fix: retain stdio target across agent replacement

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

@@ -95,13 +95,20 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
   const welcome = config.welcome ?? 'ready.'
   const { input, output, exit } = runtime
 
-  // This app owns exactly one pre-created agent. Hold the live object directly:
+  // This app owns one root agent. Hold the live object directly: its per-run id
-  // its per-run id is intentionally fresh, while `main` remains only the
+  // is intentionally fresh, while `main` remains only the terminal's fixed
-  // terminal's fixed display label.
+  // display label. HMR may publish the replacement before old teardown emits
-  let target: Agent | undefined = ctx.agents.list()[0]
+  // disposed, so a target disposal reselects the surviving root from the live
-  ctx.on('agent/created', (agent) => { target ??= agent })
+  // registry instead of leaving the terminal detached. Fork children carry
+  // parentSession lineage and must never become the terminal target.
+  const rootAgent = (): Agent | undefined =>
+    ctx.agents.list().find(agent => agent.session.header.parentSession === undefined)
+  let target: Agent | undefined = rootAgent()
+  ctx.on('agent/created', (agent) => {
+    if (target === undefined && agent.session.header.parentSession === undefined) target = agent
+  })
   ctx.on('agent/disposed', (agent) => {
-    if (target === agent) target = undefined
+    if (target === agent) target = rootAgent()
   })
 
   // Transcript rendering off the durable `session/event` feed — the assistant

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

@@ -230,6 +230,27 @@ describe('createStdioChat rendering', () => {
     expect(out.text()).toContain('[main turn 1] ')
   })
 
+  it('retargets a surviving root when HMR publishes it before disposing the old root', async () => {
+    const { ctx, input } = await setup()
+    const oldRoot = makeAgent('old-root')
+    const child = makeAgent('child')
+    ;(child.session.header as { parentSession?: string }).parentSession = oldRoot.id
+    const replacement = makeAgent('replacement')
+    const disposeOld = ctx.agents.register(oldRoot)
+    ctx.agents.register(child)
+    ctx.agents.register(replacement)
+
+    // The replacement's created edge arrived while oldRoot was still targeted.
+    // Once oldRoot is removed, registry order is child then replacement; the
+    // UI must skip the surviving child and route input to the replacement root.
+    disposeOld()
+    input.feed('after hmr')
+    await new Promise(resolve => setImmediate(resolve))
+
+    expect(child.sent).toEqual([])
+    expect(replacement.sent).toEqual([[{ type: 'text', text: 'after hmr' }]])
+  })
+
   it('renders tool/call and tool/result session events', async () => {
     const { ctx, out } = await setup()
     const session = {} as Session