fix(custom-model): keep the launch model out of the stored config

Quick-start wrote a selection's launchModel into the session's stored
<Mode>Config, which is what a clear falls back to, so a clear relaunched
the CLI with the endpoint's model id on its native backend. Only API
callers can reach that, since no page sends a clear.

The first launch now takes the forced model from the session's selection,
through the same _withCustomModelLaunchModel() the respawn path uses, so
the stored config keeps the model chosen at create. That also carries the
model onto a new tmux session for a selection applied to a running
session, which the create path used to lose.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/routes/quick-start-custom-model.test.ts

@@ -204,6 +204,34 @@ describe('POST /api/quick-start: customModel (one-shot custom-model launch)', ()
     expect(restartSpy).not.toHaveBeenCalled();
   });
 
+  it("leaves the caller's own model in the stored config, which is what a later clear falls back to", async () => {
+    const res = await quickStart({
+      caseName: 'cm-pi-own-model',
+      mode: 'pi',
+      piConfig: { model: 'anthropic/claude-x' },
+      customModel: { endpointId: 'ep1', modelId: 'qwen3' },
+    });
+
+    expect(res.statusCode).toBe(200);
+    const { sessionId } = res.json();
+    const session = ctx.sessions.get(sessionId) as unknown as Session;
+    expect(session.getCustomModelForPersist()?.launchModel).toBe('custom/qwen3');
+    expect(session.toState().piConfig?.model).toBe('anthropic/claude-x');
+  });
+
+  it('stores no model at all for a caller that chose none', async () => {
+    const res = await quickStart({
+      caseName: 'cm-codex-no-model',
+      mode: 'codex',
+      customModel: { endpointId: 'ep1', modelId: 'qwen3' },
+    });
+
+    expect(res.statusCode).toBe(200);
+    const session = ctx.sessions.get(res.json().sessionId) as unknown as Session;
+    expect(session.getCustomModelForPersist()?.launchModel).toBe('qwen3');
+    expect(session.toState().codexConfig?.model).toBeUndefined();
+  });
+
   it('404s for an unknown endpoint id', async () => {
     const res = await quickStart({
       caseName: 'cm-ghost',

test/session-custom-model-restart.test.ts

@@ -195,6 +195,53 @@ describe('restartCli() must not kill a working pane', () => {
 });
 
 describe('launchModel reaches the CLI through its own launch param', () => {
+  /** A session the route just built: no pane yet, so its launch is `createSession`. */
+  function freshSession(mode: SessionMode, extra: Record<string, unknown> = {}) {
+    mkdirSync(workingDir, { recursive: true });
+    const mux = new TmuxManager();
+    const create = vi.spyOn(mux, 'createSession');
+    vi.spyOn(mux, 'muxSessionExists').mockReturnValue(true);
+    const respawn = vi.spyOn(mux, 'respawnPane').mockResolvedValue(4242);
+    const session = new Session({ workingDir, mode, mux, useMux: true, ...extra });
+    sessions.push(session);
+    return { session, create, respawn };
+  }
+
+  it('the first launch carries it too, and a clear relaunches on the model the session was created with', async () => {
+    const { session, create, respawn } = freshSession('pi', { piConfig: { model: 'anthropic/claude-x' } });
+    session.setCustomModel(
+      { endpointId: 'ep', modelId: 'qwen3', envKeys: ['HOME'], configDir: '/tmp/cfg', launchModel: 'custom/qwen3' },
+      { HOME: '/tmp/cfg' }
+    );
+    await session.startInteractive();
+    expect(create.mock.calls[0][0].piConfig?.model).toBe('custom/qwen3');
+    expect(session.toState().piConfig?.model).toBe('anthropic/claude-x');
+
+    session.setCustomModel(undefined);
+    expect(await session.restartCli()).toBe(true);
+    expect(respawn.mock.calls[0][0].piConfig?.model).toBe('anthropic/claude-x');
+  });
+
+  it('a clear on a session created with no model relaunches with none', async () => {
+    const { session, create, respawn } = freshSession('codex');
+    session.setCustomModel(
+      { endpointId: 'ep', modelId: 'qwen3', envKeys: ['CODEX_HOME'], configDir: '/tmp/cfg', launchModel: 'qwen3' },
+      { CODEX_HOME: '/tmp/cfg' }
+    );
+    await session.startInteractive();
+    expect(create.mock.calls[0][0].codexConfig?.model).toBe('qwen3');
+
+    session.setCustomModel(undefined);
+    expect(await session.restartCli()).toBe(true);
+    expect(respawn.mock.calls[0][0].codexConfig?.model).toBeUndefined();
+  });
+
+  it('leaves the launch untouched without a selection', async () => {
+    const { session, create } = freshSession('pi', { piConfig: { model: 'anthropic/claude-x', thinking: 'low' } });
+    await session.startInteractive();
+    expect(create.mock.calls[0][0].piConfig).toEqual({ model: 'anthropic/claude-x', thinking: 'low' });
+  });
+
   it('pi: the selection forces piConfig.model on the respawn options, leaving the stored config alone', async () => {
     const { session, respawn } = liveSession('pi', { piConfig: { model: 'anthropic/claude-x', thinking: 'low' } });
     session.setCustomModel(