts';
+import type { ExponentialBackoffCallbackParameters, ExponentialBackoffOptions, ExponentialBackoffRunOptions } from './exponential-backoff.ts';
+
+/**
+ * Options for the ExponentialBackoffExternallyAbortable class
+ *
+ * @extends Partial<ExponentialBackoffOptions>
+ * @property abortSignal - The abort signal to use for this instance
+ */
+export type ExponentialBackoffExternallyAbortableOptions = Partial<ExponentialBackoffOptions> & {
+    abortSignal: AbortSignal;
+};
+
+/**
+ * An exponential backoff that can be stopped by calling `.abort()` or by passing an
+ * `abortSignal` to the constructor.
+ *
+ * @remarks One instance can run many tasks. Aborting it stops retries for every run.
+ */
+export class ExponentialBackoffExternallyAbortable {
+    readonly #abortController = new AbortController();
+    readonly #exponentialBackoff: ExponentialBackoff;
+
+    constructor(options: Partial<ExponentialBackoffExternallyAbortableOptions> = {}) {
+        const { abortSignal, ...backoffOptions } = options;
+
+        this.#exponentialBackoff = new ExponentialBackoff(backoffOptions);
+
+        // Listen for the provided abort signal to be aborted
+        abortSignal?.addEventListener(
+            'abort',
+            () => {
+                this.#abortController.abort(new ExternallyAbortedExponentialBackoffExternalSignalAbortedError(abortSignal.reason));
+            },
+            {
+                once: true,
+                // Clean up this listener if the internal abort controller is aborted
+                signal: this.#abortController.signal,
+            },
+        );
+
+        // If the abort signal is already aborted, immediately abort the internal abort controller
+        if (abortSignal?.aborted) {
+            this.#abortController.abort(new ExternallyAbortedExponentialBackoffExternalSignalAbortedError(abortSignal.reason));
+        }
+    }
+
+    /**
+     * Run the function with exponential backoff
+     *
+     * If the function fails but we have not hit the max attempts, the error will be passed to the onError callback
+     * and the function will be retried with an exponential delay
+     *
+     * If every attempt fails, an ExponentialBackoffMaxRetriesHitError will be thrown
+     * with all errors from the task function.
+     *
+     * @param taskFn - The function to run
+     * @param options - Options that control this run
+     *
+     * @throws An {@link ExponentialBackoffMaxRetriesHitError} with all the errors that were thrown by the task function
+     * @throws An {@link ExponentialBackoffStoppedRetriesError} if the abort signal is activated
+     * @throws An {@link ExternallyAbortedExponentialBackoffExternalSignalAbortedError}
+     *         if the abort signal that was provided during construction is activated
+     * @throws An {@link ExternallyAbortedExponentialBackoffInternalSignalAbortedError}
+     *         if {@link ExponentialBackoffExternallyAbortable.abort} is called on this class
+     *
+     * @returns The result of the function
+     */
+    public run<T>(
+        taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
+        options: Partial<ExponentialBackoffRunOptions> = {},
+    ): Promise<T> {
+        let signal = this.#abortController.signal;
+
+        // If a signal was provided for this run, compose it with the instance-wide signal
+        if (options.signal !== undefined) {
+            signal = AbortSignal.any([ signal, options.signal ]);
+        }
+
+        // Call the exponential backoff run method with the composed signal
+        return this.#exponentialBackoff.run(taskFn, { ...options, signal });
+    }
+
+    /**
+     * Stops retries for all current and future runs.
+     *
+     * @param reason - The reason for stopping retries
+     */
+    public abort(reason: unknown): void {
+        this.#abortController.abort(new ExternallyAbortedExponentialBackoffInternalSignalAbortedError(reason));
+    }
+}

source/exponential-backoff/exponential-backoff.ts

@@ -0,0 +1,364 @@
+import {
+    ExponentialBackoffStoppedRetriesError,
+    ExponentialBackoffMaxRetriesHitError,
+    ExponentialBackoffNonIntegerError,
+    ExponentialBackoffNumberTooSmallError,
+    ExponentialBackoffNumberOutOfBoundsError,
+    ExponentialBackoffNumberNotFiniteError,
+} from './errors.ts';
+import { isWithinBounds } from '../misc.ts';
+
+export type ExponentialBackoffOptions = {
+
+    /**
+     * The maximum delay between attempts in milliseconds
+     */
+    maxDelay: number;
+
+    /**
+     * The maximum number of attempts. Passing 0 will result in infinite attempts.
+     */
+    maxAttempts: number;
+
+    /**
+     * The base delay between attempts in milliseconds
+     */
+    baseDelay: number;
+
+    /**
+     * The growth rate of the delay
+     */
+    growthRate: number;
+
+    /**
+     * The jitter of the delay as a percentage of growthRate. The jitter is subtracted from the delay.
+     */
+    jitter: number;
+};
+
+/**
+ * The function to call to stop the retries.
+ * This mimics the AbortSignal.abort function by taking in a reason for stopping
+ *
+ * @param reason - The reason for stopping the retries.
+ */
+export type ExponentialBackoffStopRetriesFunction = (reason: unknown) => void;
+
+/**
+ * The parameters for the task function
+ *
+ * @param stopRetries - The function to call to stop the retries
+ */
+export type ExponentialBackoffCallbackParameters = {
+    stopRetries: ExponentialBackoffStopRetriesFunction;
+};
+
+/**
+ * Options that control a single exponential-backoff run.
+ */
+export type ExponentialBackoffRunOptions = {
+
+    /**
+     * Called after each failed task attempt.
+     */
+    onError?: ((error: Error) => void) | undefined;
+
+    /**
+     * Stops pending delays and prevents future attempts when aborted.
+     */
+    signal?: AbortSignal | undefined;
+};
+
+/**
+ * Options accepted by the static exponential-backoff run helper.
+ */
+export type ExponentialBackoffStaticRunOptions = Partial<ExponentialBackoffOptions> & ExponentialBackoffRunOptions;
+
+/**
+ * Exponential backoff is a technique used to retry a function after a delay.
+ *
+ * The delay increases exponentially with each attempt, up to a maximum delay.
+ *
+ * The jitter is a random amount of time subtracted from the delay to prevent thundering herd problems.
+ *
+ * The growth rate is the factor by which the delay increases with each attempt.
+ */
+export class ExponentialBackoff {
+    readonly #options: ExponentialBackoffOptions;
+
+    /**
+     * Creates a new exponential-backoff instance.
+     *
+     * Unspecified options use the defaults listed below.
+     *
+     * @param options - Exponential-backoff configuration overrides.
+     * @param options.maxDelay - Maximum delay between retries. Default: `10_000` ms.
+     * @param options.maxAttempts - Maximum number of attempts; `0` retries indefinitely. Default: `10`.
+     * @param options.baseDelay - Delay used as the basis for the first retry. Default: `1_000` ms.
+     * @param options.growthRate - Multiplier applied to the delay after each attempt. Default: `2`.
+     * @param options.jitter - Maximum proportional reduction subtracted from each delay (0–1). Default: `0.1`.
+     *
+     * @throws An {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
+     * @throws An {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
+     * @throws An {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
+     * @throws An {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
+     */
+    constructor(options: Partial<ExponentialBackoffOptions> = {}) {
+        this.#options = {
+            maxDelay: 10_000,
+            maxAttempts: 10,
+            baseDelay: 1_000,
+            growthRate: 2,
+            jitter: 0.1,
+            ...options,
+        };
+
+        ExponentialBackoff.validateOptions(this.#options);
+    }
+
+    /**
+     * Create a new ExponentialBackoff instance
+     *
+     * @param config - The configuration for the exponential backoff
+     *
+     * @throws An {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
+     * @throws An {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
+     * @throws An {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
+     * @throws An {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
+     *
+     * @returns The ExponentialBackoff instance
+     */
+    public static from(config?: Partial<ExponentialBackoffOptions>): ExponentialBackoff {
+        const backoff = new ExponentialBackoff(config);
+
+        return backoff;
+    }
+
+    /**
+     * Run the function with exponential backoff
+     *
+     * @param taskFn - The function to run
+     * @param options - Backoff configuration and options for this run
+     *
+     * @throws An {@link ExponentialBackoffMaxRetriesHitError} with all the errors that were thrown by the task function
+     * @throws An {@link ExponentialBackoffStoppedRetriesError} if the abort signal is activated
+     *
+     * @returns The result of the function
+     */
+    public static run<T>(
+        taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
+        options: Partial<ExponentialBackoffStaticRunOptions> = {},
+    ): Promise<T> {
+        // Grab onError and signal from the options
+        const { onError, signal, ...backoffOptions } = options;
+        const backoff = ExponentialBackoff.from(backoffOptions);
+
+        return backoff.run(taskFn, { onError, signal });
+    }
+
+    /**
+     * Validate the options for the exponential backoff
+     *
+     * @param options - The options to validate
+     *
+     * @throws {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
+     * @throws {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
+     * @throws {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
+     * @throws {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
+     */
+    public static validateOptions(options: ExponentialBackoffOptions): void {
+        /** Validate the value is finite, throwing an {@link ExponentialBackoffInvalidInfiniteIntegerError} if the value is infinite */
+        const assertIsFinite = (key: string, value: number): void => {
+            if (!Number.isFinite(value)) {
+                throw new ExponentialBackoffNumberNotFiniteError(key, value);
+            }
+        };
+
+        /** Validate the value is an integer, throwing a {@link ExponentialBackoffNonIntegerError} if it is not an integer */
+        const assertIsInteger = (key: string, value: number): void => {
+            if (!Number.isInteger(value)) {
+                throw new ExponentialBackoffNonIntegerError(key, value);
+            }
+        };
+
+        /** Validate the value is greater than the minimum, throwing a {@link ExponentialBackoffNumberTooSmallError} if it is not */
+        const assertIsHigherThan = (key: string, value: number, min: number): void => {
+            if (value < min) {
+                throw new ExponentialBackoffNumberTooSmallError(key, value, min);
+            }
+        };
+
+        /** Validate the value is within the bounds, throwing a {@link ExponentialBackoffNumberOutOfBoundsError} if it is not within the bounds */
+        const assertIsWithinBounds = (key: string, value: number, min: number, max: number): void => {
+            if (!isWithinBounds(value, min, max)) {
+                throw new ExponentialBackoffNumberOutOfBoundsError(key, value, min, max);
+            }
+        };
+
+        // Validate the max delay
+        assertIsFinite('maxDelay', options.maxDelay);
+        assertIsHigherThan('maxDelay', options.maxDelay, 0);
+
+        // Validate the max attempts
+        assertIsFinite('maxAttempts', options.maxAttempts);
+        assertIsInteger('maxAttempts', options.maxAttempts);
+        assertIsHigherThan('maxAttempts', options.maxAttempts, 0);
+
+        // Validate the base delay
+        assertIsFinite('baseDelay', options.baseDelay);
+        assertIsHigherThan('baseDelay', options.baseDelay, 0);
+
+        // Validate the growth rate
+        assertIsFinite('growthRate', options.growthRate);
+        assertIsHigherThan('growthRate', options.growthRate, 0);
+
+        // Validate the jitter
+        assertIsFinite('jitter', options.jitter);
+        assertIsWithinBounds('jitter', options.jitter, 0, 1);
+    }
+
+    /**
+     * Run the function with exponential backoff
+     *
+     * If the function fails but we have not hit the max attempts, the error will be passed to the onError callback
+     * and the function will be retried with an exponential delay
+     *
+     * If every attempt fails, an ExponentialBackoffMaxRetriesHitError will be thrown
+     * with all errors from the task function.
+     *
+     * @param taskFn - The function to run
+     * @param options - Options that control this run
+     *
+     * @throws An {@link ExponentialBackoffMaxRetriesHitError} with all the errors that were thrown by the task function
+     * @throws An {@link ExponentialBackoffStoppedRetriesError} if the abort signal is activated
+     *
+     * @returns The result of the function
+     */
+    public async run<T>(
+        taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
+        options: ExponentialBackoffRunOptions = {},
+    ): Promise<T> {
+        const { onError, signal: suppliedSignal } = options;
+
+        // The task can stop its own retries without owning the signal supplied by the caller.
+        const abortController = new AbortController();
+        const stopRetries = abortController.abort.bind(abortController);
+
+        // Start with the abort controller signal
+        const signals = [ abortController.signal ];
+
+        // If we received a signal from the caller, add it to the signals array
+        if (suppliedSignal !== undefined) {
+            signals.push(suppliedSignal);
+        }
+
+        // Compose the signals together
+        const signal = AbortSignal.any(signals);
+
+        // If the composed signal is already aborted, throw an error
+        if (signal.aborted) {
+            throw new ExponentialBackoffStoppedRetriesError(signal.reason);
+        }
+
+        // Initialize an empty array to store the errors
+        const errors: Error[] = [];
+
+        // Initialize the attempt counter
+        let attempt = 0;
+
+        // If the max attempts is 0, we should continue indefinitely.
+        const unlimitedAttempts = this.#options.maxAttempts === 0;
+
+        // Loop until we succeed, hit the max attempts, or the abort signal is activated
+        while (true) {
+            try {
+                // Await the promise before returning so its execution context remains in the try-catch
+                // If we didn't await, this `run` function would successfully return and any errors would not be caught here.
+                return await taskFn({ stopRetries });
+            } catch (error) {
+                // Store the error in case we fail every attempt
+                const errorInstance = error instanceof Error ? error : new Error(`${error}`);
+                onError?.(errorInstance);
+
+                // If we have unlimited attempts, don't append this to the errors array to prevent a memory leak.
+                if (!unlimitedAttempts) {
+                    errors.push(errorInstance);
+                }
+            }
+
+            // Check if the abort signal has been activated
+            if (signal.aborted) {
+                // Throw an error if the abort signal has been activated
+                throw new ExponentialBackoffStoppedRetriesError(signal.reason);
+            }
+
+            // Calculate the count for next attempt. Do this now so we can exit before waiting and before running the next attempt.
+            const nextAttemptCount = attempt + 1;
+            const nextAttemptExceedsMaxAttempts = nextAttemptCount >= this.#options.maxAttempts;
+
+            // If the next attempt exceeds the max attempts, break out of the loop
+            if (!unlimitedAttempts && nextAttemptExceedsMaxAttempts) {
+                break;
+            }
+
+            // Wait before going to the next attempt
+            const delay = this.#calculateDelay(this.#options, attempt);
+
+            // Wait for the delay or the abort signal
+            await new Promise<void>((resolve, reject) => {
+                // eslint-disable-next-line prefer-const
+                let timeout: ReturnType<typeof setTimeout>;
+
+                // Handle the abort signal
+                const abortHandler = (): void => {
+                    clearTimeout(timeout);
+                    reject(new ExponentialBackoffStoppedRetriesError(signal.reason));
+                };
+
+                // Handle the timeout
+                const timeoutHandler = (): void => {
+                    signal.removeEventListener('abort', abortHandler);
+                    resolve(undefined);
+                };
+
+                // Set the timeout
+                timeout = setTimeout(timeoutHandler, delay);
+
+                // Add the abort handler to the abort signal
+                signal.addEventListener('abort', abortHandler, { once: true });
+            });
+
+            attempt++;
+        }
+
+        // We completed the loop without ever succeeding. Throw an ExponentialBackoffMaxRetriesHitError with all the errors we got
+        throw new ExponentialBackoffMaxRetriesHitError(errors);
+    }
+
+    /**
+     * Calculate the delay before we should attempt to retry
+     *
+     * @param options - The configuration for the exponential backoff
+     * @param attempt - The current attempt number
+     * @returns The time in milliseconds before another attempt should be made
+     */
+    #calculateDelay(options: ExponentialBackoffOptions, attempt: number): number {
+        // Get the power of the growth rate
+        const power = options.growthRate ** attempt;
+
+        // Get the delay before jitter or limit
+        const rawDelay = options.baseDelay * power;
+
+        // Cap the delay to the maximum. Do this before the jitter so jitter does not become larger than delay
+        const cappedDelay = Math.min(rawDelay, options.maxDelay);
+
+        // Get a random number for the amount to "jitter" the delay by
+        const jitterAmount = Math.random();
+
+        // Calculate the jitter
+        const jitter = jitterAmount * options.jitter * cappedDelay;
+
+        // Subtract the jitter from the delay
+        return cappedDelay - jitter;
+    }
+}

source/exponential-backoff/index.ts

@@ -0,0 +1,3 @@
+export * from './errors.ts';
+export * from './exponential-backoff.ts';
+export * from './exponential-backoff-externally-aborted.ts';

source/index.ts

@@ -1,13 +1,10 @@
+export * from './errors.ts';
+export * from './event-emitter.ts';
+export * from './exponential-backoff/index.ts';
 export * from './extended-json.ts';
+export * from './invitation/index.ts';
+export * from './misc.ts';
 export * from './script.ts';
 export * from './sse-session/index.ts';
-export * from './template/errors.ts';
-export * from './template/identifier.ts';
-export * from './template/parser.ts';
-export * from './template/schemas.ts';
+export * from './template/index.ts';
 export * from './cash-assembly/index.ts';
-export * from './cash-assembly/errors.ts';
-export * from './cash-assembly/defaults.ts';
-
-// Only exporting serializeTemplate as deserializeTemplate is only used internally and parseTemplate should be used instead.
-export { serializeTemplate } from './template/serialization.ts';

source/invitation/errors.ts

@@ -0,0 +1,46 @@
+/* eslint-disable max-classes-per-file */
+
+/**
+ * Thrown when a value does not satisfy the XOInvitation schema.
+ */
+export class InvitationInvalidError extends Error {
+    constructor(details: string) {
+        const defaultMessage = `Invitation invalid, expected a value matching the XOInvitation type, but got: ${details}`;
+        super(defaultMessage);
+        this.name = 'InvitationInvalidError';
+    }
+}
+
+/**
+ * Thrown when invitation share JSON cannot be parsed.
+ */
+export class InvitationJsonMalformedError extends Error {
+    /**
+     * Parser failure forwarded from JSON parsing.
+     */
+    readonly reason: string;
+
+    constructor(reason: string) {
+        const defaultMessage = `Invitation JSON malformed: expected a valid JSON string, but got: ${reason}`;
+        super(defaultMessage);
+        this.name = 'InvitationJsonMalformedError';
+        this.reason = reason;
+    }
+}
+
+/**
+ * Thrown when invitation serialization fails.
+ */
+export class InvitationSerializationFailedError extends Error {
+    /**
+     * Serializer failure forwarded from the serialization path.
+     */
+    readonly reason: string;
+
+    constructor(reason: string) {
+        const defaultMessage = `Invitation serialization failed: ${reason}`;
+        super(defaultMessage);
+        this.name = 'InvitationSerializationFailedError';
+        this.reason = reason;
+    }
+}

source/invitation/index.ts

@@ -0,0 +1,4 @@
+export * from './errors.ts';
+export * from './parser.ts';
+export * from './schemas.ts';
+export * from './serialization.ts';

source/invitation/parser.ts

@@ -0,0 +1,27 @@
+import type { XOInvitation } from '@xo-cash/types';
+import { buildErrorDescription } from '../errors.ts';
+import { InvitationInvalidError } from './errors.ts';
+import { xoInvitationSchema } from './schemas.ts';
+
+/**
+ * Validates an untyped value against the XOInvitation schema and returns it as a typed invitation.
+ *
+ * The schema parser returns a rebuilt copy whose keys follow the schema order. Commit signatures cover the JSON text of
+ * each commit's data, and that text depends on key order, so a rebuilt copy no longer matches its signatures. Only the
+ * value that was received keeps the order it was signed with.
+ *
+ * The received value itself is returned, not a copy, so the key order its commit signatures were made over is kept.
+ *
+ * @param {unknown} invitationValue - Value to validate, such as the output of Extended JSON parsing.
+ * @returns {XOInvitation} - The validated invitation, as the same object that was passed in.
+ * @throws {@link InvitationInvalidError} - When any field is missing, has the wrong type, or is not part of the invitation type.
+ */
+export const parseInvitation = (invitationValue: unknown): XOInvitation => {
+    const parseResult = xoInvitationSchema.safeParse(invitationValue);
+
+    if (parseResult.success === false) {
+        throw new InvitationInvalidError(buildErrorDescription(parseResult.error.issues));
+    }
+
+    return invitationValue as XOInvitation;
+};

source/invitation/schemas.ts

@@ -0,0 +1,241 @@
+/* eslint-disable @stylistic/newline-per-chained-call */
+import type { XOInvitation } from '@xo-cash/types';
+import { z } from 'zod';
+import { satoshisSchema, uint8ArraySchema, xoTemplateNftCapabilitySchema } from '../template/schemas.ts';
+
+/**
+ * Validation schema for a reference to an input or output in an earlier commit that a later item extends.
+ *
+ * ```
+ * {
+ *   "inputs|outputs": [
+ *     {
+ *       "mergesWith": { "commitIdentifier": "...", "itemIndex": 0 }  ← this schema
+ *     }
+ *   ]
+ * }
+ * ```
+ */
+export const xoInvitationMergesWithReferenceSchema = z
+    .object({
+        commitIdentifier: z.string().describe('The commit that holds the item being extended.'),
+        itemIndex: z.number().int().nonnegative().describe('The position of the extended item inside that commit.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for the template action a commit is resolved against when it differs from the invitation root.
+ */
+export const xoInvitationActionReferenceSchema = z
+    .object({
+        templateIdentifier: z.string().describe('The template that defines the action.'),
+        actionIdentifier: z.string().describe('The action inside that template.'),
+    })
+    .strict();
+
+// ============================================================
+// Commit data
+// ============================================================
+
+/**
+ * Validation schema for a variable value. Matches every value type an invitation variable can hold.
+ */
+export const xoInvitationVariableValueSchema = z
+    .union([ z.bigint(), z.boolean(), z.number(), z.string() ])
+    .describe('A variable value expressed as a bigint, boolean, number, or string.');
+
+/**
+ * Validation schema for a variable supplied by a commit.
+ */
+export const xoInvitationVariableSchema = z
+    .object({
+        variableIdentifier: z.string().describe('The template variable this value is for.'),
+        value: xoInvitationVariableValueSchema,
+        roleIdentifier: z.string().exactOptional().describe('The role this variable belongs to.'),
+        mergesWith: xoInvitationMergesWithReferenceSchema.exactOptional(),
+    })
+    .strict();
+
+/**
+ * Validation schema for a cashtoken.
+ */
+export const outputTokenSchema = z
+    .object({
+        category: uint8ArraySchema.describe('The token category.'),
+        amount: z.bigint().describe('The fungible token amount.'),
+        nft: z
+            .object({
+                capability: xoTemplateNftCapabilitySchema,
+                commitment: uint8ArraySchema.describe('The NFT commitment.'),
+            })
+            .strict()
+            .exactOptional(),
+    })
+    .strict();
+
+/**
+ * Validation schema for a cashtoken supplied by a commit output. Every field is optional because a later commit may
+ * supply the fields an earlier commit left unset.
+ */
+export const xoInvitationTokenSchema = z
+    .object({
+        amount: z.bigint().exactOptional().describe('The fungible token amount.'),
+        category: uint8ArraySchema.exactOptional().describe('The token category.'),
+        nft: z
+            .object({
+                capability: xoTemplateNftCapabilitySchema.exactOptional(),
+                commitment: uint8ArraySchema.exactOptional().describe('The NFT commitment.'),
+            })
+            .strict()
+            .exactOptional(),
+    })
+    .strict();
+
+/**
+ * Validation schema for a value one participant resolved from its own template key, such as a public key or a signature.
+ */
+export const xoInvitationResolutionSchema = z
+    .object({
+        resolutionIdentifier: z.string().describe('The key operation this value resolves, such as someKey.public_key.'),
+        value: uint8ArraySchema.describe('The resolved bytes.'),
+        roleIdentifier: z.string().exactOptional().describe('The role whose key this value was resolved from.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for a transaction input supplied by a commit.
+ */
+export const xoInvitationInputSchema = z
+    .object({
+        outpointIndex: z.number().int().nonnegative().exactOptional().describe('The index of the spent output in its transaction.'),
+        outpointTransactionHash: uint8ArraySchema.exactOptional().describe('The hash of the transaction that holds the spent output.'),
+        sequenceNumber: z.number().int().nonnegative().exactOptional().describe('The sequence number of this input.'),
+        unlockingBytecode: uint8ArraySchema.exactOptional().describe('The script that unlocks the spent output.'),
+        inputIdentifier: z.string().exactOptional().describe('The template input this input fills.'),
+        transactionIndex: z.number().int().nonnegative().exactOptional().describe('The position this input must take in the final transaction.'),
+        roleIdentifier: z.string().exactOptional().describe('The role this input belongs to.'),
+        resolutions: z.array(xoInvitationResolutionSchema).exactOptional(),
+        mergesWith: xoInvitationMergesWithReferenceSchema.exactOptional(),
+        valueSatoshis: satoshisSchema.exactOptional(),
+        token: outputTokenSchema.exactOptional(),
+    })
+    .strict();
+
+/**
+ * Validation schema for a transaction output supplied by a commit.
+ */
+export const xoInvitationOutputSchema = z
+    .object({
+        lockingBytecode: uint8ArraySchema.exactOptional().describe('The locking bytecode of this output.'),
+        valueSatoshis: satoshisSchema.exactOptional(),
+        token: xoInvitationTokenSchema.exactOptional(),
+        outputIdentifier: z.string().exactOptional().describe('The template output this output fills.'),
+        transactionIndex: z.number().int().nonnegative().exactOptional().describe('The position this output must take in the final transaction.'),
+        secretIdentifier: z.string().exactOptional().describe('The secret used to derive this output.'),
+        roleIdentifier: z.string().exactOptional().describe('The role this output belongs to.'),
+        mergesWith: xoInvitationMergesWithReferenceSchema.exactOptional(),
+    })
+    .strict();
+
+/**
+ * Validation schema for the data a commit supplies.
+ */
+export const xoInvitationCommitDataSchema = z
+    .object({
+        transactionVersion: z.number().int().nonnegative().exactOptional().describe('The transaction version.'),
+        transactionLocktime: z.number().int().nonnegative().exactOptional().describe('The transaction locktime.'),
+        variables: z.array(xoInvitationVariableSchema).exactOptional(),
+        inputs: z.array(xoInvitationInputSchema).exactOptional(),
+        outputs: z.array(xoInvitationOutputSchema).exactOptional(),
+        resolutions: z.array(xoInvitationResolutionSchema).exactOptional(),
+        roleIdentifier: z.string().exactOptional().describe('The role the committing entity takes in the action with this commit.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for one signed commit in an invitation.
+ */
+export const xoInvitationCommitSchema = z
+    .object({
+        commitIdentifier: z.string().describe('The identifier of this commit.'),
+        // The type declares this key on every commit, but serialization drops it on the first commit where it is undefined.
+        // Prefault restores the missing key, in this case it's undefined.
+        previousCommitIdentifier: z.string().optional().prefault(undefined).describe('The commit this one follows. Undefined on the first commit.'),
+        entityIdentifier: z.string().describe('The entity that made this commit.'),
+        createdAtTimestamp: z.number().int().nonnegative().describe('When this commit was created, as a Unix timestamp in milliseconds.'),
+        actionReference: xoInvitationActionReferenceSchema.exactOptional(),
+        data: xoInvitationCommitDataSchema,
+        signature: z.string().describe('The signature over this commit.'),
+        expiresAtTimestamp: z.number().int().nonnegative().describe('When this commit expires, as a Unix timestamp in milliseconds.'),
+        memo: z.string().exactOptional().describe('A free text note attached to this commit.'),
+    })
+    .strict();
+
+// ============================================================
+// Results
+// ============================================================
+
+/**
+ * Validation schema for a transaction result that has been created but not broadcast.
+ */
+export const xoInvitationTransactionCreatedResultSchema = z
+    .object({
+        transactionHash: z.string().describe('The hash of the created transaction.'),
+        createdAtTimestamp: z.number().int().nonnegative().describe('When the transaction was created, as a Unix timestamp in milliseconds.'),
+        createdByEntityIdentifier: z.string().describe('The entity that created the transaction.'),
+        createdBySignature: z.string().describe('The signature over the created result.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for a transaction result that has been created and broadcast.
+ */
+export const xoInvitationTransactionBroadcastedResultSchema = xoInvitationTransactionCreatedResultSchema
+    .extend({
+        broadcastedAtTimestamp: z.number().int().nonnegative().describe('When the transaction was broadcasted, as a Unix timestamp in milliseconds.'),
+        broadcastedByEntityIdentifier: z.string().describe('The entity that broadcasted the transaction.'),
+        broadcastedBySignature: z.string().describe('The signature over the broadcasted result.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for a data result created on an invitation.
+ */
+export const xoInvitationCreatedResultSchema = z
+    .object({
+        value: xoInvitationVariableValueSchema,
+        createdAtTimestamp: z.number().int().nonnegative().describe('When the value was created, as a Unix timestamp in milliseconds.'),
+        createdByEntityIdentifier: z.string().describe('The entity that created the value.'),
+        createdBySignature: z.string().describe('The signature over the created value.'),
+    })
+    .strict();
+
+/**
+ * Validation schema for the results created on an invitation.
+ */
+export const xoInvitationResultSchema = z
+    .object({
+        transactions: z
+            .record(z.string(), z.union([ xoInvitationTransactionBroadcastedResultSchema, xoInvitationTransactionCreatedResultSchema ]))
+            .exactOptional(),
+        data: z.record(z.string(), xoInvitationCreatedResultSchema).exactOptional(),
+    })
+    .strict();
+
+// ============================================================
+// Invitation
+// ============================================================
+
+/**
+ * Validation schema for an invitation.
+ */
+export const xoInvitationSchema: z.ZodType<XOInvitation> = z
+    .object({
+        invitationIdentifier: z.string().describe('The identifier of this invitation.'),
+        createdAtTimestamp: z.number().int().nonnegative().describe('When this invitation was created, as a Unix timestamp in milliseconds.'),
+        templateIdentifier: z.string().describe('The template this invitation uses.'),
+        actionIdentifier: z.string().describe('The template action this invitation performs.'),
+        commits: z.array(xoInvitationCommitSchema),
+        result: xoInvitationResultSchema.exactOptional(),
+    })
+    .strict();

source/invitation/serialization.ts

@@ -0,0 +1,46 @@
+import type { XOInvitation } from '@xo-cash/types';
+import { fromExtendedJson, toExtendedJson } from '../extended-json.ts';
+import { InvitationJsonMalformedError, InvitationSerializationFailedError } from './errors.ts';
+import { parseInvitation } from './parser.ts';
+
+/**
+ * Serializes an invitation for sharing. Encodes bigint and Uint8Array fields using Extended JSON.
+ *
+ * @param invitation Shared invitation to export.
+ * @returns JSON string that {@link deserializeInvitation} can restore.
+ * @throws {InvitationSerializationFailedError} When JSON serialization fails.
+ */
+export const serializeInvitation = (invitation: XOInvitation): string => {
+    try {
+        return toExtendedJson(invitation);
+    } catch (serializationError) {
+        const reason = serializationError instanceof Error ? serializationError.message : 'unknown error while serializing invitation';
+
+        throw new InvitationSerializationFailedError(reason);
+    }
+};
+
+/**
+ * Deserializes a shared invitation JSON string into an invitation.
+ *
+ * Every field is checked against its declared type, so the returned invitation can be trusted to match
+ * {@link XOInvitation}.
+ *
+ * @param serializedInvitation JSON produced by {@link serializeInvitation}.
+ * @returns Parsed invitation whose fields match their declared types.
+ * @throws {InvitationJsonMalformedError} When the string is not valid JSON.
+ * @throws {InvitationInvalidError} When the parsed value does not match the XOInvitation type.
+ */
+export const deserializeInvitation = (serializedInvitation: string): XOInvitation => {
+    let parsedValue: unknown;
+
+    try {
+        parsedValue = fromExtendedJson(serializedInvitation);
+    } catch (parsingError) {
+        const reason = parsingError instanceof Error ? parsingError.message : 'unknown error while deserializing invitation';
+
+        throw new InvitationJsonMalformedError(reason);
+    }
+
+    return parseInvitation(parsedValue);
+};

source/misc.ts

@@ -0,0 +1,51 @@
+import type { DeeplyReadonly } from './types';
+
+/**
+ * Recursively freezes an object by iterating over all properties and freezing them.
+ * @param obj - The object to freeze.
+ * @returns The frozen object.
+ */
+export const deepFreeze = <T>(value: T): DeeplyReadonly<T> => {
+    if (value !== null && (typeof value === 'object' || typeof value === 'function')) {
+        for (const key of Reflect.ownKeys(value)) {
+            const descriptor = Reflect.getOwnPropertyDescriptor(value, key);
+
+            if (descriptor && 'value' in descriptor) {
+                deepFreeze(descriptor.value);
+            }
+        }
+
+        Object.freeze(value);
+    }
+
+    return value;
+};
+
+/**
+ * Validate the value is within the bounds, returning true if it is within the bounds, false otherwise
+ *
+ * @param value - The value to validate
+ * @param min - The minimum value
+ * @param max - The maximum value
+ *
+ * @returns True if the value is within the bounds, false otherwise
+ */
+export const isWithinBounds = (value: number, min: number, max: number): boolean => {
+    if (value < min || value > max) {
+        return false;
+    }
+
+    return true;
+};
+
+/**
+ * Converts a non-error to an error
+ * @param error - The error to convert
+ * @returns The error
+ */
+export const normalizeError = (error: unknown): Error => {
+    // Convert the error to an error if it is not already an error
+    const errorInstance = error instanceof Error ? error : new Error(`${error}`);
+
+    return errorInstance;
+};

source/sse-session/constants.ts

@@ -32,3 +32,26 @@ export const SSE_TRAILING_NEWLINE_REGEX = /\n$/;
  * buffered partial lines between streamed chunks before the next parse call.
  */
 export const NEW_LINE = '\n';
+
+/**
+ * Default exponential backoff options for the SSE session.
+ */
+export const SSE_SESSION_EXPONENTIAL_BACKOFF_DEFAULTS = {
+    baseDelay: 250,
+    maxDelay: 10000,
+    maxAttempts: 0,
+    growthRate: 1.3,
+    jitter: 0.3,
+} as const;
+
+/**
+ * Default attempt reconnect flag for the SSE session.
+ * @default true
+ */
+export const SSE_SESSION_ATTEMPT_RECONNECT_DEFAULT = true;
+
+/**
+ * Default persistent flag for the SSE session.
+ * @default false
+ */
+export const SSE_SESSION_PERSISTENT_DEFAULT = false;

source/sse-session/errors.ts

@@ -0,0 +1,31 @@
+/* eslint-disable max-classes-per-file */
+
+/**
+ * Error thrown when a response body is null
+ */
+export class ResponseBodyNullError extends Error {
+    constructor() {
+        super('Response body is null');
+        this.name = 'ResponseBodyNullError';
+    }
+}
+
+/**
+ * Error thrown when an HTTP error occurs
+ */
+export class HTTPError extends Error {
+    constructor(status: number, message: string) {
+        super(`HTTP error! Status: ${status} - ${message}`);
+        this.name = 'HTTPError';
+    }
+}
+
+/**
+ * Error thrown when a plugin's unsubscribe function is not a function
+ */
+export class SSESessionUnsubscribePluginNotAFunctionError extends Error {
+    constructor(pluginName: string) {
+        super(`${pluginName}'s unsubscribe function is not a function`);
+        this.name = 'SSESessionUnsubscribePluginNotAFunctionError';
+    }
+}

source/sse-session/index.ts

@@ -1,2 +1,4 @@
 export * from './async-push-iterator.ts';
+export * from './types.ts';
+export * from './sse-session.ts';
 export * from './sse-event-parser.ts';

source/sse-session/sse-event-parser.ts

@@ -79,7 +79,7 @@ export class SSEEventParser {
      * @returns Zero or more complete parsed SSE events from this chunk.
      */
     public parseEvents(chunk: Uint8Array): SSEvent[] {
-        const lines = this.getBufferedLines(chunk);
+        const lines = this.#getBufferedLines(chunk);
 
         const eventLines = lines.slice(0, -1);
 
@@ -91,7 +91,7 @@ export class SSEEventParser {
             // A blank line indicates the end of an event. If we have received data, we can complete the event
             if (line === '') {
                 if (event.data !== undefined) {
-                    events.push(this.completeEvent(event));
+                    events.push(this.#completeEvent(event));
                     event = {};
                     processedLineCount = index + 1;
                 }
@@ -99,10 +99,10 @@ export class SSEEventParser {
                 continue;
             }
 
-            this.parseLine(line, event);
+            this.#parseLine(line, event);
         }
 
-        this.storeRemainingLines(lines, processedLineCount);
+        this.#storeRemainingLines(lines, processedLineCount);
 
         return events;
     }
@@ -114,7 +114,7 @@ export class SSEEventParser {
      * Accepts `\r\n`, `\r`, and `\n` line endings so events parse correctly
      * regardless of server or platform conventions.
      */
-    private getBufferedLines(chunk: Uint8Array): string[] {
+    #getBufferedLines(chunk: Uint8Array): string[] {
         this.#messageBuffer += this.#textDecoder.decode(chunk, { stream: true });
 
         return this.#messageBuffer.split(SSE_LINE_ENDINGS);
@@ -126,7 +126,7 @@ export class SSEEventParser {
      * Lines without a colon are ignored. A single optional space after the colon
      * is stripped from the field value, per the SSE spec.
      */
-    private parseLine(line: string, event: Partial<SSEvent>): void {
+    #parseLine(line: string, event: Partial<SSEvent>): void {
         const colonIndex = line.indexOf(':');
         if (colonIndex === -1) return;
 
@@ -150,7 +150,7 @@ export class SSEEventParser {
                 return;
 
             case 'retry':
-                this.parseRetry(value, event);
+                this.#parseRetry(value, event);
 
                 return;
         }
@@ -161,7 +161,7 @@ export class SSEEventParser {
      *
      * Non-numeric values are ignored rather than failing the parse.
      */
-    private parseRetry(value: string, event: Partial<SSEvent>): void {
+    #parseRetry(value: string, event: Partial<SSEvent>): void {
         const retry = parseInt(value, 10);
 
         if (!isNaN(retry)) {
@@ -175,7 +175,7 @@ export class SSEEventParser {
      * Trims a trailing newline from multi-line `data` values so callers receive
      * the payload without an extra line break at the end.
      */
-    private completeEvent(event: Partial<SSEvent>): SSEvent {
+    #completeEvent(event: Partial<SSEvent>): SSEvent {
         return {
             ...event,
             data: event.data?.replace(SSE_TRAILING_NEWLINE_REGEX, ''),
@@ -188,7 +188,7 @@ export class SSEEventParser {
      * Only lines that were fully processed (through a completed event boundary)
      * are discarded; the remainder is re-encoded into {@link messageBuffer}.
      */
-    private storeRemainingLines(lines: string[], processedLineCount: number): void {
+    #storeRemainingLines(lines: string[], processedLineCount: number): void {
         this.#messageBuffer = lines.slice(processedLineCount).join(NEW_LINE);
     }
 }

source/sse-session/sse-session.ts

@@ -0,0 +1,635 @@
+import type { SSESessionOptions, SSESessionEventMap, SSEvent, SSESessionEventCallbacks, SSESessionOperationContext } from './types.ts';
+import { SSE_SESSION_EXPONENTIAL_BACKOFF_DEFAULTS, SSE_SESSION_ATTEMPT_RECONNECT_DEFAULT, SSE_SESSION_PERSISTENT_DEFAULT } from './constants.ts';
+
+import { EventEmitter, type OffCallback } from '../event-emitter.ts';
+import { ExponentialBackoff } from '../exponential-backoff/index.ts';
+import { normalizeError } from '../misc.ts';
+
+import { SSEEventParser } from './sse-event-parser.ts';
+import { AsyncPushIterator } from './async-push-iterator.ts';
+import { HTTPError, ResponseBodyNullError, SSESessionUnsubscribePluginNotAFunctionError } from './errors.ts';
+
+/**
+ * A fetch-based Server-Sent Events (SSE) client with reconnect and optional
+ * browser tab visibility handling.
+ *
+ * Each session maintains one HTTP streaming connection at a time. Incoming
+ * bytes are parsed into {@link SSEvent} objects and delivered through two
+ * surfaces:
+ *
+ * - **Events** — `"connected"`, `"message"`, `"disconnected"`, `"error"`,
+ *   and `"closed"` on the session itself (extends {@link EventEmitter}).
+ * - **Messages** — {@link messages}, an async iterable for `for await...of`
+ *   consumers.
+ *
+ * Typical usage:
+ *
+ * ```ts
+ * const session = await SSESession.create("/events");
+ *
+ * session.on("message", (event) => console.log(event.data));
+ *
+ * for await (const event of session.messages) {
+ *   handle(event);
+ * }
+ * ```
+ *
+ * ## Lifecycle
+ *
+ * - {@link connect} opens (or reopens) the transport. It resolves once the
+ *   HTTP stream is established; reading continues in the background.
+ * - {@link disconnect} stops the in-flight fetch without ending the session.
+ *   Used internally for tab visibility. The {@link messages} iterator stays
+ *   open so an existing consumer resumes when the tab becomes visible again.
+ * - {@link close} aborts the transport, closes {@link messages}, and emits
+ *   `"closed"`.
+ *
+ * Automatic reconnect is controlled by {@link SSESessionOptions.persistent}
+ * (server closed the stream) and
+ * {@link SSESessionOptions.attemptReconnect} (transport error).
+ *
+ * ## Connection supersession
+ *
+ * Each {@link connect} will create a new controller if one does not exist. Otherwise,
+ * it will return immediately.
+ */
+export class SSESession extends EventEmitter<SSESessionEventMap> {
+    /**
+     * Creates a session and waits until the first connection is established.
+     *
+     * @param url - The SSE endpoint URL.
+     * @param options - Configuration merged with instance defaults.
+     * @returns A connected session.
+     * @throws When the initial connection cannot be established.
+     */
+    static async create(url: string, options: Partial<SSESessionOptions & SSESessionEventCallbacks> = {}): Promise<SSESession> {
+        const session = new SSESession(url, options);
+        await session.connect();
+
+        return session;
+    }
+
+    /**
+     * Enables SSE resume semantics by sending `Last-Event-ID` on reconnect.
+     *
+     * Listens for incoming `"message"` events and remembers the most recent
+     * {@link SSEvent.id}. On every subsequent connect or reconnect, the session's
+     * {@link onRequest} hook is wrapped so that header is attached when an id is
+     * known, allowing the server to replay only events the client has not yet
+     * received.
+     *
+     * The existing {@link onRequest} callback is preserved and runs after the
+     * header is applied, so auth or other header mutations continue to work.
+     *
+     * Since this relies on listening to the `"message"` event, this should ideally
+     * be called before "{@link connect} is called so no messages (and no ids) are missed.
+     *
+     * ```ts
+     * const session = new SSESession(url);
+     * SSESession.addLastEventIdReconnect(session);
+     * await session.connect();
+     * // Reconnects send Last-Event-ID once an event with an id is received.
+     * ```
+     *
+     * @param session - The session to instrument.
+     * @returns The same session, for chaining.
+     */
+    static addLastEventIdReconnect(session: SSESession): { session: SSESession; removeListener?: OffCallback } {
+        // If the plugin is already installed, we return the session and the remove listener function.
+        if (session.plugins.has(SSESession.addLastEventIdReconnect)) {
+            // Get the remove listener function from the session.
+            const removeListener = session.plugins.get(SSESession.addLastEventIdReconnect) as OffCallback;
+
+            // If the plugin was added with a non-function listener, we throw an error.
+            if (typeof removeListener !== 'function') {
+                throw new SSESessionUnsubscribePluginNotAFunctionError('addLastEventIdReconnect');
+            }
+
+            // If the plugin is already installed, we return the session and the remove listener function.
+            return { session, removeListener };
+        }
+
+        // Store the last event id so we can send it on reconnect.
+        let lastEventId: string | undefined;
+
+        // Define the message listener that will store the last event id and begin listening for 'message' events.
+        const messageListener = (event: SSEvent): void => {
+            // If the event has no id, we should not update the lastEventId and instead keep the old one.
+            lastEventId = event.id ?? lastEventId;
+        };
+
+        session.on('message', messageListener);
+
+        // Store the original onRequest callback so we can extend its behavior to write the reconnect header.
+        const originalOnRequest = session.options.onRequest;
+
+        // Whether we actively send the Last-Event-ID header on reconnect.
+        // Because the `onRequest` callback may be extended further by other code, we can't just roll-back to the `originalOnRequest`
+        // Instead, we just use this flag to determine if we should write the Last-Event-ID header.
+        let enabled = true;
+
+        // Extend the onRequest callback to write the Last-Event-ID header if it is enabled.
+        session.options.onRequest = async (request: RequestInit): Promise<RequestInit> => {
+            if (lastEventId && enabled) {
+                request.headers = { ...request.headers, 'Last-Event-ID': lastEventId };
+            }
+
+            return originalOnRequest(request);
+        };
+
+        // Define the remove listener function that will disable the Last-Event-ID header on reconnect.
+        const removeListener = (): void => {
+            enabled = false;
+            session.off('message', messageListener);
+
+            // If the remove listener function is the same as the existing remove listener function, we remove the plugin from the session.
+            // This protects against the `removeListener` function being called onto after a new instance of this plugin is added
+            const existingRemoveListener = session.plugins.get(SSESession.addLastEventIdReconnect) as OffCallback;
+            if (existingRemoveListener === removeListener) {
+                session.plugins.delete(SSESession.addLastEventIdReconnect);
+            }
+        };
+
+        // Store the plugin in the session.
+        session.plugins.set(SSESession.addLastEventIdReconnect, removeListener);
+
+        return { session, removeListener };
+    }
+
+    /**
+     * Pauses and resumes a session based on browser tab visibility.
+     *
+     * Uses the Page Visibility API (`document.visibilitychange`):
+     *
+     * - **hidden** — {@link disconnect} stops the active fetch. {@link messages}
+     *   stays open; `"disconnected"` fires but `"closed"` does not.
+     * - **visible** — {@link connect} re-establishes the stream if needed.
+     *
+     * This controller will not re-connect if the session was disconnected by something else.
+     *
+     * No-op in non-browser environments where `document` is undefined.
+     *
+     * @param session - The session to manage.
+     */
+    static addBrowserVisibilityHandler(session: SSESession): { session: SSESession; removeListener?: OffCallback } {
+        if (typeof document === 'undefined') return { session };
+
+        // If the plugin is already installed, we return the session and the remove listener function.
+        if (session.plugins.has(SSESession.addBrowserVisibilityHandler)) {
+            // Get the remove listener function from the session.
+            const removeListener = session.plugins.get(SSESession.addBrowserVisibilityHandler) as OffCallback;
+
+            // If the plugin was added with a non-function listener, we throw an error.
+            if (typeof removeListener !== 'function') {
+                throw new SSESessionUnsubscribePluginNotAFunctionError('addBrowserVisibilityHandler');
+            }
+
+            // If the plugin is already installed, we return the session and the remove listener function.
+            return { session, removeListener };
+        }
+
+        // Create a listener that will reconnect the session
+        // This listener is added when the hidden->disconnect listener disconnects the session, but will not be added
+        // if the session was disconnected by something else, like a manual disconnect, server-disconnect, or another plugin.
+        const reconnectOnVisible = async (): Promise<void> => {
+            if (document.visibilityState === 'visible') {
+                // Remove this listener
+                document.removeEventListener('visibilitychange', reconnectOnVisible);
+
+                // Reconnect the session
+                await session.connect();
+            }
+        };
+
+        // Create a listener that will disable the reconnect listener if the session is disconnected by something else.
+        const disableOnExternalDisconnect = ({ source }: SSESessionOperationContext): void => {
+            // If the source is not the browser visibility handler, we remove the listener.
+            if (source !== SSESession.addBrowserVisibilityHandler) {
+                document.removeEventListener('visibilitychange', reconnectOnVisible);
+            }
+        };
+
+        // Create a listener that will live as-long as this session is not closed
+        // This listener will create a new 'on visible' listener that will reconnect the session when it disconnects the session.
+        // This prevents it from trying to reconnect the session if the session was disconnected by something else
+        const handleVisibilityChange = async (): Promise<void> => {
+            // If the session is not connected, we do nothing.
+            if (!session.active) {
+                return;
+            }
+
+            // If the session is connected and the visibility is hidden, we disconnect it.
+            if (document.visibilityState === 'hidden') {
+                // Add the listener for connected event to reconnect the session.
+                document.addEventListener('visibilitychange', reconnectOnVisible);
+
+                // Disconnect the session.
+                session.disconnect({ source: SSESession.addBrowserVisibilityHandler });
+            }
+        };
+
+        // Add the listener for visibility change.
+        document.addEventListener('visibilitychange', handleVisibilityChange);
+
+        // Add a listener for `beforeDisconnect` event to disable the reconnect listener if the session is disconnected by something else.
+        session.on('beforeDisconnect', disableOnExternalDisconnect);
+
+        // Remove the listener for visibility change.
+        const removeListener = (): void => {
+            document.removeEventListener('visibilitychange', handleVisibilityChange);
+            document.removeEventListener('visibilitychange', reconnectOnVisible);
+
+            session.off('beforeDisconnect', disableOnExternalDisconnect);
+
+            // If the remove listener function is the same as the existing remove listener function, we remove the plugin from the session.
+            // This protects against the `removeListener` function being called onto after a new instance of this plugin is added
+            const existingRemoveListener = session.plugins.get(SSESession.addBrowserVisibilityHandler) as OffCallback;
+            if (existingRemoveListener === removeListener) {
+                session.plugins.delete(SSESession.addBrowserVisibilityHandler);
+            }
+        };
+
+        // Store the plugin in the session.
+        session.plugins.set(SSESession.addBrowserVisibilityHandler, removeListener);
+
+        // Return the session and the remove listener function.
+        return { session, removeListener };
+    }
+
+    /** SSE endpoint URL for this session. */
+    readonly #url: string;
+
+    /**
+     * Per-instance configuration.
+     *
+     * Defaults live on the instance field (not a shared static) so each session
+     * gets its own {@link SSEEventParser} and {@link ExponentialBackoff}.
+     */
+    public options: SSESessionOptions = {
+        // Fetch options
+        fetch: (...args) => fetch(...args),
+        method: 'GET',
+        headers: {
+            Accept: 'text/event-stream',
+            'Cache-Control': 'no-cache',
+        },
+        body: new FormData(),
+
+        // callback to mutate the fetch options before the fetch is made.
+        onRequest: (request) => Promise.resolve(request),
+
+        // Retry the initial fetch.
+        // Use the spread operator so it's impossible for us to override the default options. (default is exponential backoff)
+        retry: new ExponentialBackoff({
+            ...SSE_SESSION_EXPONENTIAL_BACKOFF_DEFAULTS,
+        }),
+
+        // Reconnection options
+        attemptReconnect: SSE_SESSION_ATTEMPT_RECONNECT_DEFAULT,
+        persistent: SSE_SESSION_PERSISTENT_DEFAULT,
+
+        // Event parser options
+        eventParser: new SSEEventParser(),
+    };
+
+    /**
+     * Registered plugins for the session.
+     * This can be used to prevent duplicate plugins from being added to the session.
+     * The value for a plugin is arbitrary, for example a function that removes the plugin.
+     */
+    public readonly plugins: Map<unknown, unknown> = new Map();
+
+    /** AbortController for the currently active fetch, if any. */
+    #connectionController: AbortController | null = null;
+
+    /** The server's requested retry interval in milliseconds (per SSE spec) if any */
+    #retryInterval: number | null = null;
+
+    /**
+     * Asynchronous stream of parsed SSE events for the active connection.
+     *
+     * Stays open across {@link disconnect} and automatic reconnects so an existing
+     * `for await` consumer keeps receiving events after visibility resumes.
+     *
+     * Closes when:
+     * - the server ends the stream and {@link SSESessionOptions.persistent}
+     *   is false,
+     * - {@link close} is called, or
+     * - a transport error occurs with
+     *   {@link SSESessionOptions.attemptReconnect} disabled.
+     *
+     * A later {@link connect} replaces this with a new iterator when the
+     * previous one was closed. Consumers should read from `session.messages`
+     * rather than caching a reference across terminal disconnects.
+     */
+    public messages: AsyncPushIterator<SSEvent> = new AsyncPushIterator<SSEvent>();
+
+    public constructor(url: string, options: Partial<SSESessionOptions & SSESessionEventCallbacks> = {}) {
+        super();
+
+        const { onConnected, onDisconnected, onError, ...restOptions } = options;
+
+        this.#url = url;
+        this.options = {
+            ...this.options,
+            ...restOptions,
+            // Shallow merge would drop default headers when options.headers is set.
+            headers: { ...this.options.headers, ...options.headers },
+        };
+
+        // Add the onConnected, onDisconnected, and onError as event listeners to the session.
+        // We accept these as options to the constructor so that we can both instantiate the session and add the event listeners at the same time,
+        // avoiding any potential race-conditions or async/await issues.
+        if (onConnected) {
+            this.on('connected', onConnected);
+        }
+
+        if (onDisconnected) {
+            this.on('disconnected', onDisconnected);
+        }
+
+        if (onError) {
+            this.on('error', onError);
+        }
+    }
+
+    /**
+     * Returns true if the session is active by checking if the controller is not null.
+     */
+    public get active(): boolean {
+        return this.#connectionController !== null;
+    }
+
+    /**
+     * Connects or reconnects to the SSE endpoint.
+     *
+     * Resolves once the HTTP stream is established and `"connected"` has been
+     * emitted. Body reading continues asynchronously in the background via the
+     * internal `#readStream` method.
+     *
+     * @throws When the fetch retry policy exhausts attempts or the connection
+     *   is superseded before the reader is handed off (in the latter case the
+     *   promise resolves without throwing).
+     */
+    public async connect(): Promise<void> {
+        // If there is already a controller present, we are already connected.
+        if (this.#connectionController) return;
+
+        // Prepare for a fresh transport. Parser state from an abandoned connection
+        // must not bleed into the next one; reopen messages if a prior terminal
+        // close ended the consumer's iteration loop.
+        this.#resetEventParser();
+        this.#ensureMessageStreamOpen();
+
+        const connectionController = new AbortController();
+        this.#connectionController = connectionController;
+
+        const { method, headers, body } = this.options;
+
+        // Create the fetch options.
+        const fetchOptions: RequestInit = {
+            method,
+            headers: headers || {},
+            signal: connectionController.signal,
+            cache: 'no-store',
+        };
+
+        // If the method is POST, we will set the body of the fetch options.
+        // NOTE: RequestInit doesn't allow body to be `undefined`, so if its false-y we will set it to null.
+        if (method === 'POST') {
+            fetchOptions.body = body || null;
+        }
+
+        // Create a reader for the response body.
+        let reader: ReadableStreamDefaultReader<Uint8Array>;
+
+        try {
+            // Retry the fetch using the provided retry policy. (default is exponential backoff)
+            reader = await this.options.retry.run(() => this.#createReader(fetchOptions), {
+                signal: connectionController.signal,
+            });
+        } catch (error) {
+            // A newer `connect()` superseded this attempt — leave state to the winner.
+            if (this.#connectionController !== connectionController) return;
+
+            // Reset the controller to null to allow for reconnection if needed.
+            this.#connectionController = null;
+
+            // Normalize the error so it can be passed to the event listeners.
+            const normalizedError = normalizeError(error);
+
+            // Emit the disconnected and error events.
+            this.emit('disconnected', { reason: 'error', error: normalizedError });
+            this.emit('error', normalizedError);
+
+            // Close the message stream so `for await...of` consumers resolve.
+            this.#closeMessageStream();
+
+            // Throw the error so the caller can handle it.
+            throw error;
+        }
+
+        // Connection succeeded but was already replaced (for example disconnect during fetch).
+        if (this.#connectionController !== connectionController) {
+            await reader.cancel();
+
+            return;
+        }
+
+        this.emit('connected', undefined);
+
+        // Fire-and-forget: connect() resolves while the stream is consumed.
+        // Data is emitted as a `message` event and from the `messages` iterator.
+        this.#readStream(reader, connectionController).catch((error) => {
+            this.emit('error', error);
+        });
+    }
+
+    /**
+     * Disconnects only the currently active transport.
+     *
+     * `beforeDisconnect` is emitted on every invocation, even when no
+     * transport is currently active. This allows observers to react to
+     * an explicit disconnect operation.
+     *
+     * @param context - Context describing the operation initiator.
+     * @emits `beforeDisconnect` with the supplied operation context.
+     * @emits `disconnected` with reason `"disconnect"` when an active
+     * transport is actually disconnected.
+     */
+    public disconnect(context: SSESessionOperationContext = {}): void {
+        this.emit('beforeDisconnect', context);
+
+        if (this.#connectionController) {
+            // Grab the current controller to ensure we are aborting the correct one.
+            const connectionController = this.#connectionController;
+            this.#connectionController = null;
+
+            // Invalidate any in-flight read loop and fetch for this transport.
+            connectionController.abort();
+
+            // Emit the disconnected event.
+            this.emit('disconnected', {
+                reason: 'disconnect',
+                source: context.source,
+            });
+        }
+
+        // Reset the event parser buffer so data is not carried through resulting in malformed events.
+        this.#resetEventParser();
+    }
+
+    /**
+     * Terminates the session and disables attached visibility handling until
+     * the same instance is manually {@link connect connected} again.
+     *
+     * Closes {@link messages} and emits `"closed"`.
+     */
+    public close(): void {
+        // Disconnect the session.
+        this.disconnect();
+
+        // Close the message stream so `for await...of` consumers resolve.
+        this.#closeMessageStream();
+
+        // Emit the closed event.
+        this.emit('closed', undefined);
+    }
+
+    /**
+     * Performs the HTTP request and returns a reader for the response body.
+     *
+     * {@link SSESessionOptions.onRequest} may mutate headers (for example auth
+     * tokens or `Last-Event-ID`) before the fetch runs.
+     */
+    async #createReader(fetchOptions: RequestInit): Promise<ReadableStreamDefaultReader<Uint8Array>> {
+        const requestOptions = await this.options.onRequest(fetchOptions);
+        const response = await this.options.fetch(this.#url, requestOptions);
+
+        // Handle a bad response from the server
+        if (!response.ok) {
+            // Get the response code and text.
+            const responseCode = response.status;
+            const responseText = await response.text();
+
+            // Create a new error and emit it.
+            const error = new HTTPError(responseCode, responseText);
+            this.emit('error', error);
+
+            // Throw the error so the caller can handle it.
+            throw error;
+        }
+
+        // Handle a response with no body (SSE still uses the response body)
+        if (!response.body) {
+            const error = new ResponseBodyNullError();
+            this.emit('error', error);
+            throw error;
+        }
+
+        // Return the reader for the response body.
+        return response.body.getReader();
+    }
+
+    /**
+     * Reads bytes from an established stream until it ends, errors, or is
+     * superseded by a newer connection.
+     */
+    async #readStream(reader: ReadableStreamDefaultReader<Uint8Array>, connectionController: AbortController): Promise<void> {
+        try {
+            while (this.#connectionController === connectionController) {
+                const { done, value } = await reader.read();
+
+                // `disconnect()` or a newer connect() may have landed while we were awaiting.
+                if (this.#connectionController !== connectionController) return;
+
+                // If the stream is done, we've been disconnected.
+                // We need to clean up and either complete or reconnect the session.
+                if (done) {
+                    // Reset the controller to null to allow for reconnection if needed.
+                    this.#connectionController = null;
+
+                    // Emit the disconnected event.
+                    this.emit('disconnected', { reason: 'remote' });
+
+                    // If the session is persistent, we will attempt to reconnect.
+                    // NOTE: in very rare cases, a malfunctioning server can make this code turn into an infinite loop.
+                    if (this.options.persistent) {
+                        // Server closed gracefully — reopen unless the consumer opted out.
+                        await this.connect();
+                    } else {
+                        // Close the message stream so `for await...of` consumers resolve.
+                        this.#closeMessageStream();
+                    }
+
+                    return;
+                }
+
+                // Some environments yield `{ done: false, value: undefined }`.
+                if (!value) continue;
+
+                // Emit the parsed events to the event listeners and the messages iterator.
+                for (const event of this.options.eventParser.parseEvents(value)) {
+                    // If the event has a retry interval, we will store it.
+                    if (event.retry) {
+                        this.#retryInterval = event.retry;
+                    }
+
+                    this.emit('message', event);
+                    this.messages.push(event);
+                }
+            }
+        } catch (error) {
+            // If the controller is different, we already started a new connection and it would be confusing to handle this error.
+            // The controller can also be null here if the session was disconnected.
+            if (connectionController !== this.#connectionController) return;
+
+            // Invalidate the current controller to allow for reconnection if needed
+            this.#connectionController = null;
+
+            const normalizedError = normalizeError(error);
+            this.emit('disconnected', { reason: 'error', error: normalizedError });
+
+            // Expected path for `disconnect()` — do not treat as an error or reconnect.
+            if (connectionController.signal.aborted) return;
+
+            this.emit('error', normalizedError);
+
+            // If the session is configured to attempt reconnect, we will attempt to reconnect.
+            if (this.options.attemptReconnect) {
+                // If the server has requested a retry interval, we will wait for it before attempting to reconnect.
+                if (this.#retryInterval) {
+                    await new Promise((resolve) => setTimeout(resolve, this.#retryInterval!));
+                }
+
+                // Attempt to reconnect.
+                await this.connect();
+            } else {
+                // Close the message stream so `for await...of` consumers resolve.
+                this.#closeMessageStream();
+            }
+        }
+    }
+
+    /** Clears partial SSE frames left over from an abandoned transport. */
+    #resetEventParser(): void {
+        this.options.eventParser.reset();
+    }
+
+    /**
+     * Creates a new {@link messages} iterator when the previous one was closed
+     * by a terminal disconnect or server stream end.
+     */
+    #ensureMessageStreamOpen(): void {
+        if (!this.messages.closed) return;
+
+        this.messages = new AsyncPushIterator<SSEvent>();
+    }
+
+    /** Ends the message iteration loop for the current connection span. */
+    #closeMessageStream(): void {
+        if (this.messages.closed) return;
+
+        this.messages.close();
+    }
+}

source/sse-session/types.ts

@@ -1,3 +1,173 @@
+import type { ExponentialBackoffRunOptions } from '../exponential-backoff/exponential-backoff';
+
+export type SSERequestInit = {
+
+    /**
+     * The HTTP method to use.
+     */
+    method: 'GET' | 'POST';
+
+    /**
+     * Request headers sent on every connect and reconnect.
+     */
+    headers?: Record<string, string>;
+
+    /**
+     * Request body for POST-based SSE endpoints.
+     */
+    body?: string | FormData;
+};
+
+/**
+ * The fetch function to use.
+ *
+ * NOTE: This is compatible with Browser/Node's native "fetch" function.
+ *       We use this in place of "typeof fetch" so that we can accept non-standard URLs ("url" is a "string" here).
+ *       For example, a LibP2P adapter might not use a standardized URL format (and might only include "path").
+ *       This would cause a type error as native fetch expects type "URL".
+ */
+export type SSERequestFunction = {
+    fetch: (url: string, options: RequestInit) => Promise<Response>;
+};
+
+/**
+ * Lifecycle hooks invoked by {@link SSESession} during connect, read, and teardown.
+ */
+export type SSESessionCallbacks = {
+
+    /**
+     * Called before each fetch so callers can attach auth headers, cookies, or
+     * a `Last-Event-ID` for resume semantics.
+     */
+    onRequest: (request: RequestInit) => Promise<RequestInit>;
+};
+
+export type SSESessionEventCallbacks = {
+
+    /**
+     * Called after the HTTP stream is established and before body reading begins.
+     */
+    onConnected: () => void;
+
+    /**
+     * Called when the active transport ends — including {@link SSESession.disconnect},
+     * server stream completion, and errors. Not paired with {@link SSESessionEventCallbacks.onConnected}
+     * when the initial connect never succeeds.
+     */
+    onDisconnected: () => void;
+
+    /**
+     * Called on fetch or read failures. Not invoked for intentional
+     * {@link SSESession.disconnect} disconnects.
+     */
+    onError: (error: Error) => void;
+};
+
+export type SSESessionRetryInterface = {
+
+    /**
+     * Retry policy used while establishing the HTTP connection in
+     * {@link SSESession.connect}. Defaults to {@link ExponentialBackoff} with
+     * unlimited attempts.
+     */
+    retry: {
+        run<T>(fn: () => Promise<T>, options?: ExponentialBackoffRunOptions): Promise<T>;
+    };
+};
+
+export interface SSEParser {
+
+    /**
+     * Incremental SSE frame parser for the response body.
+     *
+     * {@link SSEEventParser.reset} is called by the session when abandoning a
+     * transport so partial frames do not carry over to the next connection.
+     */
+    eventParser: {
+        parseEvents(buffer: Uint8Array): SSEvent[];
+        reset(): void;
+    };
+}
+
+export type SSELifecycleOptions = {
+
+    /**
+     * When true, {@link SSESession} calls {@link SSESession.connect} again after
+     * a transport **error** (not an intentional disconnect).
+     * @default true
+     */
+    attemptReconnect: boolean;
+
+    /**
+     * When true, {@link SSESession} calls {@link SSESession.connect} again after
+     * the **server** closes the stream normally (`done`).
+     * @default false
+     */
+    persistent: boolean;
+};
+
+/**
+ * Context from the source that triggered the event.
+ */
+export type SSESessionOperationContext = {
+    source?: unknown;
+};
+
+/**
+ * Discriminated context for the reason the session was disconnected internally.
+ */
+export type SSESessionDisconnectInternalContext = {
+    reason: 'disconnect';
+} & SSESessionOperationContext;
+
+/**
+ * Discriminated context for the reason the session was disconnected remotely.
+ */
+export type SSESessionDisconnectRemoteContext = {
+    reason: 'remote';
+};
+
+/**
+ * Discriminated context for the reason the session was disconnected due to an error.
+ */
+export type SSESessionDisconnectErrorContext = {
+    reason: 'error';
+    error: Error;
+};
+
+/**
+ * Discriminated context for the reason the session was disconnected.
+ */
+export type SSESessionDisconnectContext = SSESessionDisconnectInternalContext | SSESessionDisconnectRemoteContext | SSESessionDisconnectErrorContext;
+
+/**
+ * Events emitted by {@link SSESession}.
+ *
+ * - `"connected"` — HTTP stream established.
+ * - `"message"` — A complete SSE event was parsed.
+ * - `"disconnected"` — The active transport ended (including {@link SSESession.disconnect}).
+ * - `"error"` — An unexpected fetch or read failure.
+ * - `"closed"` — {@link SSESession.close} was called.
+ */
+export type SSESessionEventMap = {
+    connected: void;
+    beforeDisconnect: SSESessionOperationContext;
+    disconnected: SSESessionDisconnectContext;
+    error: Error;
+    message: SSEvent;
+    closed: void;
+};
+
+/**
+ * Configuration for {@link SSESession}.
+ */
+export type SSESessionOptions = SSESessionCallbacks
+    & SSERequestInit
+    & SSERequestFunction
+    & SSESessionRetryInterface
+    & SSELifecycleOptions
+    & SSEParser;
+
 /**
  * Represents a Server-Sent Event.
  */

source/template/errors.ts

@@ -1,37 +1,5 @@
 /* eslint-disable max-classes-per-file */
 
-import type { $ZodIssue } from 'zod/v4/core';
-
-/**
- * Formats the Zod validation failures into a single string with one line each: "- <field>: <message>" and top level failures
- * with no field path show as "(root)" for better readability.
- *
- * @param issues The Zod validation failures to format.
- * @returns A human readable error string for better debugging.
- */
-export const buildErrorDescription = (issues: $ZodIssue[]): string => {
-    // Initialize an empty array to store the formatted lines.
-    const lines: string[] = [];
-
-    // Iterate over the issues and format them into a string.
-    for (const issue of issues) {
-        // Get the issue path.
-        const issuePath = issue.path.length > 0 ? issue.path.join('.') : '(root)';
-
-        // The prefix that Zod adds to messages.
-        const messagePrefix = 'Invalid input: ';
-
-        // Remove the prefix for better readability.
-        const issueMessage = issue.message.startsWith(messagePrefix) ? issue.message.slice(messagePrefix.length) : issue.message;
-
-        // Add the formatted line to the array.
-        lines.push(`- ${issuePath}: ${issueMessage}`);
-    }
-
-    // Return the formatted string.
-    return `\n${lines.join('\n')}`;
-};
-
 /**
  * Thrown when the provided template does not satisfy the XOTemplate schema.
  */

source/template/index.ts

@@ -0,0 +1,7 @@
+export * from './errors.ts';
+export * from './identifier.ts';
+export * from './parser.ts';
+export * from './schemas.ts';
+
+// Only exporting serializeTemplate as deserializeTemplate is only used internally and parseTemplate should be used instead.
+export { serializeTemplate } from './serialization.ts';

source/template/parser.ts

@@ -1,6 +1,7 @@
 import type { XOTemplate } from '@xo-cash/types';
 import { xoTemplateSchema } from './schemas.ts';
-import { TemplateInvalidError, buildErrorDescription } from './errors.ts';
+import { buildErrorDescription } from '../errors.ts';
+import { TemplateInvalidError } from './errors.ts';
 import { deserializeTemplate, serializeTemplate } from './serialization.ts';
 
 /**

source/template/schemas.ts

@@ -1,5 +1,5 @@
 /* eslint-disable @stylistic/newline-per-chained-call */
-import { BchVmVersions, XOTemplateBaseTypes, XOTemplatePrimitiveTypes, XOTemplateLockingTypes, XOTemplateNftCapabilities } from '@xo-cash/types';
+import { BchVmVersions, XOTemplatePrimitiveTypes, XOTemplateLockingTypes, XOTemplateNftCapabilities } from '@xo-cash/types';
 import { z } from 'zod';
 
 // ============================================================
@@ -58,12 +58,6 @@ export const xoTemplateNftCapabilitySchema = z.enum(XOTemplateNftCapabilities);
  */
 export const xoTemplateLockingTypeSchema = z.enum(XOTemplateLockingTypes);
 
-/**
- * Validation schema for a base type identifier.
- * Accepts values from XOTemplateBaseTypes for the `type` field on constants, variables, and data.
- */
-export const xoTemplateBaseTypeSchema = z.enum(XOTemplateBaseTypes);
-
 /**
  * Validation schema for a primitive type identifier.
  * Accepts values from XOTemplatePrimitiveTypes for the `hint` field on constants, variables, and data.
@@ -521,6 +515,11 @@ export const xoTemplateLockingScriptRoleSchema = xoTemplateViewPropertiesSchema
         actions: z.array(xoTemplateActionIntentSchema).optional().describe('List of action references available to this role.'),
         balance: xoTemplateAssetAmountsSchema.partial().optional().describe('Estimated ownership in the optional set of asset amounts specified.'),
         selectable: z.boolean().optional().describe('Whether outputs locked to this script should be available for coin selection.'),
+        relevant: z
+            .union([ z.boolean(), z.string() ])
+            .optional()
+            .describe('Whether this output or locking script should be tracked by the engine, When omitted, the engine treats this field as true.'
+                    + 'Accepts true, false, or a CashASM expression that evaluates to a boolean relevance value.'),
         privacy: z
             .union([ z.number(), z.string() ])
             .optional()
@@ -738,9 +737,9 @@ export const xoTemplateTransactionSchema = xoTemplateViewPropertiesSchema
  */
 export const xoTemplateConstantSchema = xoTemplateViewPropertiesSchema
     .extend({
-        type: xoTemplateBaseTypeSchema.describe('The data type of this constant.'),
+        type: xoTemplatePrimitiveTypeSchema.describe('The data type of this constant.'),
         value: z.unknown().describe('The value of this constant.'),
-        hint: xoTemplatePrimitiveTypeSchema.optional().describe('An optional hint to help apps and users understand what this constant represents.'),
+        hint: z.unknown().optional().describe('An optional hint to help apps and users understand what this constant represents.'),
     })
     .strict();
 
@@ -757,9 +756,9 @@ export const xoTemplateConstantSchema = xoTemplateViewPropertiesSchema
  */
 export const xoTemplateDataSchema = z
     .object({
-        type: xoTemplateBaseTypeSchema.describe('The data type of this data field.'),
+        type: xoTemplatePrimitiveTypeSchema.describe('The data type of this data field.'),
         value: z.unknown().describe('The value for this data field.'),
-        hint: xoTemplatePrimitiveTypeSchema.optional().describe('An optional hint to help apps and users understand this data field.'),
+        hint: z.unknown().optional().describe('An optional hint to help apps and users understand this data field.'),
     })
     .strict();
 
@@ -795,8 +794,8 @@ export const xoTemplateImportDefaultValueSchema = xoTemplateIntentSchema
  */
 export const xoTemplateVariableSchema = xoTemplateViewPropertiesSchema
     .extend({
-        type: xoTemplateBaseTypeSchema.optional().describe('The data type of this variable.'),
-        hint: xoTemplatePrimitiveTypeSchema.optional().describe('A hint to help users understand what value to provide.'),
+        type: xoTemplatePrimitiveTypeSchema.optional().describe('The data type of this variable.'),
+        hint: z.unknown().optional().describe('A hint to help users understand what value to provide.'),
 
         // A neutral intent that the engine uses to populate the default value for this variable.
         // View properties (name, description, icon) may contain CashASM expressions that the
@@ -864,6 +863,10 @@ export const xoTemplateIconSchema = xoTemplateViewPropertiesSchema
 export const xoTemplateDefaultsSchema = z
     .object({
         change: xoTemplateOutputIntentSchema.optional().describe('Instructions for how to construct automated change output.'),
+        address: xoTemplateLockingScriptIntentSchema
+            .optional()
+            .describe('Configure the default intent structure for creating receiving addresses / locking scripts.'),
+        receive: xoTemplateOutputIntentSchema.optional().describe('Configure the default intent structure for creating receive outputs.'),
     })
     .strict();
 

source/types.ts

@@ -0,0 +1,8 @@
+/**
+ * A deeply readonly type.
+ * @template T - The type to make deeply readonly.
+ * @returns The deeply readonly type.
+ */
+export type DeeplyReadonly<T> = {
+    readonly [K in keyof T]: T[K] extends (...args: never[]) => unknown ? T[K] : DeeplyReadonly<T[K]>;
+};

test/event-emitter.test.ts

@@ -0,0 +1,828 @@
+import { expect, test, vi } from 'vitest';
+import { EventEmitter } from '../source/event-emitter.ts';
+
+import type { DeeplyReadonly } from '../source/types.ts';
+import { deepFreeze } from '../source/misc.ts';
+
+/** Simple event map used across these tests. */
+type TestEvents = {
+    message: string;
+    count: number;
+    readonly: Readonly<{ value: number }>;
+    deeplyReadonly: DeeplyReadonly<{ nested: { value: number } }>;
+};
+
+/**
+ * Tests that EventEmitter invokes listeners when an event is emitted.
+ */
+const testEventEmitterCallsListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    // Register the listener and emit an event.
+    emitter.on('message', listener);
+    emitter.emit('message', 'hello');
+
+    // Expect the listener to have been called with the emitted payload.
+    expect(listener).toHaveBeenCalledOnce();
+    expect(listener).toHaveBeenCalledWith('hello');
+};
+
+/**
+ * Tests that EventEmitter invokes all listeners registered for the same event.
+ */
+const testEventEmitterCallsMultipleListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const firstListener = vi.fn();
+    const secondListener = vi.fn();
+
+    // Register two listeners for the same event type.
+    emitter.on('count', firstListener);
+    emitter.on('count', secondListener);
+    emitter.emit('count', 42);
+
+    // Expect both listeners to receive the same payload.
+    expect(firstListener).toHaveBeenCalledOnce();
+    expect(firstListener).toHaveBeenCalledWith(42);
+    expect(secondListener).toHaveBeenCalledOnce();
+    expect(secondListener).toHaveBeenCalledWith(42);
+};
+
+/**
+ * Tests that EventEmitter only invokes listeners registered for the emitted event type.
+ */
+const testEventEmitterCallsOnlyMatchingListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const messageListener = vi.fn();
+    const countListener = vi.fn();
+
+    // Register listeners on different event types.
+    emitter.on('message', messageListener);
+    emitter.on('count', countListener);
+
+    // Emit only the message event.
+    emitter.emit('message', 'hello');
+
+    // Expect only the matching listener to have been called.
+    expect(messageListener).toHaveBeenCalledOnce();
+    expect(countListener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that EventEmitter.emit returns false when no listeners are registered.
+ */
+const testEventEmitterEmitReturnsFalseWithNoListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+
+    const hasListeners = emitter.emit('message', 'hello');
+
+    // Expect emit to report that nobody was listening.
+    expect(hasListeners).toBe(false);
+};
+
+/**
+ * Tests that EventEmitter.emit returns true when listeners are registered.
+ */
+const testEventEmitterEmitReturnsTrueWithListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+
+    emitter.on('message', vi.fn());
+
+    const hasListeners = emitter.emit('message', 'hello');
+
+    // Expect emit to report that at least one listener was invoked.
+    expect(hasListeners).toBe(true);
+};
+
+/**
+ * Tests that EventEmitter.emit continues after a listener throws an error.
+ */
+const testEventEmitterEmitContinuesAfterListenerThrows = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const secondListener = vi.fn();
+
+    // Spy on the console.error method
+    const consoleErrorSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+    emitter.on('message', (): void => {
+        throw new Error('listener failure');
+    });
+    emitter.on('message', secondListener);
+
+    emitter.emit('message', 'hello');
+
+    expect(secondListener).toHaveBeenCalledOnce();
+    expect(secondListener).toHaveBeenCalledWith('hello');
+
+    expect(consoleErrorSpy).toHaveBeenCalledOnce();
+    expect(consoleErrorSpy).toHaveBeenCalledWith(new Error('listener failure'));
+
+    // Remove the mock on console.error
+    consoleErrorSpy.mockRestore();
+};
+
+/**
+ * Tests that it supports Object.freeze() frozen payloads.
+ */
+const testEventEmitterSupportsObjectFreezeFrozenPayloads = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+        const payload = Object.freeze({ value: 42 });
+
+        emitter.on('readonly', listener, 100);
+        emitter.emit('readonly', payload);
+
+        // @ts-expect-error - We are trying to mutate a frozen payload.
+        expect(() => (payload.value = 43)).toThrow();
+
+        // Advance the timer to ensure the debounce timer goes off
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Expect the listener to have been called with the frozen payload.
+        expect(listener).toHaveBeenCalledOnce();
+        expect(listener).toHaveBeenCalledWith(payload);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that it supports deepFreeze() frozen payloads.
+ */
+const testEventEmitterSupportsDeepFreezeFrozenPayloads = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+        const payload = deepFreeze({ nested: { value: 42 } });
+
+        emitter.on('deeplyReadonly', listener, 100);
+        emitter.emit('deeplyReadonly', payload);
+
+        // @ts-expect-error - We are trying to mutate a frozen payload.
+        expect(() => (payload.nested.value = 43)).toThrow();
+
+        // Advance the timer to ensure the debounce timer goes off
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Expect the listener to have been called with the frozen payload.
+        expect(listener).toHaveBeenCalledOnce();
+        expect(listener).toHaveBeenCalledWith(payload);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that the off callback returned by on() removes the listener.
+ */
+const testEventEmitterOffCallbackRemovesListener = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    // on() returns an off callback that removes the listener.
+    const off = emitter.on('message', listener);
+    emitter.emit('message', 'first');
+
+    // Unsubscribe before emitting again.
+    off();
+    emitter.emit('message', 'second');
+
+    // Expect the listener to have only received the first event.
+    expect(listener).toHaveBeenCalledOnce();
+    expect(listener).toHaveBeenCalledWith('first');
+};
+
+/**
+ * Tests that off() removes a listener when given the same function reference.
+ */
+const testEventEmitterOffRemovesListenerByReference = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    emitter.on('message', listener);
+    emitter.off('message', listener);
+    emitter.emit('message', 'hello');
+
+    // Expect the listener to have been removed before the emit.
+    expect(listener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that off() removes all listeners for an event type when no listener is provided.
+ */
+const testEventEmitterOffRemovesAllListenersForEventType = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    emitter.on('message', listener);
+    emitter.off('message');
+
+    expect(listener).not.toHaveBeenCalled();
+    expect(emitter.emit('message', 'hello')).toBe(false);
+    expect(emitter.emit('count', 42)).toBe(false);
+};
+
+/**
+ * Tests that off() does nothing when given an unknown listener reference.
+ */
+const testEventEmitterOffIgnoresUnknownListener = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    emitter.on('message', listener);
+
+    // Try to remove a different function reference.
+    emitter.off('message', vi.fn());
+    emitter.emit('message', 'hello');
+
+    // Expect the original listener to still receive the event.
+    expect(listener).toHaveBeenCalledOnce();
+};
+
+/**
+ * Tests that off() does nothing when called for an event type with no listeners.
+ */
+const testEventEmitterOffIgnoresUnregisteredEventType = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    // Call off without ever registering this listener.
+    emitter.off('message', listener);
+
+    expect(listener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that once() listeners are invoked only one time.
+ */
+const testEventEmitterOnceListenerFiresOnce = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    emitter.once('message', listener);
+    emitter.emit('message', 'first');
+    emitter.emit('message', 'second');
+
+    // Expect the listener to auto-unsubscribe after the first emit.
+    expect(listener).toHaveBeenCalledOnce();
+    expect(listener).toHaveBeenCalledWith('first');
+};
+
+/**
+ * Tests that once() can be added when regular listeners already exist for the event type.
+ */
+const testEventEmitterOnceWorksWithExistingListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const existingListener = vi.fn();
+    const onceListener = vi.fn();
+
+    // Register a regular listener first so the event type already exists in the map.
+    emitter.on('message', existingListener);
+    emitter.once('message', onceListener);
+    emitter.emit('message', 'hello');
+
+    expect(existingListener).toHaveBeenCalledOnce();
+    expect(onceListener).toHaveBeenCalledOnce();
+};
+
+/**
+ * Tests that the off callback returned by once() removes the listener before it fires.
+ */
+const testEventEmitterOnceOffCallbackRemovesListener = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    const off = emitter.once('message', listener);
+
+    // Unsubscribe before the event is ever emitted.
+    off();
+    emitter.emit('message', 'hello');
+
+    expect(listener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that debounced listeners do not receive the debounced event if the listener is removed.
+ */
+const testEventEmitterOffCancelsPendingDebouncedCallback = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        const off = emitter.on('message', listener, 100);
+        emitter.emit('message', 'first');
+
+        off();
+        emitter.emit('message', 'second');
+        await vi.advanceTimersByTimeAsync(100);
+
+        expect(listener).not.toHaveBeenCalled();
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that removeAllListeners() clears every registered listener.
+ */
+const testEventEmitterRemoveAllListeners = (): void => {
+    const emitter = new EventEmitter<TestEvents>();
+    const messageListener = vi.fn();
+    const countListener = vi.fn();
+
+    emitter.on('message', messageListener);
+    emitter.on('count', countListener);
+    emitter.removeAllListeners();
+
+    // Emit on both event types after clearing all listeners.
+    emitter.emit('message', 'hello');
+    emitter.emit('count', 1);
+
+    expect(messageListener).not.toHaveBeenCalled();
+    expect(countListener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that removeAllListeners() cancels a pending debounced callback.
+ */
+const testEventEmitterRemoveAllListenersCancelsPendingDebouncedCallback = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        // Arm a debounce timer, then clear every listener before it expires.
+        emitter.on('message', listener, 100);
+        emitter.emit('message', 'should not arrive');
+        emitter.removeAllListeners();
+
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Cleared listeners must not receive delayed debounced delivery.
+        expect(listener).not.toHaveBeenCalled();
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that timeouts are eventually cleared when removeAllListeners is called.
+ */
+const testEventEmitterTimeoutsAreClearedAfterTheyRunWhenRemoveAllListenersIsCalled = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        emitter.on('message', listener, 100);
+        emitter.emit('message', 'pending');
+
+        // We expect there to be a timer since we emitted a debounced event
+        expect(vi.getTimerCount()).toBe(1);
+
+        // Remove all listeners
+        emitter.removeAllListeners();
+
+        // Advance the timer to ensure the debounce timer is cleared.
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Make sure the listener was not called
+        expect(listener).not.toHaveBeenCalled();
+
+        // Make sure the timer was cleared
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that timeouts are eventually cleared when off is called.
+ */
+const testEventEmitterTimeoutsAreClearedWhenOffIsCalled = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        const off = emitter.on('message', listener, 100);
+        emitter.emit('message', 'pending');
+
+        // We expect there to be a timer since we emitted a debounced event
+        expect(vi.getTimerCount()).toBe(1);
+
+        // Cancel the listener
+        off();
+
+        // Advance the timer to ensure the debounce timer is cleared.
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Make sure the listener was not called
+        expect(listener).not.toHaveBeenCalled();
+
+        // Make sure the timer was cleared
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that waitFor() resolves when a matching event is emitted.
+ */
+const testEventEmitterWaitForResolvesOnMatch = async (): Promise<void> => {
+    const emitter = new EventEmitter<TestEvents>();
+
+    // Wait until an event matches the predicate.
+    const waitPromise = emitter.waitFor('count', (payload) => payload === 42);
+
+    // Emit a non-matching event first, then the matching one.
+    emitter.emit('count', 41);
+    emitter.emit('count', 42);
+
+    await expect(waitPromise).resolves.toBe(42);
+};
+
+/**
+ * Tests that waitFor() ignores non-matching events while other listeners still receive them.
+ */
+const testEventEmitterWaitForIgnoresNonMatchingEvents = async (): Promise<void> => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    const waitPromise = emitter.waitFor('message', (payload) => payload === 'done');
+
+    // A regular listener should still receive every emit while waitFor filters.
+    emitter.on('message', listener);
+    emitter.emit('message', 'pending');
+    emitter.emit('message', 'done');
+
+    await expect(waitPromise).resolves.toBe('done');
+    expect(listener).toHaveBeenCalledTimes(2);
+};
+
+/**
+ * Tests that waitFor() rejects when the timeout expires.
+ */
+const testEventEmitterWaitForRejectsOnTimeout = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+
+        const waitPromise = emitter.waitFor('message', () => true, 100);
+
+        // Attach the rejection handler before advancing timers so the rejection is handled.
+        const assertion = expect(waitPromise).rejects.toThrow('Timeout waiting for event "message"');
+
+        await vi.advanceTimersByTimeAsync(100);
+
+        await assertion;
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that waitFor() clears its timeout when it resolves before expiry.
+ */
+const testEventEmitterWaitForClearsTimeoutOnResolve = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+
+        // Register waitFor with a timeout, then resolve it before the timer fires.
+        const waitPromise = emitter.waitFor('message', (payload) => payload === 'done', 100);
+
+        emitter.emit('message', 'done');
+
+        await expect(waitPromise).resolves.toBe('done');
+
+        // If clearTimeout was not called, advancing past the timeout would reject the promise.
+        await vi.advanceTimersByTimeAsync(100);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that waitFor() removes its listener after resolving.
+ */
+const testEventEmitterWaitForRemovesListenerAfterResolve = async (): Promise<void> => {
+    const emitter = new EventEmitter<TestEvents>();
+
+    const waitPromise = emitter.waitFor('message', () => true);
+
+    emitter.emit('message', 'first');
+    await expect(waitPromise).resolves.toBe('first');
+
+    // Register a second waitFor so we can verify the first listener was cleaned up.
+    const secondWaitPromise = emitter.waitFor('message', (payload) => payload === 'second');
+
+    // Emit a payload that only the second waitFor should accept.
+    emitter.emit('message', 'ignored');
+
+    // Track whether the second waitFor resolves too early.
+    let resolvedEarly = false;
+    /* eslint-disable-next-line */
+    secondWaitPromise.then(() => {
+        resolvedEarly = true;
+    });
+
+    // Yield so any premature resolution would have a chance to run.
+    await Promise.resolve();
+    expect(resolvedEarly).toBe(false);
+
+    emitter.emit('message', 'second');
+    await expect(secondWaitPromise).resolves.toBe('second');
+};
+
+/**
+ * Tests that the first debounced emit does not call clearTimeout.
+ */
+const testEventEmitterDebouncedFirstEmitDoesNotClearTimeout = (): void => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const clearTimeoutSpy = vi.spyOn(globalThis, 'clearTimeout');
+        const listener = vi.fn();
+
+        emitter.on('message', listener, 100);
+        emitter.emit('message', 'first');
+
+        // The first emit starts the debounce timer; there is nothing to clear yet.
+        expect(clearTimeoutSpy).not.toHaveBeenCalled();
+        expect(listener).not.toHaveBeenCalled();
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that debounced on() listeners receive only the last payload after the debounce window.
+ */
+const testEventEmitterDebouncedOnListener = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        emitter.on('message', listener, 100);
+
+        // Emit several events in quick succession.
+        emitter.emit('message', 'first');
+        emitter.emit('message', 'second');
+        emitter.emit('message', 'third');
+
+        // Expect the listener to not have fired yet.
+        expect(listener).not.toHaveBeenCalled();
+
+        // Advance past the debounce window.
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Expect only the last payload to have been delivered.
+        expect(listener).toHaveBeenCalledOnce();
+        expect(listener).toHaveBeenCalledWith('third');
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that repeated debounced emits reset the debounce timer.
+ */
+const testEventEmitterDebouncedTimerResetsOnRepeatedEmits = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        emitter.on('count', listener, 100);
+        emitter.emit('count', 1);
+
+        // Advance halfway through the debounce window and emit again.
+        await vi.advanceTimersByTimeAsync(50);
+        emitter.emit('count', 2);
+        await vi.advanceTimersByTimeAsync(50);
+
+        // The timer was reset, so the listener should not have fired yet.
+        expect(listener).not.toHaveBeenCalled();
+
+        // Advance the remaining time for the reset timer to expire.
+        await vi.advanceTimersByTimeAsync(50);
+
+        expect(listener).toHaveBeenCalledOnce();
+        expect(listener).toHaveBeenCalledWith(2);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that a debounce time of zero behaves like a normal listener.
+ */
+const testEventEmitterZeroDebounceDoesNotDebounce = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        // A debounce time of zero should behave like a normal listener.
+        emitter.on('message', listener, 0);
+        emitter.emit('message', 'first');
+        emitter.emit('message', 'second');
+
+        expect(listener).toHaveBeenCalledTimes(2);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that debounced once() listeners fire once with the last payload.
+ */
+const testEventEmitterDebouncedOnceListener = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const listener = vi.fn();
+
+        emitter.once('message', listener, 100);
+        emitter.emit('message', 'first');
+        emitter.emit('message', 'second');
+
+        await vi.advanceTimersByTimeAsync(100);
+
+        // Expect the debounced once listener to fire once with the last payload.
+        expect(listener).toHaveBeenCalledOnce();
+        expect(listener).toHaveBeenCalledWith('second');
+
+        // Emit again after the debounce window; the once listener should stay removed.
+        emitter.emit('message', 'third');
+        await vi.advanceTimersByTimeAsync(100);
+
+        expect(listener).toHaveBeenCalledOnce();
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that the `waitFor` method rejects if the predicate function throws
+ */
+const testEventEmitterWaitForRejectsOnPredicateError = async (): Promise<void> => {
+    const emitter = new EventEmitter<TestEvents>();
+    const listener = vi.fn();
+
+    const waitPromise = emitter.waitFor('message', () => {
+        throw new Error('predicate error');
+    });
+
+    emitter.emit('message', 'hello');
+
+    await expect(waitPromise).rejects.toThrow('predicate error');
+    expect(listener).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that waitFor() removes its listener after the predicate function throws an error.
+ */
+const testEventEmitterWaitForRemovesListenerAfterPredicateError = async (): Promise<void> => {
+    const emitter = new EventEmitter<TestEvents>();
+
+    // Create a predicate function that throws an error.
+    const predicate = vi.fn().mockImplementation(() => {
+        throw new Error('predicate error');
+    });
+
+    // Wait for the predicate function to throw an error.
+    const waitPromise = emitter.waitFor('message', predicate);
+
+    // Emit an event, expecting the predicate function to throw an error.
+    emitter.emit('message', 'hello');
+    await expect(waitPromise).rejects.toThrow('predicate error');
+
+    // Expect the predicate function to have been called once.
+    expect(predicate).toHaveBeenCalledTimes(1);
+
+    // A later emit must not invoke the failed waitFor predicate again.
+    emitter.emit('message', 'again');
+
+    // Expect the predicate function to still have been called once.
+    expect(predicate).toHaveBeenCalledTimes(1);
+};
+
+/**
+ * Tests that waitFor() clears its timeout when the predicate function throws an error.
+ */
+const testEventEmitterWaitForClearsTimeoutAfterPredicateError = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const emitter = new EventEmitter<TestEvents>();
+        const clearTimeoutSpy = vi.spyOn(globalThis, 'clearTimeout');
+
+        const waitPromise = emitter.waitFor(
+            'message',
+            (): boolean => {
+                throw new Error('predicate error');
+            },
+            100,
+        );
+
+        emitter.emit('message', 'hello');
+        await expect(waitPromise).rejects.toThrow('predicate error');
+
+        // The timeout scheduled for waitFor must be cleared on predicate failure.
+        expect(clearTimeoutSpy).toHaveBeenCalled();
+
+        // Advancing past the original timeout must not produce a second rejection path.
+        await vi.advanceTimersByTimeAsync(100);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+const runTests = async (): Promise<void> => {
+    test('EventEmitter: calls listeners when an event is emitted', testEventEmitterCallsListeners);
+    test('EventEmitter: calls multiple listeners for the same event', testEventEmitterCallsMultipleListeners);
+    test('EventEmitter: only calls listeners for the emitted event type', testEventEmitterCallsOnlyMatchingListeners);
+    test('EventEmitter: returns false when emitting with no listeners', testEventEmitterEmitReturnsFalseWithNoListeners);
+    test('EventEmitter: returns true when emitting with listeners', testEventEmitterEmitReturnsTrueWithListeners);
+    test('EventEmitter: continues after a listener throws an error', testEventEmitterEmitContinuesAfterListenerThrows);
+    test('EventEmitter: supports Object.freeze() frozen payloads', testEventEmitterSupportsObjectFreezeFrozenPayloads);
+    test('EventEmitter: supports deepFreeze() frozen payloads', testEventEmitterSupportsDeepFreezeFrozenPayloads);
+    test('EventEmitter: stops calling a listener after its off callback is invoked', testEventEmitterOffCallbackRemovesListener);
+    test('EventEmitter: removes a listener when off is called with the same reference', testEventEmitterOffRemovesListenerByReference);
+    test(
+        'EventEmitter: removes all listeners for an event type when off is called with no listener',
+        testEventEmitterOffRemovesAllListenersForEventType,
+    );
+    test('EventEmitter: ignores off when the listener reference is unknown', testEventEmitterOffIgnoresUnknownListener);
+    test('EventEmitter: ignores off for an event type with no listeners', testEventEmitterOffIgnoresUnregisteredEventType);
+    test('EventEmitter: calls a once listener only one time', testEventEmitterOnceListenerFiresOnce);
+    test('EventEmitter: registers once when listeners already exist', testEventEmitterOnceWorksWithExistingListeners);
+    test('EventEmitter: stops a once listener after its off callback is invoked', testEventEmitterOnceOffCallbackRemovesListener);
+    test(
+        'EventEmitter: debounced listeners do not receive the debounced event if the listener is removed',
+        testEventEmitterOffCancelsPendingDebouncedCallback,
+    );
+    test('EventEmitter: removes all listeners when removeAllListeners is called', testEventEmitterRemoveAllListeners);
+    test(
+        'EventEmitter: cancels a pending debounced callback when removeAllListeners is called',
+        testEventEmitterRemoveAllListenersCancelsPendingDebouncedCallback,
+    );
+    test(
+        'EventEmitter: timeouts are cleared after they run when removeAllListeners is called',
+        testEventEmitterTimeoutsAreClearedAfterTheyRunWhenRemoveAllListenersIsCalled,
+    );
+    test('EventEmitter: timeouts are cleared when off is called', testEventEmitterTimeoutsAreClearedWhenOffIsCalled);
+    test('EventEmitter: resolves waitFor when a matching event is emitted', testEventEmitterWaitForResolvesOnMatch);
+    test('EventEmitter: ignores non-matching events while waiting with waitFor', testEventEmitterWaitForIgnoresNonMatchingEvents);
+    test('EventEmitter: rejects waitFor when the timeout is reached', testEventEmitterWaitForRejectsOnTimeout);
+    test('EventEmitter: clears the timeout when waitFor resolves before expiry', testEventEmitterWaitForClearsTimeoutOnResolve);
+    test('EventEmitter: removes the waitFor listener after it resolves', testEventEmitterWaitForRemovesListenerAfterResolve);
+    test('EventEmitter: does not clear a timeout on the first debounced emit', testEventEmitterDebouncedFirstEmitDoesNotClearTimeout);
+    test('EventEmitter: debounces on listeners', testEventEmitterDebouncedOnListener);
+    test('EventEmitter: resets the debounce timer on repeated emits', testEventEmitterDebouncedTimerResetsOnRepeatedEmits);
+    test('EventEmitter: does not debounce when debounceMilliseconds is zero', testEventEmitterZeroDebounceDoesNotDebounce);
+    test('EventEmitter: debounces once listeners and invokes them only once', testEventEmitterDebouncedOnceListener);
+    test('EventEmitter: rejects waitFor when the predicate function throws', testEventEmitterWaitForRejectsOnPredicateError);
+    test(
+        'EventEmitter: removes the waitFor listener after it rejects due to predicate error',
+        testEventEmitterWaitForRemovesListenerAfterPredicateError,
+    );
+    test('EventEmitter: clears the timeout when waitFor rejects due to predicate error', testEventEmitterWaitForClearsTimeoutAfterPredicateError);
+};
+
+await runTests();

test/exponential-backoff/exponential-backoff-externally-aborted.test.ts

@@ -0,0 +1,307 @@
+import { expect, test, vi } from 'vitest';
+import { ExponentialBackoffExternallyAbortable } from '../../source/exponential-backoff/exponential-backoff-externally-aborted.ts';
+import type { ExponentialBackoffExternallyAbortableOptions } from '../../source/exponential-backoff/exponential-backoff-externally-aborted.ts';
+import {
+    ExponentialBackoffNumberTooSmallError,
+    ExponentialBackoffStoppedRetriesError,
+    ExternallyAbortedExponentialBackoffExternalSignalAbortedError,
+    ExternallyAbortedExponentialBackoffInternalSignalAbortedError,
+} from '../../source/exponential-backoff/errors.ts';
+import { testExponentialBackoff } from './exponential-backoff-run.shared.ts';
+
+/**
+ * Tests that invalid constructor options are still rejected by the inner ExponentialBackoff.
+ */
+const testExternallyAbortedExponentialBackoffConstructorValidatesOptions = (): void => {
+    expect(() => new ExponentialBackoffExternallyAbortable({ maxAttempts: -1 })).toThrow(ExponentialBackoffNumberTooSmallError);
+};
+
+/**
+ * Tests that {@link ExponentialBackoffExternallyAbortable.abort} before `run` prevents the task from
+ * ever being invoked and rejects with a stopped-retries error whose cause is the internal abort error.
+ */
+const testExternallyAbortedExponentialBackoffAbortBeforeRun = async (): Promise<void> => {
+    const taskFn = vi.fn(async () => 'should not run');
+    const onError = vi.fn();
+    const abortReason = 'stopped by abort()';
+
+    const backoff = new ExponentialBackoffExternallyAbortable({
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 3,
+    });
+
+    // Abort before any attempt so the wrapper should short-circuit on the first inner call.
+    backoff.abort(abortReason);
+
+    try {
+        await backoff.run(taskFn, { onError });
+        expect.fail('Expected ExponentialBackoffStoppedRetriesError to be thrown');
+    } catch (error) {
+        expect(error).toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        expect((error as ExponentialBackoffStoppedRetriesError).cause).toBeInstanceOf(ExternallyAbortedExponentialBackoffInternalSignalAbortedError);
+        expect(((error as ExponentialBackoffStoppedRetriesError).cause as Error).cause).toEqual(new Error(abortReason));
+    }
+
+    // The user task must not run; the wrapper throws before forwarding to it.
+    expect(taskFn).not.toHaveBeenCalled();
+
+    expect(onError).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that {@link ExponentialBackoffExternallyAbortable.abort} during a retry delay takes effect
+ * immediately without calling the task again.
+ */
+const testExternallyAbortedExponentialBackoffAbortBetweenAttempts = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const taskFn = vi.fn(async () => {
+            throw new Error('retry me');
+        });
+        const onError = vi.fn();
+        const abortReason = new Error('stopped between attempts');
+
+        const backoff = new ExponentialBackoffExternallyAbortable({
+            baseDelay: 1_000,
+            jitter: 0,
+            maxAttempts: 0,
+        });
+
+        const errorPromise = backoff.run(taskFn, { onError }).catch((error: unknown) => error);
+
+        // First attempt runs immediately and fails, then a 1000ms delay is scheduled.
+        await Promise.resolve();
+        expect(taskFn).toHaveBeenCalledTimes(1);
+
+        await vi.advanceTimersByTimeAsync(500);
+        backoff.abort(abortReason);
+
+        const error = await errorPromise;
+        expect(error).toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        expect((error as ExponentialBackoffStoppedRetriesError).cause).toBeInstanceOf(ExternallyAbortedExponentialBackoffInternalSignalAbortedError);
+        expect(((error as ExponentialBackoffStoppedRetriesError).cause as Error).cause).toBe(abortReason);
+
+        // The user task ran only on the first attempt; the aborted second attempt never forwarded to it.
+        expect(taskFn).toHaveBeenCalledTimes(1);
+
+        expect(onError).toHaveBeenCalledOnce();
+        expect(onError.mock.calls[0][0].message).toBe('retry me');
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+    }
+};
+
+/**
+ * Tests that a constructor `abortSignal` that is already aborted prevents the task from running.
+ */
+const testExternallyAbortedExponentialBackoffAbortSignalAlreadyAborted = async (): Promise<void> => {
+    const taskFn = vi.fn(async () => 'should not run');
+    const onError = vi.fn();
+    const abortReason = 'signal already aborted';
+
+    const abortController = new AbortController();
+    abortController.abort(abortReason);
+
+    const backoff = new ExponentialBackoffExternallyAbortable({
+        abortSignal: abortController.signal,
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 3,
+    });
+
+    try {
+        await backoff.run(taskFn, { onError });
+        expect.fail('Expected ExponentialBackoffStoppedRetriesError to be thrown');
+    } catch (error) {
+        expect(error).toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        expect((error as ExponentialBackoffStoppedRetriesError).cause).toBeInstanceOf(ExternallyAbortedExponentialBackoffExternalSignalAbortedError);
+        expect(((error as ExponentialBackoffStoppedRetriesError).cause as Error).cause).toEqual(new Error(abortReason));
+    }
+
+    expect(taskFn).not.toHaveBeenCalled();
+    expect(onError).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that aborting the constructor `abortSignal` interrupts the current retry delay.
+ */
+const testExternallyAbortedExponentialBackoffAbortSignalBetweenAttempts = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const taskFn = vi.fn(async () => {
+            throw new Error('retry me');
+        });
+        const onError = vi.fn();
+        const abortReason = new Error('signal aborted between attempts');
+
+        const abortController = new AbortController();
+        const backoff = new ExponentialBackoffExternallyAbortable({
+            abortSignal: abortController.signal,
+            baseDelay: 1_000,
+            jitter: 0,
+            maxAttempts: 0,
+        });
+
+        const errorPromise = backoff.run(taskFn, { onError }).catch((error: unknown) => error);
+
+        await Promise.resolve();
+        expect(taskFn).toHaveBeenCalledTimes(1);
+
+        await vi.advanceTimersByTimeAsync(500);
+        abortController.abort(abortReason);
+
+        const error = await errorPromise;
+        expect(error).toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        expect((error as ExponentialBackoffStoppedRetriesError).cause).toBeInstanceOf(ExternallyAbortedExponentialBackoffExternalSignalAbortedError);
+        expect(((error as ExponentialBackoffStoppedRetriesError).cause as Error).cause).toBe(abortReason);
+
+        expect(taskFn).toHaveBeenCalledTimes(1);
+        expect(onError).toHaveBeenCalledOnce();
+        expect(onError.mock.calls[0][0].message).toBe('retry me');
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+    }
+};
+
+/**
+ * Tests that a single {@link ExponentialBackoffExternallyAbortable.abort} stops every concurrent `run` on that instance.
+ */
+const testExternallyAbortedExponentialBackoffAbortStopsConcurrentRuns = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const firstTaskFn = vi.fn(async () => {
+            throw new Error('first run failed');
+        });
+        const secondTaskFn = vi.fn(async () => {
+            throw new Error('second run failed');
+        });
+        const abortReason = new Error('stop all runs');
+
+        // One instance is shared across concurrent runs, matching the class remark that abort stops all of them.
+        const backoff = new ExponentialBackoffExternallyAbortable({
+            baseDelay: 1_000,
+            jitter: 0,
+            maxAttempts: 0,
+        });
+
+        // Handle both rejections before aborting the shared instance.
+        const firstErrorPromise = backoff.run(firstTaskFn).catch((error: unknown) => error);
+        const secondErrorPromise = backoff.run(secondTaskFn).catch((error: unknown) => error);
+
+        await Promise.resolve();
+        expect(firstTaskFn).toHaveBeenCalledTimes(1);
+        expect(secondTaskFn).toHaveBeenCalledTimes(1);
+
+        backoff.abort(abortReason);
+
+        await expect(firstErrorPromise).resolves.toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        await expect(secondErrorPromise).resolves.toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+
+        expect(firstTaskFn).toHaveBeenCalledTimes(1);
+        expect(secondTaskFn).toHaveBeenCalledTimes(1);
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+    }
+};
+
+/**
+ * Tests that a signal supplied for one run composes with the instance-wide abort sources.
+ */
+const testExternallyAbortedExponentialBackoffRunSignalInterruptsDelay = async (): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        const abortReason = new Error('stop this run');
+        const abortController = new AbortController();
+        const taskFn = vi.fn().mockRejectedValue(new Error('retry me'));
+        const backoff = new ExponentialBackoffExternallyAbortable({
+            baseDelay: 1_000,
+            jitter: 0,
+            maxAttempts: 0,
+        });
+
+        const errorPromise = backoff.run(taskFn, { signal: abortController.signal }).catch((error: unknown) => error);
+
+        await Promise.resolve();
+        expect(taskFn).toHaveBeenCalledOnce();
+
+        abortController.abort(abortReason);
+
+        const error = await errorPromise;
+        expect(error).toBeInstanceOf(ExponentialBackoffStoppedRetriesError);
+        expect((error as ExponentialBackoffStoppedRetriesError).cause).toBe(abortReason);
+        expect(taskFn).toHaveBeenCalledOnce();
+        expect(vi.getTimerCount()).toBe(0);
+    } finally {
+        vi.useRealTimers();
+    }
+};
+
+/**
+ * Tests that a successful run is unaffected by a later abort of the shared instance.
+ */
+const testExternallyAbortedExponentialBackoffSuccessfulRunIgnoresLaterAbort = async (): Promise<void> => {
+    const taskFn = vi.fn(async () => 'success');
+
+    const backoff = new ExponentialBackoffExternallyAbortable({
+        baseDelay: 0,
+        jitter: 0,
+    });
+
+    const result = await backoff.run(taskFn);
+
+    expect(result).toBe('success');
+    expect(taskFn).toHaveBeenCalledOnce();
+
+    // Aborting after success must not throw retroactively; it only affects subsequent runs.
+    backoff.abort(new Error('too late'));
+};
+
+const runTests = async (): Promise<void> => {
+    const factory = (options?: Partial<ExponentialBackoffExternallyAbortableOptions>): ExponentialBackoffExternallyAbortable => {
+        return new ExponentialBackoffExternallyAbortable(options);
+    };
+
+    // Shared retry/delay/stopRetries contract, run against an adapter around instance run.
+    testExponentialBackoff('ExternallyAbortedExponentialBackoff', factory);
+
+    // Behavior that only exists on the externally-aborted wrapper.
+    test(
+        'ExternallyAbortedExponentialBackoff: constructor still validates options',
+        testExternallyAbortedExponentialBackoffConstructorValidatesOptions,
+    );
+    test(
+        'ExternallyAbortedExponentialBackoff: abort before run stops without calling the task',
+        testExternallyAbortedExponentialBackoffAbortBeforeRun,
+    );
+    test(
+        'ExternallyAbortedExponentialBackoff: abort between attempts interrupts the delay',
+        testExternallyAbortedExponentialBackoffAbortBetweenAttempts,
+    );
+    test(
+        'ExternallyAbortedExponentialBackoff: abortSignal already aborted stops without calling the task',
+        testExternallyAbortedExponentialBackoffAbortSignalAlreadyAborted,
+    );
+    test(
+        'ExternallyAbortedExponentialBackoff: abortSignal between attempts interrupts the delay',
+        testExternallyAbortedExponentialBackoffAbortSignalBetweenAttempts,
+    );
+    test('ExternallyAbortedExponentialBackoff: abort stops every concurrent run', testExternallyAbortedExponentialBackoffAbortStopsConcurrentRuns);
+    test(
+        'ExternallyAbortedExponentialBackoff: a per-run signal interrupts the delay',
+        testExternallyAbortedExponentialBackoffRunSignalInterruptsDelay,
+    );
+    test(
+        'ExternallyAbortedExponentialBackoff: successful run is unaffected by a later abort',
+        testExternallyAbortedExponentialBackoffSuccessfulRunIgnoresLaterAbort,
+    );
+};
+
+await runTests();

test/exponential-backoff/exponential-backoff-run.shared.ts

@@ -0,0 +1,516 @@
+import { describe, expect, test, vi } from 'vitest';
+
+import type {
+    ExponentialBackoffCallbackParameters,
+    ExponentialBackoffOptions,
+    ExponentialBackoffRunOptions,
+} from '../../source/exponential-backoff/index.ts';
+
+import {
+    ExponentialBackoffMaxRetriesHitError,
+    ExponentialBackoffNumberNotFiniteError,
+    ExponentialBackoffStoppedRetriesError,
+} from '../../source/exponential-backoff/index.ts';
+
+/**
+ * The common run interface implemented by ExponentialBackoff and ExponentialBackoffExternallyAbortable.
+ */
+export type ExponentialBackoffInstance = {
+    run: <T>(
+        taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
+        options?: Partial<ExponentialBackoffRunOptions>,
+    ) => Promise<T>;
+};
+
+/**
+ * Factory function passed into the tests to create an {@link ExponentialBackoffInstance}.
+ */
+export type ExponentialBackoffFactory = (options?: Partial<ExponentialBackoffOptions>) => ExponentialBackoffInstance;
+
+/**
+ * Tests the happy path: the wrapped function succeeds immediately and no retry machinery runs.
+ */
+const testExponentialBackoffSucceedsOnFirstAttempt = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Always resolves — never enters the catch/retry branch.
+    const resolveFn = vi.fn(async () => 'success');
+    const onError = vi.fn();
+
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+    };
+
+    const result = await exponentialBackoffFactory(options).run(resolveFn, { onError });
+
+    expect(result).toBe('success');
+    expect(resolveFn).toHaveBeenCalledOnce();
+    expect(onError).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that retries continue across multiple failures until the function eventually resolves.
+ */
+const testExponentialBackoffRetriesUntilSuccess = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Three invocations: two rejections then a success on the third call.
+    const tripleRejectFn = vi
+        .fn()
+        .mockRejectedValueOnce(new Error('attempt 1'))
+        .mockRejectedValueOnce(new Error('attempt 2'))
+        .mockResolvedValueOnce('success');
+
+    // maxAttempts: 5 is high enough that we stop because fn succeeded, not because we hit the cap.
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 5,
+    };
+
+    const result = await exponentialBackoffFactory(options).run(tripleRejectFn);
+
+    expect(result).toBe('success');
+    expect(tripleRejectFn).toHaveBeenCalledTimes(3);
+};
+
+/**
+ * Tests that the onError callback is invoked once for every failed attempt, including the last one
+ * before an ExponentialBackoffMaxRetriesHitError is thrown to the caller.
+ */
+const testExponentialBackoffCallsOnErrorForEachFailure = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    const error = new Error('temporary failure');
+
+    // Always rejects with the same error — we will exhaust all attempts.
+    const rejectFn = vi.fn().mockRejectedValue(error);
+    const onError = vi.fn();
+
+    // maxAttempts: 3 means three tries total, all of which will fail.
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 3,
+    };
+
+    await expect(exponentialBackoffFactory(options).run(rejectFn, { onError })).rejects.toThrow(ExponentialBackoffMaxRetriesHitError);
+
+    expect(onError).toHaveBeenCalledTimes(3);
+    expect(onError).toHaveBeenCalledWith(error);
+};
+
+/**
+ * Tests that when all attempts are exhausted the caller receives an ExponentialBackoffMaxRetriesHitError
+ * with every task error preserved in order on the cause.
+ */
+const testExponentialBackoffThrowsMaxRetriesHitErrorWhenExhausted = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    const firstError = new Error('first');
+    const lastError = new Error('last');
+
+    // Two distinct errors so we can prove both are collected, not just the last one.
+    const doubleRejectFn = vi.fn().mockRejectedValueOnce(firstError)
+.mockRejectedValueOnce(lastError);
+
+    try {
+        const options = {
+            baseDelay: 0,
+            jitter: 0,
+            maxAttempts: 2,
+        };
+
+        await exponentialBackoffFactory(options).run(doubleRejectFn, { onError: vi.fn() });
+        expect.fail('Expected ExponentialBackoffMaxRetriesHitError to be thrown');
+    } catch (error) {
+        expect(error).toBeInstanceOf(ExponentialBackoffMaxRetriesHitError);
+        expect((error as ExponentialBackoffMaxRetriesHitError).cause).toEqual([ firstError, lastError ]);
+    }
+
+    expect(doubleRejectFn).toHaveBeenCalledTimes(2);
+};
+
+/**
+ * Tests that rejections which are not Error instances are coerced to Error before onError runs,
+ * so callers always observe a consistent error type in the callback.
+ */
+const testExponentialBackoffWrapsNonErrorThrows = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Reject with a plain string — not an Error subclass.
+    const rejectedFn = vi.fn().mockRejectedValue('not-an-error');
+    const onError = vi.fn();
+
+    // Single attempt — we fail fast and inspect what onError received.
+    try {
+        const options = {
+            baseDelay: 0,
+            jitter: 0,
+            maxAttempts: 1,
+        };
+
+        await exponentialBackoffFactory(options).run(rejectedFn, { onError });
+        expect.fail('Expected ExponentialBackoffMaxRetriesHitError to be thrown');
+    } catch (error) {
+        expect(error).toBeInstanceOf(ExponentialBackoffMaxRetriesHitError);
+        const [ wrappedError ] = (error as ExponentialBackoffMaxRetriesHitError).cause as Error[];
+        expect(wrappedError).toBeInstanceOf(Error);
+        expect(wrappedError.message).toBe('not-an-error');
+    }
+
+    expect(onError).toHaveBeenCalledOnce();
+    expect(onError.mock.calls[0][0]).toBeInstanceOf(Error);
+    expect(onError.mock.calls[0][0].message).toBe('not-an-error');
+};
+
+/**
+ * Tests that when the task function succeeds and the abort signal is aborted, the result is returned
+ * and the onError callback is not called.
+ */
+const testExponentialBackoffRunSuccessAndAbortSignal = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Define the function which aborts the exponential backoff and succeeds
+    const abortAndSucceedFn = vi.fn(({ stopRetries }) => {
+        stopRetries(new Error('retry me'));
+
+        return Promise.resolve('success');
+    });
+    const onErrorFn = vi.fn();
+
+    // Run the exponential backoff with the function and the onError callback
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+    };
+
+    const result = await exponentialBackoffFactory(options).run(abortAndSucceedFn, { onError: onErrorFn });
+
+    // Expect the result to be the success message
+    expect(result).toBe('success');
+    expect(abortAndSucceedFn).toHaveBeenCalledOnce();
+
+    // Expect the onError callback to not have been called
+    expect(onErrorFn).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that when the abort signal is aborted with an error, an ExponentialBackoffStoppedRetriesError is thrown
+ * with the error as the message.
+ */
+const testExponentialBackoffRunWithAbortSignal = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Define the function which aborts the exponential backoff and throws an error
+    const abortAndThrowFn = vi.fn(({ stopRetries }) => {
+        stopRetries(new Error('exponential backoff aborted'));
+        throw new Error('error message');
+    });
+    const onErrorFn = vi.fn();
+
+    // Define the expected error
+    const expectedError = new ExponentialBackoffStoppedRetriesError(new Error('exponential backoff aborted'));
+
+    // Run the exponential backoff with the function and the onError callback and expect the error to be thrown
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+    };
+
+    await expect(exponentialBackoffFactory(options).run(abortAndThrowFn, { onError: onErrorFn })).rejects.toThrow(expectedError);
+
+    // Expect the onError callback to have been called once with the error
+    expect(onErrorFn).toHaveBeenCalledOnce();
+    expect(onErrorFn.mock.calls[0][0]).toBeInstanceOf(Error);
+    expect(onErrorFn.mock.calls[0][0].message).toBe('error message');
+
+    // Expect the function to have been called once and not to have resolved
+    expect(abortAndThrowFn).toHaveBeenCalledOnce();
+    expect(abortAndThrowFn).not.toHaveResolved();
+};
+
+/**
+ * Tests that when the abort signal is aborted with a string, an ExponentialBackoffStoppedRetriesError is thrown
+ * with the string as the message.
+ */
+const testExponentialBackoffRunAbortedStringCreatesError = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    // Define the function which aborts the exponential backoff and throws an error
+    const abortAndThrowStringFn = vi.fn(({ stopRetries }) => {
+        stopRetries('exponential backoff aborted');
+
+        // eslint-disable-next-line
+        throw 'error message';
+    });
+    const onErrorFn = vi.fn();
+
+    // Define the expected error, Note that we "stopRetries" with just a string, not an error. They are treated equivalently.
+    const expectedError = new ExponentialBackoffStoppedRetriesError(new Error('exponential backoff aborted'));
+
+    // Run the exponential backoff with the function and the onError callback and expect the error to be thrown
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+    };
+
+    await expect(exponentialBackoffFactory(options).run(abortAndThrowStringFn, { onError: onErrorFn })).rejects.toThrow(expectedError);
+
+    // Expect the onError callback to have been called once with the error
+    expect(onErrorFn).toHaveBeenCalledOnce();
+    expect(onErrorFn.mock.calls[0][0]).toBeInstanceOf(Error);
+    expect(onErrorFn.mock.calls[0][0].message).toBe('error message');
+
+    // Expect the function to have been called once and not to have resolved
+    expect(abortAndThrowStringFn).toHaveBeenCalledOnce();
+    expect(abortAndThrowStringFn).not.toHaveResolved();
+};
+
+/**
+ * Tests that the delay is aborted when the abort signal is activated.
+ */
+const testExponentialBackoffSignalInterruptsDelay = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    vi.useFakeTimers();
+
+    try {
+        let abort: (reason: unknown) => void;
+
+        const taskFn = vi.fn(async ({ stopRetries }) => {
+            abort = stopRetries;
+            throw new Error('error message');
+        });
+
+        // Start the exponential backoff and wait for it to complete
+        const options = {
+            baseDelay: 1000,
+            jitter: 0,
+            maxAttempts: 0,
+        };
+
+        const result = exponentialBackoffFactory(options).run(taskFn, { onError: vi.fn() });
+
+        // Advance the timer by 500 (mid delay)
+        await vi.advanceTimersByTimeAsync(500);
+
+        // Make sure the abort function is defined (That the taskFn was called)
+        if (!abort!) {
+            throw new Error('abort is not defined');
+        }
+
+        // Check that the abort function is defined
+        expect(abort).toBeDefined();
+
+        // Abort the exponential backoff
+        abort?.(new Error('exponential backoff aborted'));
+
+        // Expect the result to be rejected with an ExponentialBackoffStoppedRetriesError
+        await expect(result).rejects.toThrow(ExponentialBackoffStoppedRetriesError);
+    } finally {
+        vi.useRealTimers();
+    }
+};
+
+/**
+ * Tests that the exponential backoff is cancelled if the abort signal was activated before run()
+ */
+const testExponentialBackoffAlreadyAbortedSignalPreventsRun = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    const abortController = new AbortController();
+    const abortSignal = abortController.signal;
+    abortController.abort(new Error('already aborted'));
+
+    const taskFn = vi.fn();
+
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+    };
+
+    const promise = exponentialBackoffFactory(options).run(taskFn, { onError: vi.fn(), signal: abortSignal });
+
+    await expect(promise).rejects.toThrow(ExponentialBackoffStoppedRetriesError);
+
+    expect(taskFn).not.toHaveBeenCalled();
+};
+
+/**
+ * Tests that maxAttempts: 0 disables the attempt cap so retries continue until the function succeeds.
+ */
+const testExponentialBackoffRetriesIndefinitelyWhenMaxAttemptsIsZero = async (factory: ExponentialBackoffFactory): Promise<void> => {
+    // Four invocations: three failures then success — would exceed a cap of 3 if one existed.
+    const tripleRejectThenResolveFn = vi
+        .fn()
+        .mockRejectedValueOnce(new Error('attempt 1'))
+        .mockRejectedValueOnce(new Error('attempt 2'))
+        .mockRejectedValueOnce(new Error('attempt 3'))
+        .mockResolvedValueOnce('eventually');
+
+    const options = {
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 0,
+    };
+
+    const result = await factory(options).run(tripleRejectThenResolveFn, { onError: vi.fn() });
+
+    expect(result).toBe('eventually');
+    expect(tripleRejectThenResolveFn).toHaveBeenCalledTimes(4);
+};
+
+/**
+ * Tests the delay formula: each retry waits baseDelay * growthRate^attemptIndex milliseconds
+ * (with jitter disabled so the math is exact).
+ */
+const testExponentialBackoffIncreasesDelayExponentially = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    vi.useFakeTimers();
+    vi.spyOn(Math, 'random').mockReturnValue(0.5);
+
+    try {
+        const doubleRejectThenResolveFn = vi
+            .fn()
+            .mockRejectedValueOnce(new Error('attempt 1'))
+            .mockRejectedValueOnce(new Error('attempt 2'))
+            .mockResolvedValueOnce('success');
+
+        const options = {
+            baseDelay: 100,
+            growthRate: 2,
+            jitter: 0,
+            maxDelay: 10_000,
+            maxAttempts: 5,
+        };
+
+        const promise = exponentialBackoffFactory(options).run(doubleRejectThenResolveFn, { onError: vi.fn() });
+
+        // Attempt 0 fires synchronously on the first microtask tick.
+        await Promise.resolve();
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(1);
+
+        // After attempt 0 fails, delay = 100 * 2^0 = 100ms before attempt 1.
+        await vi.advanceTimersByTimeAsync(100);
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(2);
+
+        // After attempt 1 fails, delay = 100 * 2^1 = 200ms before attempt 2.
+        await vi.advanceTimersByTimeAsync(200);
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(3);
+
+        await expect(promise).resolves.toBe('success');
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that computed delay never exceeds maxDelay even when exponential growth would go higher.
+ */
+const testExponentialBackoffCapsDelayAtMaxDelay = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    vi.useFakeTimers();
+    vi.spyOn(Math, 'random').mockReturnValue(0.5);
+
+    try {
+        const doubleRejectThenResolveFn = vi
+            .fn()
+            .mockRejectedValueOnce(new Error('attempt 1'))
+            .mockRejectedValueOnce(new Error('attempt 2'))
+            .mockResolvedValueOnce('success');
+
+        const options = {
+            baseDelay: 1_000,
+            growthRate: 4,
+            jitter: 0,
+            maxDelay: 2_000,
+            maxAttempts: 5,
+        };
+
+        const promise = exponentialBackoffFactory(options).run(doubleRejectThenResolveFn, { onError: vi.fn() });
+
+        await Promise.resolve();
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(1);
+
+        // attempt 0: 1000 * 4^0 = 1000ms, below the 2000ms cap.
+        await vi.advanceTimersByTimeAsync(1_000);
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(2);
+
+        // attempt 1: uncapped would be 4000ms but maxDelay clamps to 2000ms.
+        await vi.advanceTimersByTimeAsync(2_000);
+        expect(doubleRejectThenResolveFn).toHaveBeenCalledTimes(3);
+
+        await expect(promise).resolves.toBe('success');
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that jitter subtracts up to jitter * cappedDelay from the capped delay based on Math.random.
+ */
+const testExponentialBackoffAppliesJitter = async (exponentialBackoffFactory: ExponentialBackoffFactory): Promise<void> => {
+    vi.useFakeTimers();
+    // random = 1 → full 10% reduction: 1000 - (1 * 0.1 * 1000) = 900ms.
+    vi.spyOn(Math, 'random').mockReturnValue(1);
+
+    try {
+        const rejectThenResolveFn = vi.fn().mockRejectedValueOnce(new Error('attempt 1'))
+.mockResolvedValueOnce('success');
+
+        const options = {
+            baseDelay: 1_000,
+            growthRate: 1,
+            jitter: 0.1,
+            maxDelay: 10_000,
+            maxAttempts: 3,
+        };
+
+        const promise = exponentialBackoffFactory(options).run(rejectThenResolveFn, { onError: vi.fn() });
+
+        await Promise.resolve();
+        expect(rejectThenResolveFn).toHaveBeenCalledTimes(1);
+
+        // Advancing 899ms is one ms short of the jittered delay; 900ms triggers the retry.
+        await vi.advanceTimersByTimeAsync(899);
+        expect(rejectThenResolveFn).toHaveBeenCalledTimes(1);
+
+        await vi.advanceTimersByTimeAsync(1);
+        expect(rejectThenResolveFn).toHaveBeenCalledTimes(2);
+
+        await expect(promise).resolves.toBe('success');
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that passing undefined into the constructor does not cause an error during spread
+ */
+const testExponentialBackoffConstructorDoesNotCauseErrorDuringSpread = async (factory: ExponentialBackoffFactory): Promise<void> => {
+    const options = {
+        baseDelay: undefined,
+        growthRate: undefined,
+        jitter: undefined,
+        maxDelay: undefined,
+        maxAttempts: undefined,
+    };
+
+    // We expect an error during validation as undefined is not a finite number, not an issue with the spread operator
+    // @ts-expect-error - Passing undefined is allowed if the exactOptionalPropertyTypes option is set to false in TS Compiler options.
+    expect(() => factory(options)).toThrow(ExponentialBackoffNumberNotFiniteError);
+};
+
+export const testExponentialBackoff = (name: string, exponentialBackoffFactory: ExponentialBackoffFactory): void => {
+    describe(name, () => {
+        test('ExponentialBackoff: returns the result on first success', () =>
+            testExponentialBackoffSucceedsOnFirstAttempt(exponentialBackoffFactory));
+        test('ExponentialBackoff: retries until the function succeeds', () => testExponentialBackoffRetriesUntilSuccess(exponentialBackoffFactory));
+        test('ExponentialBackoff: calls onError for each failed attempt', () =>
+            testExponentialBackoffCallsOnErrorForEachFailure(exponentialBackoffFactory));
+        test('ExponentialBackoff: throws ExponentialBackoffMaxRetriesHitError when max attempts are exhausted', () =>
+            testExponentialBackoffThrowsMaxRetriesHitErrorWhenExhausted(exponentialBackoffFactory));
+        test('ExponentialBackoff: wraps non-Error throws before calling onError', () =>
+            testExponentialBackoffWrapsNonErrorThrows(exponentialBackoffFactory));
+        test('ExponentialBackoff: succeeds and aborts with abort signal', () =>
+            testExponentialBackoffRunSuccessAndAbortSignal(exponentialBackoffFactory));
+        test('ExponentialBackoff: aborts with abort signal', () => testExponentialBackoffRunWithAbortSignal(exponentialBackoffFactory));
+        test('ExponentialBackoff: aborts with aborted string creates error', () =>
+            testExponentialBackoffRunAbortedStringCreatesError(exponentialBackoffFactory));
+        test('ExponentialBackoff: aborts with abort signal, interrupts delay', () =>
+            testExponentialBackoffSignalInterruptsDelay(exponentialBackoffFactory));
+        test('ExponentialBackoff: already aborted signal prevents run', () =>
+            testExponentialBackoffAlreadyAbortedSignalPreventsRun(exponentialBackoffFactory));
+        test('ExponentialBackoff: retries indefinitely when maxAttempts is 0', () =>
+            testExponentialBackoffRetriesIndefinitelyWhenMaxAttemptsIsZero(exponentialBackoffFactory));
+        test('ExponentialBackoff: increases delay exponentially between attempts', () =>
+            testExponentialBackoffIncreasesDelayExponentially(exponentialBackoffFactory));
+        test('ExponentialBackoff: caps delay at maxDelay', () => testExponentialBackoffCapsDelayAtMaxDelay(exponentialBackoffFactory));
+        test('ExponentialBackoff: subtracts jitter from the capped delay', () => testExponentialBackoffAppliesJitter(exponentialBackoffFactory));
+        test('ExponentialBackoff: constructor does not cause an error during spread', () =>
+            testExponentialBackoffConstructorDoesNotCauseErrorDuringSpread(exponentialBackoffFactory));
+    });
+};

test/exponential-backoff/exponential-backoff.test.ts

@@ -0,0 +1,259 @@
+import { expect, test, vi } from 'vitest';
+
+import { testExponentialBackoff } from './exponential-backoff-run.shared.ts';
+
+import { ExponentialBackoff } from '../../source/exponential-backoff/exponential-backoff.ts';
+import type { ExponentialBackoffOptions } from '../../source/exponential-backoff/exponential-backoff.ts';
+
+/**
+ * A valid options object that satisfies {@link ExponentialBackoff.validateOptions}.
+ */
+const validExponentialBackoffOptions = {
+    maxDelay: 10_000,
+    maxAttempts: 10,
+    baseDelay: 1_000,
+    growthRate: 2,
+    jitter: 0.1,
+};
+
+/**
+ * Tests the {@link ExponentialBackoff.from} factory and subsequent instance {@link ExponentialBackoff.run}
+ * as an alternative to the static helper.
+ */
+const testExponentialBackoffFromAndInstanceRun = async (): Promise<void> => {
+    const successfullyResolve = vi.fn(async () => 42);
+
+    // from() is a convenience constructor; run() on the result should behave like the static path.
+    const backoff = ExponentialBackoff.from({
+        baseDelay: 0,
+        jitter: 0,
+    });
+
+    const result = await backoff.run(successfullyResolve);
+
+    expect(result).toBe(42);
+    expect(successfullyResolve).toHaveBeenCalledOnce();
+};
+
+/**
+ * Tests that the static {@link ExponentialBackoff.run} helper creates a throwaway instance
+ * with library defaults (including the default 1000ms base delay) when no options are passed.
+ */
+const testExponentialBackoffRunUsesDefaultOptions = async (): Promise<void> => {
+    // Fake timers let us advance time without waiting real seconds between retries.
+    vi.useFakeTimers();
+    // Pin Math.random to 0 so jitter does not reduce the default delay.
+    vi.spyOn(Math, 'random').mockReturnValue(0);
+
+    try {
+        // The wrapped function fails on its first invocation and succeeds on the second.
+        // That forces ExponentialBackoff.run down the retry path using default options.
+        const rejectThenResolveFn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
+.mockResolvedValueOnce('static-result');
+
+        // Call the static helper with no onError and no options — defaults apply entirely.
+        const promise = ExponentialBackoff.run(rejectThenResolveFn);
+
+        // Yield one microtask so the first (immediate) attempt completes and schedules the retry timer.
+        await Promise.resolve();
+        expect(rejectThenResolveFn).toHaveBeenCalledTimes(1);
+
+        // Default baseDelay is 1000ms; advancing less would not trigger the retry yet.
+        await vi.advanceTimersByTimeAsync(1_000);
+
+        // The retry should have succeeded and returned the resolved value from the mock.
+        await expect(promise).resolves.toBe('static-result');
+        expect(rejectThenResolveFn).toHaveBeenCalledTimes(2);
+    } finally {
+        vi.useRealTimers();
+        vi.restoreAllMocks();
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.run} accepts a partial options object and merges it
+ * with defaults, still retrying when only some fields are overridden.
+ */
+const testExponentialBackoffRunWithPartialOptions = async (): Promise<void> => {
+    // Same fail-then-succeed pattern; we only care that partial options still enable a retry.
+    const rejectThenResolveFn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
+.mockResolvedValueOnce('done');
+
+    // baseDelay/jitter of 0 skip real waiting; maxAttempts: 3 gives headroom for one retry.
+    // onError is omitted to verify the default no-op handler is used.
+    const result = await ExponentialBackoff.run(rejectThenResolveFn, {
+        baseDelay: 0,
+        jitter: 0,
+        maxAttempts: 3,
+    });
+
+    expect(result).toBe('done');
+    expect(rejectThenResolveFn).toHaveBeenCalledTimes(2);
+};
+
+/**
+ * Tests that calling {@link ExponentialBackoff.run} on a constructed instance applies
+ * the instance's stored options when no per-run options are supplied.
+ */
+const testExponentialBackoffInstanceRunUsesDefaultOnError = async (): Promise<void> => {
+    const rejectThenResolveFn = vi.fn().mockRejectedValueOnce(new Error('retry me'))
+.mockResolvedValueOnce('instance-result');
+
+    // Options live on the instance; run(fn) should read them instead of static defaults.
+    const backoff = new ExponentialBackoff({ baseDelay: 0, jitter: 0, maxAttempts: 3 });
+
+    const result = await backoff.run(rejectThenResolveFn);
+
+    expect(result).toBe('instance-result');
+    expect(rejectThenResolveFn).toHaveBeenCalledTimes(2);
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} accepts valid options, including boundary values of 0 and 1.
+ */
+const testExponentialBackoffValidateOptionsAcceptsValidOptions = (): void => {
+    const validCases = [
+        validExponentialBackoffOptions,
+        {
+            ...validExponentialBackoffOptions,
+            maxDelay: 0,
+            maxAttempts: 0,
+            baseDelay: 0,
+            growthRate: 0,
+            jitter: 0,
+        },
+        {
+            ...validExponentialBackoffOptions,
+            jitter: 1,
+        },
+    ] as const;
+
+    for (const options of validCases) {
+        expect(() => ExponentialBackoff.validateOptions(options)).not.toThrow();
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} rejects negative numeric options.
+ */
+const testExponentialBackoffValidateOptionsRejectsNegativeValues = (): void => {
+    // Define our test cases with each value being less than 0
+    const negativeCases = [
+        { field: 'maxDelay', value: -1 },
+        { field: 'maxAttempts', value: -1 },
+        { field: 'baseDelay', value: -1 },
+        { field: 'growthRate', value: -1 },
+    ] as const;
+
+    // Iterate through the test cases and expect an error to be thrown
+    for (const { field, value } of negativeCases) {
+        expect(() =>
+            ExponentialBackoff.validateOptions({
+                ...validExponentialBackoffOptions,
+                [field]: value,
+            })).toThrow(`Exponential backoff option "${field}" is too small. Must be at least 0`);
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} rejects jitter below 0 or above 1.
+ */
+const testExponentialBackoffValidateOptionsRejectsInvalidJitter = (): void => {
+    // Define our test cases with each value being less than 0 or greater than 1
+    const invalidJitterCases: Array<{ value: number }> = [{ value: -0.1 }, { value: 1.1 }];
+
+    // Iterate through the test cases and expect an error to be thrown
+    for (const { value } of invalidJitterCases) {
+        expect(() =>
+            ExponentialBackoff.validateOptions({
+                ...validExponentialBackoffOptions,
+                jitter: value,
+            })).toThrow('Exponential backoff option "jitter" is out of bounds. Must be between 0 and 1');
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} rejects non-finite values such as Infinity.
+ */
+const testExponentialBackoffValidateOptionsRejectsNonFiniteValues = (): void => {
+    // Define our test cases with each value being Infinity
+    const nonFiniteCases = [
+        { field: 'maxDelay', value: Infinity },
+        { field: 'maxAttempts', value: Infinity },
+        { field: 'baseDelay', value: Infinity },
+        { field: 'growthRate', value: Infinity },
+        { field: 'jitter', value: Infinity },
+    ] as const;
+
+    // Iterate through the test cases and expect an error to be thrown
+    for (const { field, value } of nonFiniteCases) {
+        expect(() =>
+            ExponentialBackoff.validateOptions({
+                ...validExponentialBackoffOptions,
+                [field]: value,
+            })).toThrow(`Exponential backoff option "${field}" is invalid. Must be a finite number`);
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} rejects non-integer values.
+ */
+const testExponentialBackoffValidateOptionsRejectsNonIntegerValues = (): void => {
+    // Define our test cases with each value being a non-integer
+    const nonIntegerCases = [{ field: 'maxAttempts', value: 1.5 }] as const;
+
+    // Iterate through the test cases and expect an error to be thrown
+    for (const { field, value } of nonIntegerCases) {
+        expect(() =>
+            ExponentialBackoff.validateOptions({
+                ...validExponentialBackoffOptions,
+                [field]: value,
+            })).toThrow(`Exponential backoff option "${field}" is invalid. Must be an integer`);
+    }
+};
+
+/**
+ * Tests that {@link ExponentialBackoff.validateOptions} rejects NaN, which is also non-finite.
+ */
+const testExponentialBackoffValidateOptionsRejectsNaN = (): void => {
+    // Define our test cases with each value being NaN
+    const nanCases = [
+        { field: 'maxDelay', value: Number.NaN },
+        { field: 'maxAttempts', value: Number.NaN },
+        { field: 'baseDelay', value: Number.NaN },
+        { field: 'growthRate', value: Number.NaN },
+        { field: 'jitter', value: Number.NaN },
+    ] as const;
+
+    // Iterate through the test cases and expect an error to be thrown
+    for (const { field, value } of nanCases) {
+        expect(() =>
+            ExponentialBackoff.validateOptions({
+                ...validExponentialBackoffOptions,
+                [field]: value,
+            })).toThrow(`Exponential backoff option "${field}" is invalid. Must be a finite number`);
+    }
+};
+
+const runTests = async (): Promise<void> => {
+    const exponentialBackoffFactory = (options?: Partial<ExponentialBackoffOptions>): ExponentialBackoff => {
+        return new ExponentialBackoff(options);
+    };
+
+    // Test the shared ExponentialBackoff contract.
+    testExponentialBackoff('ExponentialBackoff', exponentialBackoffFactory);
+
+    // APIs that only exist on ExponentialBackoff itself.
+    test('ExponentialBackoff: works via from and instance run', testExponentialBackoffFromAndInstanceRun);
+    test('ExponentialBackoff: uses the default options when no options are passed', testExponentialBackoffRunUsesDefaultOptions);
+    test('ExponentialBackoff: accepts partial options and merges them with defaults', testExponentialBackoffRunWithPartialOptions);
+    test('ExponentialBackoff: uses the instance default onError when omitted', testExponentialBackoffInstanceRunUsesDefaultOnError);
+    test('ExponentialBackoff.validateOptions: accepts valid options', testExponentialBackoffValidateOptionsAcceptsValidOptions);
+    test('ExponentialBackoff.validateOptions: rejects negative values', testExponentialBackoffValidateOptionsRejectsNegativeValues);
+    test('ExponentialBackoff.validateOptions: rejects invalid jitter', testExponentialBackoffValidateOptionsRejectsInvalidJitter);
+    test('ExponentialBackoff.validateOptions: rejects Infinity', testExponentialBackoffValidateOptionsRejectsNonFiniteValues);
+    test('ExponentialBackoff.validateOptions: rejects non-integer values', testExponentialBackoffValidateOptionsRejectsNonIntegerValues);
+    test('ExponentialBackoff.validateOptions: rejects NaN', testExponentialBackoffValidateOptionsRejectsNaN);
+};
+
+await runTests();

test/invitation.test.ts

@@ -0,0 +1,248 @@
+import { expect, test } from 'vitest';
+import { binToHex, generateRandomBytes, hexToBin } from '@bitauth/libauth';
+import {
+    COMMIT_IDENTIFIER_BYTE_LENGTH,
+    INVITATION_IDENTIFIER_BYTE_LENGTH,
+    TEMPLATE_IDENTIFIER_BYTE_LENGTH,
+    TOKEN_CATEGORY_BYTE_LENGTH,
+    TRANSACTION_HASH_BYTE_LENGTH,
+} from '@xo-cash/primitives';
+import type { XOInvitation } from '@xo-cash/types';
+import {
+    InvitationInvalidError,
+    InvitationJsonMalformedError,
+    InvitationSerializationFailedError,
+    deserializeInvitation,
+    parseInvitation,
+    serializeInvitation,
+} from '../source/index.ts';
+
+const invitationIdentifier = binToHex(generateRandomBytes(INVITATION_IDENTIFIER_BYTE_LENGTH));
+const templateIdentifier = binToHex(generateRandomBytes(TEMPLATE_IDENTIFIER_BYTE_LENGTH));
+const commitIdentifier = binToHex(generateRandomBytes(COMMIT_IDENTIFIER_BYTE_LENGTH));
+const outpointTransactionHash = binToHex(generateRandomBytes(TRANSACTION_HASH_BYTE_LENGTH));
+const tokenCategory = binToHex(generateRandomBytes(TOKEN_CATEGORY_BYTE_LENGTH));
+
+// Valid invitation
+const buildValidInvitation = (): XOInvitation => {
+    return {
+        invitationIdentifier,
+        createdAtTimestamp: 1700000000000,
+        templateIdentifier,
+        actionIdentifier: 'receive',
+        commits: [
+            {
+                commitIdentifier,
+                previousCommitIdentifier: undefined,
+                entityIdentifier: 'xpub661MyMwAqRbcEngHLkoUWCKMBMGeqdqCGkqtzrHaTZub9ALw2oRfHA6fQP5n5X9VHStaNTBYomkSb8BFhUGavwD3RG1qvMkEKceTavTp2Tm',
+                createdAtTimestamp: 1700000000000,
+                data: {
+                    transactionVersion: 2,
+                    transactionLocktime: 0,
+                    variables: [{ variableIdentifier: 'amount', value: 18014398509481985n, roleIdentifier: 'sender' }],
+                    inputs: [
+                        {
+                            outpointTransactionHash: hexToBin(outpointTransactionHash),
+                            outpointIndex: 0,
+                            sequenceNumber: 0,
+                            inputIdentifier: 'fundingInput',
+                            roleIdentifier: 'sender',
+                        },
+                    ],
+                    outputs: [
+                        {
+                            outputIdentifier: 'receiveOutput',
+                            valueSatoshis: 1000n,
+                            lockingBytecode: hexToBin('76a914000000000000000000000000000000000000000088ac'),
+                            token: { category: hexToBin(tokenCategory), amount: 21n },
+                            roleIdentifier: 'receiver',
+                        },
+                    ],
+                },
+                signature:
+                    '5f487c045657f3939ecfeaaacf239a7cfd44b485c2be591f5280bf0cc3a6e5fe304e8ea23311d82b2afa4f0ad7e0a6d07ec1e0b1aaee9c44097613694390966b',
+                expiresAtTimestamp: 1700003600000,
+            },
+        ],
+    };
+};
+
+/**
+ * Tests that parseInvitation accepts a valid invitation and returns the same object it received.
+ */
+const testParseInvitationReturnsReceivedObject = (): void => {
+    const invitation = buildValidInvitation();
+
+    expect(parseInvitation(invitation)).toBe(invitation);
+};
+
+/**
+ * Tests that an invitation survives a serialize and deserialize round trip.
+ */
+const testSerializeInvitationRoundTripsBigintAndBytes = (): void => {
+    const invitation = buildValidInvitation();
+
+    const restoredInvitation = deserializeInvitation(serializeInvitation(invitation));
+
+    expect(restoredInvitation).toEqual(invitation);
+
+    const [ restoredCommit ] = restoredInvitation.commits;
+    expect(restoredCommit!.data.variables![0]!.value).toBe(18014398509481985n);
+    expect(restoredCommit!.data.outputs![0]!.valueSatoshis).toBe(1000n);
+    expect(restoredCommit!.data.outputs![0]!.lockingBytecode).toBeInstanceOf(Uint8Array);
+    expect(binToHex(restoredCommit!.data.inputs![0]!.outpointTransactionHash!)).toBe(outpointTransactionHash);
+};
+
+/**
+ * Tests that parseInvitation throws InvitationInvalidError listing every missing required field.
+ */
+const testParseInvitationThrowsOnMissingRequiredFields = (): void => {
+    let thrownError: unknown;
+
+    try {
+        parseInvitation({});
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationInvalidError);
+
+    const expectedMessage =
+        'Invitation invalid, expected a value matching the XOInvitation type, but got: \n'
+        + '- invitationIdentifier: expected string, received undefined\n'
+        + '- createdAtTimestamp: expected number, received undefined\n'
+        + '- templateIdentifier: expected string, received undefined\n'
+        + '- actionIdentifier: expected string, received undefined\n'
+        + '- commits: expected array, received undefined';
+
+    expect((thrownError as InvitationInvalidError).message).toBe(expectedMessage);
+};
+
+/**
+ * Tests that parseInvitation throws InvitationInvalidError reporting every field with a wrong type or out of range value.
+ */
+const testParseInvitationThrowsOnFieldTypeViolations = (): void => {
+    const invitationWithWrongFieldTypes = { ...buildValidInvitation(), createdAtTimestamp: -1, commits: 'none' };
+
+    let thrownError: unknown;
+
+    try {
+        parseInvitation(invitationWithWrongFieldTypes);
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationInvalidError);
+
+    const expectedMessage =
+        'Invitation invalid, expected a value matching the XOInvitation type, but got: \n'
+        + '- createdAtTimestamp: Too small: expected number to be >=0\n'
+        + '- commits: expected array, received string';
+
+    expect((thrownError as InvitationInvalidError).message).toBe(expectedMessage);
+};
+
+/**
+ * Tests that parseInvitation throws InvitationInvalidError for an unrecognized property at the top level.
+ */
+const testParseInvitationThrowsOnUnknownProperties = (): void => {
+    const invitationWithUnknownProperty = { ...buildValidInvitation(), unknownProperty: 'unexpected' };
+
+    let thrownError: unknown;
+
+    try {
+        parseInvitation(invitationWithUnknownProperty);
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationInvalidError);
+
+    const expectedMessage =
+        'Invitation invalid, expected a value matching the XOInvitation type, but got: \n' + '- (root): Unrecognized key: "unknownProperty"';
+
+    expect((thrownError as InvitationInvalidError).message).toBe(expectedMessage);
+};
+
+/**
+ * Tests that parseInvitation throws InvitationInvalidError with the full path of an unrecognized property inside commit data.
+ */
+const testParseInvitationThrowsOnDeepUnknownProperties = (): void => {
+    const invitation = buildValidInvitation();
+    const [ firstCommit ] = invitation.commits;
+    const invitationWithDeepUnknown = { ...invitation, commits: [{ ...firstCommit, data: { ...firstCommit!.data, unknownData: 1 } }] };
+
+    let thrownError: unknown;
+
+    try {
+        parseInvitation(invitationWithDeepUnknown);
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationInvalidError);
+
+    const expectedMessage =
+        'Invitation invalid, expected a value matching the XOInvitation type, but got: \n' + '- commits.0.data: Unrecognized key: "unknownData"';
+
+    expect((thrownError as InvitationInvalidError).message).toBe(expectedMessage);
+};
+
+/**
+ * Tests that deserializeInvitation throws InvitationJsonMalformedError for text that is not valid JSON.
+ */
+const testDeserializeInvitationThrowsOnMalformedJson = (): void => {
+    let thrownError: unknown;
+
+    try {
+        deserializeInvitation('{"invitationIdentifier":');
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationJsonMalformedError);
+
+    const expectedMessage = 'Invitation JSON malformed: expected a valid JSON string, but got: Unexpected end of JSON input';
+
+    expect((thrownError as InvitationJsonMalformedError).message).toBe(expectedMessage);
+    expect((thrownError as InvitationJsonMalformedError).reason).toBe('Unexpected end of JSON input');
+};
+
+/**
+ * Tests that serializeInvitation throws InvitationSerializationFailedError when the value cannot be written as JSON.
+ */
+const testSerializeInvitationThrowsOnCircularValue = (): void => {
+    // A value that refers to itself has no JSON form
+    const circularInvitation: Record<string, unknown> = { ...buildValidInvitation() };
+    circularInvitation.self = circularInvitation;
+
+    let thrownError: unknown;
+
+    try {
+        serializeInvitation(circularInvitation as XOInvitation);
+    } catch (caughtError) {
+        thrownError = caughtError;
+    }
+
+    expect(thrownError).toBeInstanceOf(InvitationSerializationFailedError);
+
+    const expectedMessage =
+        'Invitation serialization failed: Converting circular structure to JSON\n'
+        + "    --> starting at object with constructor 'Object'\n"
+        + "    --- property 'self' closes the circle";
+
+    expect((thrownError as InvitationSerializationFailedError).message).toBe(expectedMessage);
+};
+
+const runTests = async (): Promise<void> => {
+    test('parseInvitation: returns the received object for a valid invitation', testParseInvitationReturnsReceivedObject);
+    test('serializeInvitation: round trips bigint and Uint8Array fields', testSerializeInvitationRoundTripsBigintAndBytes);
+    test('parseInvitation: throws InvitationInvalidError on missing required fields', testParseInvitationThrowsOnMissingRequiredFields);
+    test('parseInvitation: throws InvitationInvalidError on field type violations', testParseInvitationThrowsOnFieldTypeViolations);
+    test('parseInvitation: throws InvitationInvalidError on unknown top-level property', testParseInvitationThrowsOnUnknownProperties);
+    test('parseInvitation: throws InvitationInvalidError on unknown property inside commit data', testParseInvitationThrowsOnDeepUnknownProperties);
+    test('deserializeInvitation: throws InvitationJsonMalformedError on malformed JSON', testDeserializeInvitationThrowsOnMalformedJson);
+    test('serializeInvitation: throws InvitationSerializationFailedError on a circular value', testSerializeInvitationThrowsOnCircularValue);
+};
+
+await runTests();

test/misc.test.ts

@@ -0,0 +1,59 @@
+import { expect, test } from 'vitest';
+import { normalizeError } from '../source/misc.ts';
+
+/** Test cases for the normalizeError function. */
+const normalizeErrorTestCases = [
+    {
+        input: new Error('test error'),
+        expected: new Error('test error'),
+    },
+    {
+        input: 'test error',
+        expected: new Error('test error'),
+    },
+    {
+        input: 1,
+        expected: new Error('1'),
+    },
+    {
+        input: undefined,
+        expected: new Error('undefined'),
+    },
+    {
+        input: null,
+        expected: new Error('null'),
+    },
+    {
+        input: false,
+        expected: new Error('false'),
+    },
+    {
+        input: true,
+        expected: new Error('true'),
+    },
+    {
+        input: {},
+        expected: new Error('[object Object]'),
+    },
+    {
+        input: [],
+        expected: new Error(),
+    },
+];
+
+/**
+ * Tests that normalizeError converts non-Error throws to Error instances.
+ */
+const testNormalizeError = async (): Promise<void> => {
+    for (const testCase of normalizeErrorTestCases) {
+        const result = normalizeError(testCase.input);
+        expect(result).toBeInstanceOf(Error);
+        expect(result.message).toBe(testCase.expected.message);
+    }
+};
+
+const runTests = async (): Promise<void> => {
+    test('normalizeError: converts non-Error throws to Error instances', testNormalizeError);
+};
+
+await runTests();

test/sse-session/helpers/sse-stream.ts

@@ -0,0 +1,65 @@
+export type SseTestStreamOptions = {
+
+    /** Milliseconds to wait before enqueueing each chunk after the first. */
+    chunkDelayMs?: number;
+
+    /**
+     * When true, closes the body as soon as all initial chunks have been sent.
+     * Use this to simulate a server that sends events and then ends the stream.
+     */
+    closeWhenDone?: boolean;
+};
+
+/**
+ * Test double for an SSE HTTP response body.
+ *
+ * Enqueues fixture chunks in order and stays open until {@link close} is called,
+ * matching real servers that keep the connection alive after each event's trailing
+ * `\n\n` frame boundary.
+ */
+export class SseTestStream {
+    readonly stream: ReadableStream<Uint8Array>;
+
+    private controller: ReadableStreamDefaultController<Uint8Array> | null = null;
+
+    private closed = false;
+
+    /**
+     * @param chunks - Fixture `raw` strings, whole or split, to simulate chunk boundaries.
+     * @param options - Delivery timing and optional auto-close after the initial chunks.
+     */
+    constructor(chunks: string[], options: SseTestStreamOptions = {}) {
+        const { chunkDelayMs = 0, closeWhenDone = false } = options;
+        const encoder = new TextEncoder();
+
+        this.stream = new ReadableStream({
+            start: async (controller): Promise<void> => {
+                this.controller = controller;
+
+                for (let i = 0; i < chunks.length; i++) {
+                    if (chunkDelayMs > 0 && i > 0) {
+                        await new Promise((resolve) => setTimeout(resolve, chunkDelayMs));
+                    }
+
+                    if (this.closed) return;
+
+                    controller.enqueue(encoder.encode(chunks[i]!));
+                }
+
+                if (closeWhenDone) {
+                    this.close();
+                }
+            },
+        });
+    }
+
+    /**
+     * Ends the HTTP body the way a server closing the SSE connection would.
+     */
+    close(): void {
+        if (this.closed) return;
+
+        this.closed = true;
+        this.controller?.close();
+    }
+}

tsconfig.json

@@ -7,6 +7,7 @@
         "moduleResolution": "bundler",
         "resolveJsonModule": true,
         "allowImportingTsExtensions": true,
+        "exactOptionalPropertyTypes": true,
         "noEmit": true,
         "declaration": true,
         "declarationMap": true