fix(cli): don't brick the chat REPL when the first turn fails upstream

Observed live: turn 1 died with an anthropic 429 before the stream's `final`
frame, streamOnce resolved '' as the thread id, the REPL stored it, and every
later message sent `threadId: ""` — rejected by mcpd's z.string().min(1) with
HTTP 400. Permanently stuck: no turn could succeed again, so no `final` frame
could ever repair the id.

Two independent layers:
- streamOnce now resolves `string | undefined` — undefined when no `final`
  frame arrived — and the REPL keeps its previous thread state on undefined
  instead of overwriting it. One-shot mode skips the `(thread: ...)` footer
  when there is none to report.
- chatBody refuses to serialize an empty threadId at all, so even a leaked ''
  can never reach the wire.

Regression cover in chat-thread-brick.test.ts (7 tests), including the full
REPL chain: failed turn 1 → turn 2 body carries no threadId key. The
assertions are the direct inverse of the old behavior, so they fail pre-fix
by construction.

CLI suite 726 passed, lint clean, tsc clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dNpnBqyyz9GxfznVcX2sP

src/cli/tests/commands/chat-thread-brick.test.ts

@@ -0,0 +1,116 @@
+/**
+ * Regression: a failed first turn must not brick the REPL.
+ *
+ * Observed live: the first message died upstream (anthropic 429) before the
+ * stream's `final` frame, streamOnce resolved '' as the thread id, the REPL
+ * stored it, and every later message sent `threadId: ""` — which mcpd's
+ * `z.string().min(1)` rejects with HTTP 400. The session was permanently
+ * stuck: no turn could succeed again, so no `final` frame could ever repair
+ * the thread id.
+ *
+ * The fix has two independent layers, pinned separately below:
+ *   1. streamOnce resolves `undefined` (not '') when no `final` frame arrived,
+ *      and the REPL keeps its previous thread state on undefined;
+ *   2. chatBody never serializes an empty threadId, even if one leaks in.
+ */
+import http from 'node:http';
+import type { AddressInfo } from 'node:net';
+
+import { describe, it, expect, afterEach } from 'vitest';
+
+import { chatBody, streamOnce } from '../../src/commands/chat.js';
+import type { ChatCommandDeps, ChatSubject } from '../../src/commands/chat.js';
+import type { ApiClient } from '../../src/api-client.js';
+
+const subject: ChatSubject = {
+  kind: 'agent',
+  name: 'reviewer',
+  path: 'agents/reviewer',
+  allowSecrets: false,
+};
+
+// streamOnce only touches baseUrl + token; the ApiClient is for the
+// non-streaming path and never dereferenced here.
+function depsFor(baseUrl: string): ChatCommandDeps {
+  return { client: null as unknown as ApiClient, baseUrl, log: () => {} };
+}
+
+let server: http.Server | null = null;
+
+afterEach(async () => {
+  if (server !== null) {
+    await new Promise<void>((r) => server!.close(() => r()));
+    server = null;
+  }
+});
+
+/** Serve one SSE response body for any POST, return the base URL. */
+async function serveSse(frames: string[]): Promise<string> {
+  server = http.createServer((_req, res) => {
+    res.writeHead(200, { 'Content-Type': 'text/event-stream' });
+    for (const f of frames) res.write(`data: ${f}\n\n`);
+    res.end();
+  });
+  await new Promise<void>((r) => server!.listen(0, '127.0.0.1', r));
+  const { port } = server.address() as AddressInfo;
+  return `http://127.0.0.1:${String(port)}`;
+}
+
+describe('chatBody — threadId serialization', () => {
+  it('omits threadId when undefined', () => {
+    expect(chatBody(subject, 'hi', undefined, {})).not.toHaveProperty('threadId');
+  });
+
+  it('omits threadId when empty — the exact payload that 400s against mcpd', () => {
+    expect(chatBody(subject, 'hi', '', {})).not.toHaveProperty('threadId');
+  });
+
+  it('includes a real threadId', () => {
+    expect(chatBody(subject, 'hi', 'cthread123', {})).toHaveProperty('threadId', 'cthread123');
+  });
+});
+
+describe('streamOnce — thread id after a failed turn', () => {
+  it('resolves undefined when the stream errors before any final frame', async () => {
+    const base = await serveSse([
+      '{"type":"error","message":"anthropic stream: HTTP 429"}',
+      '[DONE]',
+    ]);
+    const resolved = await streamOnce(depsFor(base), subject, 'hi', undefined, {});
+    expect(resolved).toBeUndefined();
+  });
+
+  it('keeps the caller-supplied thread when the turn fails mid-conversation', async () => {
+    const base = await serveSse([
+      '{"type":"error","message":"upstream died"}',
+      '[DONE]',
+    ]);
+    const resolved = await streamOnce(depsFor(base), subject, 'hi', 'cexisting1', {});
+    expect(resolved).toBe('cexisting1');
+  });
+
+  it('resolves the threadId announced by the final frame', async () => {
+    const base = await serveSse([
+      '{"type":"text","delta":"pong"}',
+      '{"type":"final","threadId":"cfresh42"}',
+      '[DONE]',
+    ]);
+    const resolved = await streamOnce(depsFor(base), subject, 'hi', undefined, {});
+    expect(resolved).toBe('cfresh42');
+  });
+
+  it('REPL chain: failed turn 1 leaves turn 2 sendable (the brick)', async () => {
+    const base = await serveSse([
+      '{"type":"error","message":"anthropic stream: HTTP 429"}',
+      '[DONE]',
+    ]);
+    // Mirrors runRepl's assignment: threadId = streamOnce(...) ?? threadId
+    let threadId: string | undefined = undefined;
+    threadId = (await streamOnce(depsFor(base), subject, 'hi', threadId, {})) ?? threadId;
+
+    // Turn 2's body must be valid for mcpd: no threadId key at all.
+    const body = chatBody(subject, 'hi again', threadId, {}, true);
+    expect(body).not.toHaveProperty('threadId');
+    expect(body).toHaveProperty('message', 'hi again');
+  });
+});