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Add core source modules
Types, helpers, router, journal, SSE writer, fixture loader, HTTP server, MockOpenAI facade, CLI entry point, and public API.
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‎src/cli.ts‎

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#!/usr/bin/env node
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import { parseArgs } from "node:util";
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import { statSync } from "node:fs";
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import { resolve } from "node:path";
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import { createServer } from "./server.js";
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import { loadFixtureFile, loadFixturesFromDir } from "./fixture-loader.js";
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const HELP = `
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Usage: mock-openai [options]
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Options:
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-p, --port <number> Port to listen on (default: 4010)
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-h, --host <string> Host to bind to (default: 127.0.0.1)
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-f, --fixtures <path> Path to fixtures directory or file (default: ./fixtures)
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-l, --latency <ms> Latency in ms between SSE chunks (default: 0)
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-c, --chunk-size <chars> Chunk size in characters (default: 20)
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--help Show this help message
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`.trim();
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const { values } = parseArgs({
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options: {
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port: { type: "string", short: "p", default: "4010" },
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host: { type: "string", short: "h", default: "127.0.0.1" },
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fixtures: { type: "string", short: "f", default: "./fixtures" },
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latency: { type: "string", short: "l", default: "0" },
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"chunk-size": { type: "string", short: "c", default: "20" },
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help: { type: "boolean", default: false },
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},
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strict: true,
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});
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if (values.help) {
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console.log(HELP);
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process.exit(0);
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}
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const port = Number(values.port);
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const host = values.host!;
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const latency = Number(values.latency);
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const chunkSize = Number(values["chunk-size"]);
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const fixturePath = resolve(values.fixtures!);
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if (Number.isNaN(port) || port < 0 || port > 65535) {
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console.error(`Invalid port: ${values.port}`);
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process.exit(1);
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}
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if (Number.isNaN(latency) || latency < 0) {
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console.error(`Invalid latency: ${values.latency}`);
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process.exit(1);
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}
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if (Number.isNaN(chunkSize) || chunkSize < 1) {
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console.error(`Invalid chunk-size: ${values["chunk-size"]}`);
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process.exit(1);
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}
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async function main() {
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// Load fixtures from path (detect file vs directory)
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let fixtures;
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try {
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const stat = statSync(fixturePath);
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if (stat.isDirectory()) {
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fixtures = loadFixturesFromDir(fixturePath);
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} else {
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fixtures = loadFixtureFile(fixturePath);
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}
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} catch {
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console.error(`Fixtures path not found: ${fixturePath}`);
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process.exit(1);
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}
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console.log(`Loaded ${fixtures.length} fixture(s) from ${fixturePath}`);
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const instance = await createServer(fixtures, {
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port,
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host,
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latency,
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chunkSize,
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});
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console.log(`Mock OpenAI server listening on ${instance.url}`);
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function shutdown() {
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console.log("\nShutting down...");
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instance.server.close(() => {
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process.exit(0);
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});
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}
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process.on("SIGINT", shutdown);
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process.on("SIGTERM", shutdown);
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}
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main().catch((err) => {
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console.error(err);
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process.exit(1);
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});

‎src/fixture-loader.ts‎

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import { readFileSync, readdirSync, statSync } from "node:fs";
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import { join } from "node:path";
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import type { Fixture, FixtureFile, FixtureFileEntry } from "./types.js";
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function entryToFixture(entry: FixtureFileEntry): Fixture {
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return {
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match: {
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userMessage: entry.match.userMessage,
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toolCallId: entry.match.toolCallId,
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toolName: entry.match.toolName,
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model: entry.match.model,
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},
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response: entry.response,
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...(entry.latency !== undefined && { latency: entry.latency }),
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...(entry.chunkSize !== undefined && { chunkSize: entry.chunkSize }),
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};
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}
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export function loadFixtureFile(filePath: string): Fixture[] {
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let raw: string;
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try {
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raw = readFileSync(filePath, "utf-8");
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} catch (err) {
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console.warn(`[fixture-loader] Could not read file ${filePath}:`, err);
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return [];
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}
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let parsed: unknown;
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try {
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parsed = JSON.parse(raw);
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} catch (err) {
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console.warn(`[fixture-loader] Invalid JSON in ${filePath}:`, err);
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return [];
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}
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if (
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typeof parsed !== "object" ||
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parsed === null ||
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!Array.isArray((parsed as FixtureFile).fixtures)
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) {
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console.warn(`[fixture-loader] Missing or invalid "fixtures" array in ${filePath}`);
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return [];
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}
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return (parsed as FixtureFile).fixtures.map(entryToFixture);
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}
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export function loadFixturesFromDir(dirPath: string): Fixture[] {
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let entries: string[];
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try {
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entries = readdirSync(dirPath);
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} catch (err) {
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console.warn(`[fixture-loader] Could not read directory ${dirPath}:`, err);
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return [];
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}
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const jsonFiles: string[] = [];
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for (const name of entries) {
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const fullPath = join(dirPath, name);
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try {
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if (statSync(fullPath).isDirectory()) {
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console.warn(
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`[fixture-loader] Skipping subdirectory ${fullPath} (fixtures are not loaded recursively)`,
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);
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continue;
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}
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} catch (err) {
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const code = (err as NodeJS.ErrnoException).code;
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if (code !== "ENOENT") {
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console.warn(`[fixture-loader] Could not stat ${fullPath}:`, err);
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}
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continue;
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}
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if (name.endsWith(".json")) {
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jsonFiles.push(name);
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}
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}
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jsonFiles.sort();
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const fixtures: Fixture[] = [];
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for (const name of jsonFiles) {
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const filePath = join(dirPath, name);
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fixtures.push(...loadFixtureFile(filePath));
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}
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return fixtures;
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}

‎src/helpers.ts‎

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import { randomBytes } from "node:crypto";
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import type {
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FixtureResponse,
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TextResponse,
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ToolCallResponse,
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ErrorResponse,
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SSEChunk,
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ToolCall,
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} from "./types.js";
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export function generateId(prefix = "chatcmpl"): string {
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return `${prefix}-${randomBytes(12).toString("base64url")}`;
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}
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export function generateToolCallId(): string {
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return `call_${randomBytes(12).toString("base64url")}`;
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}
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export function isTextResponse(r: FixtureResponse): r is TextResponse {
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return "content" in r && typeof (r as TextResponse).content === "string";
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}
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export function isToolCallResponse(r: FixtureResponse): r is ToolCallResponse {
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return "toolCalls" in r && Array.isArray((r as ToolCallResponse).toolCalls);
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}
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export function isErrorResponse(r: FixtureResponse): r is ErrorResponse {
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return (
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"error" in r &&
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(r as ErrorResponse).error !== null &&
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typeof (r as ErrorResponse).error === "object"
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);
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}
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export function buildTextChunks(content: string, model: string, chunkSize: number): SSEChunk[] {
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const id = generateId();
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const created = Math.floor(Date.now() / 1000);
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const chunks: SSEChunk[] = [];
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// Role chunk
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [{ index: 0, delta: { role: "assistant", content: "" }, finish_reason: null }],
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});
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// Content chunks
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for (let i = 0; i < content.length; i += chunkSize) {
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const slice = content.slice(i, i + chunkSize);
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [{ index: 0, delta: { content: slice }, finish_reason: null }],
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});
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}
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// Finish chunk
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [{ index: 0, delta: {}, finish_reason: "stop" }],
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});
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return chunks;
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}
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export function buildToolCallChunks(
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toolCalls: ToolCall[],
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model: string,
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chunkSize: number,
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): SSEChunk[] {
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const id = generateId();
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const created = Math.floor(Date.now() / 1000);
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const chunks: SSEChunk[] = [];
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// Role chunk
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [{ index: 0, delta: { role: "assistant", content: null }, finish_reason: null }],
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});
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// Tool call chunks — one initial chunk per tool call, then argument chunks
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for (let tcIdx = 0; tcIdx < toolCalls.length; tcIdx++) {
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const tc = toolCalls[tcIdx];
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const tcId = tc.id || generateToolCallId();
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// Initial tool call chunk (id + function name)
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [
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{
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index: 0,
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delta: {
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tool_calls: [
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{
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index: tcIdx,
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id: tcId,
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type: "function",
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function: { name: tc.name, arguments: "" },
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},
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],
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},
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finish_reason: null,
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},
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],
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});
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// Argument streaming chunks
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const args = tc.arguments;
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for (let i = 0; i < args.length; i += chunkSize) {
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const slice = args.slice(i, i + chunkSize);
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [
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{
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index: 0,
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delta: {
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tool_calls: [{ index: tcIdx, function: { arguments: slice } }],
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},
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finish_reason: null,
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},
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],
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});
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}
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}
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// Finish chunk
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chunks.push({
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id,
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object: "chat.completion.chunk",
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created,
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model,
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choices: [{ index: 0, delta: {}, finish_reason: "tool_calls" }],
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});
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return chunks;
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}

‎src/index.ts‎

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// Main class
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export { MockOpenAI } from "./mock-openai.js";
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// Server
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export { createServer, type ServerInstance } from "./server.js";
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// Fixture loading
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export { loadFixtureFile, loadFixturesFromDir } from "./fixture-loader.js";
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// Journal
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export { Journal } from "./journal.js";
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// Router
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export { matchFixture } from "./router.js";
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// Helpers
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export { generateId, generateToolCallId, buildTextChunks, buildToolCallChunks } from "./helpers.js";
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// SSE
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export { writeSSEStream, writeErrorResponse } from "./sse-writer.js";
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// Types
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export type {
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ChatMessage,
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ChatCompletionRequest,
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ToolDefinition,
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FixtureMatch,
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TextResponse,
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ToolCall,
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ToolCallResponse,
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ErrorResponse,
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FixtureResponse,
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Fixture,
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FixtureFile,
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FixtureFileEntry,
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JournalEntry,
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SSEChunk,
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SSEChoice,
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SSEDelta,
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SSEToolCallDelta,
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MockServerOptions,
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ToolCallMessage,
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} from "./types.js";

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