braindump/scripts/mock-openai.ts

/**
 * Deterministic mock of an OpenAI-compatible /v1/chat/completions endpoint that
 * speaks streaming SSE, for end-to-end testing the openai-compat backend without
 * a real model. tsx-runnable:
 *
 *   PORT=11500 tsx scripts/mock-openai.ts
 *
 * Scripting (decided per request from the message history it receives):
 *  - If the latest user message contains "ESCAPE", the first assistant turn
 *    emits a write_file tool_call with path "../escape.md" (vault-escape probe).
 *  - Otherwise the first assistant turn streams some text and a write_file
 *    tool_call for projects/mock/mock.md (arguments split across fragments, and
 *    id/name only on the first fragment, to exercise streamed-tool_call parsing).
 *  - Once the history's last message is a tool result, it streams a final text
 *    and finishes with finish_reason "stop".
 *
 * It logs a compact summary of every request's message roles to stderr so an
 * e2e harness can assert that a second turn replays prior history.
 */
import http from "node:http";

const PORT = Number(process.env.PORT ?? 11500);

interface InMsg {
  role: string;
  content: string | null;
  tool_calls?: unknown[];
  tool_call_id?: string;
}

function sseChunk(obj: unknown): string {
  return `data: ${JSON.stringify(obj)}\n\n`;
}

function baseDelta(delta: unknown, finish: string | null = null) {
  return {
    id: "chatcmpl-mock",
    object: "chat.completion.chunk",
    created: Math.floor(Date.now() / 1000),
    model: "mock-model",
    choices: [{ index: 0, delta, finish_reason: finish }],
  };
}

const server = http.createServer((req, res) => {
  if (req.method !== "POST" || !req.url?.includes("/chat/completions")) {
    res.writeHead(404).end("not found");
    return;
  }
  let body = "";
  req.on("data", (c) => (body += c));
  req.on("end", () => {
    let messages: InMsg[] = [];
    try {
      messages = (JSON.parse(body).messages ?? []) as InMsg[];
    } catch {
      /* ignore */
    }

    const roles = messages.map((m) => m.role).join(",");
    const userMsgs = messages.filter((m) => m.role === "user").length;
    process.stderr.write(`[mock] request roles=[${roles}] userMessages=${userMsgs}\n`);

    const last = messages[messages.length - 1];
    const lastUser = [...messages].reverse().find((m) => m.role === "user");
    const wantEscape = typeof lastUser?.content === "string" && lastUser.content.includes("ESCAPE");

    res.writeHead(200, {
      "Content-Type": "text/event-stream",
      "Cache-Control": "no-cache",
      Connection: "keep-alive",
    });

    if (last?.role === "tool") {
      // Second iteration: finish the turn.
      res.write(sseChunk(baseDelta({ role: "assistant", content: "" })));
      res.write(sseChunk(baseDelta({ content: "Done — I captured that into the vault. " })));
      res.write(sseChunk(baseDelta({ content: "What else should we add? (1) goals? (2) stack?" })));
      res.write(sseChunk(baseDelta({}, "stop")));
      res.write("data: [DONE]\n\n");
      res.end();
      return;
    }

    // First iteration: stream text + a write_file tool_call across fragments.
    const targetPath = wantEscape ? "../escape.md" : "projects/mock/mock.md";
    const fileContent = "---\ntags:\n  - mock\n  - mock/moc\n---\n\n# Mock\n\nCaptured by the mock backend.\n\n## TODO / Ideas\n\n<!-- TODO: confirm details -->\n";
    const argsFull = JSON.stringify({ path: targetPath, content: fileContent });
    const mid = Math.floor(argsFull.length / 2);

    res.write(sseChunk(baseDelta({ role: "assistant", content: "" })));
    res.write(sseChunk(baseDelta({ content: "Got it. Let me start the note. " })));
    // tool_call fragment 1: id + name + first half of arguments.
    res.write(
      sseChunk(
        baseDelta({
          tool_calls: [
            { index: 0, id: "call_mock_1", type: "function", function: { name: "write_file", arguments: argsFull.slice(0, mid) } },
          ],
        })
      )
    );
    // tool_call fragment 2: remaining arguments only (no id/name).
    res.write(
      sseChunk(
        baseDelta({
          tool_calls: [{ index: 0, function: { arguments: argsFull.slice(mid) } }],
        })
      )
    );
    res.write(sseChunk(baseDelta({}, "tool_calls")));
    res.write("data: [DONE]\n\n");
    res.end();
  });
});

server.listen(PORT, () => {
  process.stderr.write(`[mock] listening on http://localhost:${PORT}\n`);
});