fix(mcp): start_session returns the worker's id when only the readiness call fails (#615 merge-time fix)

After quick-start succeeded, a failed wait-output readiness call (a 429 from
the waiter cap, a network blip) became an isError result without the session
id, so the model could neither prompt nor delete the worker it had just
started, and a retry started a second one. It now returns the id with
ready: null and a note, as `codeman agent spawn` does.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mcp-server.ts

@@ -624,13 +624,28 @@ export function createToolRunner(deps: ToolRunnerDeps) {
       let note = 'Ready for send_prompt.';
       const mark = composerReadyMark(mode);
       if (waitReady && mark) {
-        const { wait } = await waitCall(
-          data.sessionId,
-          { match: mark, from: 'buffer' },
-          'wait-output',
-          timeoutMs,
-          signal
-        );
+        let wait: WaitResult | undefined;
+        try {
+          ({ wait } = await waitCall(
+            data.sessionId,
+            { match: mark, from: 'buffer' },
+            'wait-output',
+            timeoutMs,
+            signal
+          ));
+        } catch (err) {
+          // The worker is already running: a failed readiness CALL (429, a network blip)
+          // must still hand the model its id, or it cannot prompt or delete it.
+          if (!(err instanceof ToolError)) throw err;
+          return structuredResult({
+            session_id: data.sessionId,
+            mode,
+            case: data.caseName ?? caseName,
+            case_path: data.casePath ?? '',
+            ready: null,
+            note: `Started, but the readiness check failed (${err.message}): read_output with source terminal before send_prompt.`,
+          });
+        }
         const outcome = waitOutcome(wait);
         if (outcome === 'exited' || outcome === 'ended') {
           ready = false;

test/mcp-server.test.ts

@@ -366,6 +366,20 @@ describe('the tools', () => {
     expect(result.structured).toMatchObject({ session_id: MINE, ready: true });
   });
 
+  it('start_session still returns the session id when only the readiness call fails', async () => {
+    const busy = fakeCodeman(undefined, {
+      'GET /api/v1/sessions/*': () => ({
+        status: 429,
+        json: { success: false, error: 'too many waiters', errorCode: 'WAIT_CAPACITY' },
+        text: '',
+      }),
+    });
+    const result = await runner(config(), busy).tools.run('start_session', { case: 'review-2', mode: 'claude' });
+    expect(result.isError).toBeFalsy();
+    expect(result.structured).toMatchObject({ session_id: MINE, ready: null });
+    expect(String(result.structured?.note)).toContain('readiness check failed');
+  });
+
   it('send_prompt refuses a newline or control byte before any request', async () => {
     const { tools, calls } = runner(config());
     for (const text of ['line one\nline two', 'esc \u001b[A', 'tab\there']) {