fix(stdio): /mode is reserved even while a question prompt is active

Review finding at the seam of two surfaces this branch added to the
same stdin: with an ask_user_question (or plan-review) prompt active,
the line handler dispatched every line as the answer first, so
'/mode plan' typed mid-question was recorded as free-text feedback —
model-visible in the tool result — and the mode never changed. Command
handling now runs before answer dispatch: the command executes, the
question stays pending and still owns the next non-command line. A
literal '/mode…' free-text answer is the trade-off deliberately spent —
a swallowed command that becomes review feedback costs far more than
that contrived answer shape.

docs/rfc/implemented/feature/2026-07-07-plan-mode.md

@@ -50,7 +50,7 @@ Mode definitions are validated plugin Config — per repo convention, changeable
 
 ### In the terminal
 
-The stdio app gains `/mode` (print the current and available modes) and `/mode <name>` (switch + banner — a command line, never sent to the model). The exit review prompts right in the terminal with no new machinery: it is an ordinary user-interaction question, so it rides the stdio provider's one-prompt-owns-stdin queue that `ask_user_question` already uses.
+The stdio app gains `/mode` (print the current and available modes) and `/mode <name>` (switch + banner — a command line, never sent to the model, and reserved even while a question prompt is active: a command is never recorded as an answer). The exit review prompts right in the terminal with no new machinery: it is an ordinary user-interaction question, so it rides the stdio provider's one-prompt-owns-stdin queue that `ask_user_question` already uses.
 
 ### Over ACP
 

packages/ui/stdio-agent/src/stdio-chat.ts

@@ -349,21 +349,21 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
     })
 
     reader.on('line', (line) => {
-      if (activeQuestion !== undefined) {
-        answerQuestion(line)
-        return
-      }
       const text = line.trim()
-      if (!text) return
-      const agent = ctx.agents.get(agentId)
-      if (!agent) {
-        ctx.logger.error('ui-stdio: agent "%s" is not running', agentId)
-        return
-      }
       if (text === '/mode' || text.startsWith('/mode ')) {
-        // A command line, never sent to the model: print or switch the session
+        // A command line, never sent to the model — and reserved even while a
-        // mode. The switch is a pending intent the mode service flushes at the
+        // question prompt is active: a command swallowed as a free-text answer
-        // next turn boundary (dsh-mode's turn-enclosure contract).
+        // would land in the tool result as model-visible feedback (the plan
+        // review is exactly such a prompt), so command handling runs before
+        // answer dispatch. The switch is a pending intent the mode service
+        // flushes at the next turn boundary (dsh-mode's turn-enclosure
+        // contract); an active question stays pending and still owns the
+        // next non-command line.
+        const agent = ctx.agents.get(agentId)
+        if (!agent) {
+          ctx.logger.error('ui-stdio: agent "%s" is not running', agentId)
+          return
+        }
         const modes = ctx.get('modes')
         if (modes === undefined) {
           output.write('session modes are not composed in this deployment\n> ')
@@ -385,6 +385,16 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
         }
         return
       }
+      if (activeQuestion !== undefined) {
+        answerQuestion(line)
+        return
+      }
+      if (!text) return
+      const agent = ctx.agents.get(agentId)
+      if (!agent) {
+        ctx.logger.error('ui-stdio: agent "%s" is not running', agentId)
+        return
+      }
       submittedWork = true
       if (agent.status === 'running') {
         agent.steer([{ type: 'text', text }])

packages/ui/stdio-agent/tests/stdio-chat.spec.ts

@@ -871,3 +871,49 @@ describe('createStdioChat /mode command', () => {
     expect(ctx.modes.get(agent)).toEqual({ current: 'default' })
   })
 })
+
+describe('createStdioChat /mode during an active question', () => {
+  it('reserves /mode while a question is pending — the command is never recorded as the answer', async () => {
+    const bundle = await setup()
+    await bundle.ctx.plugin(SystemPrompt)
+    await bundle.ctx.plugin(ToolRegistry)
+    await bundle.ctx.plugin(ModesService)
+    const agent = {
+      id: 'main' as Agent['id'],
+      status: 'idle',
+      options: {},
+      session: new RealSession(SessionId('main-session')),
+      send: () => {},
+      steer: () => {},
+    } as never as Agent
+    bundle.ctx.agents.register(agent)
+
+    const answer = bundle.ctx.userInteraction.ask({
+      questions: [{
+        id: 'plan-review',
+        header: 'Plan review',
+        question: 'Approve this plan and leave plan mode?',
+        options: [{ label: 'Approve' }, { label: 'Keep planning' }],
+      }],
+    })
+    await new Promise(r => setImmediate(r))
+
+    // The command runs as a command: the mode switches, the question stays
+    // pending (it still owns the next non-command line).
+    bundle.input.feed('/mode plan')
+    await new Promise(r => setImmediate(r))
+    expect(bundle.out.text()).toContain('mode → plan (applies from the next turn)')
+    expect(bundle.ctx.modes.get(agent)).toEqual({ current: 'default', pending: PLAN_MODE })
+
+    bundle.input.feed('1')
+    await expect(answer).resolves.toEqual({ answers: [{ id: 'plan-review', selected: ['Approve'] }] })
+  })
+
+  it('logs and drops /mode when the target agent is not running', async () => {
+    const { ctx, input } = await setup()
+    const spy = vi.spyOn(ctx.logger, 'error').mockImplementation(() => {})
+    input.feed('/mode plan')
+    await new Promise(r => setImmediate(r))
+    expect(spy).toHaveBeenCalledWith('ui-stdio: agent "%s" is not running', 'main')
+  })
+})