diff --git a/packages/ui/stdio-agent/src/stdio-chat.ts b/packages/ui/stdio-agent/src/stdio-chat.ts
index bfb58e3a57..2a4d620494 100644
--- a/packages/ui/stdio-agent/src/stdio-chat.ts
+++ b/packages/ui/stdio-agent/src/stdio-chat.ts
@@ -95,20 +95,18 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
   const welcome = config.welcome ?? 'ready.'
   const { input, output, exit } = runtime
 
-  // This app owns one root agent. Hold the live object directly: its per-run id
-  // is intentionally fresh, while `main` remains only the terminal's fixed
-  // display label. HMR may publish the replacement before old teardown emits
-  // disposed, so a target disposal reselects the surviving root from the live
-  // 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
-  })
+  // This app owns one configured agent. Hold the live object directly: its
+  // per-run id is intentionally fresh, while `main` remains only the
+  // terminal's fixed display label. At install the configured agent is the
+  // earliest registry entry (it creates any subagents later). During HMR the
+  // replacement is published after the old tree, so when old teardown finally
+  // emits disposed, the newest survivor is the replacement. Persisted
+  // parentSession lineage is deliberately irrelevant: a resumed child session
+  // can itself be this process's configured top-level agent.
+  let target: Agent | undefined = ctx.agents.list()[0]
+  ctx.on('agent/created', (agent) => { target ??= agent })
   ctx.on('agent/disposed', (agent) => {
-    if (target === agent) target = rootAgent()
+    if (target === agent) target = ctx.agents.list().at(-1)
   })
 
   // Transcript rendering off the durable `session/event` feed — the assistant
diff --git a/packages/ui/stdio-agent/tests/stdio-chat.spec.ts b/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
index 2b1df53921..2654a05680 100644
--- a/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
+++ b/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
@@ -185,6 +185,9 @@ describe('createStdioChat rendering', () => {
     await ctx.plugin(AgentRegistry)
     await ctx.plugin(UserInteractionService)
     const agent = makeAgent('main')
+    // Durable lineage does not imply runtime child ownership: the stdio app
+    // may explicitly resume a persisted fork as its one configured agent.
+    ;(agent.session.header as { parentSession?: string }).parentSession = 'persisted-parent'
     ctx.agents.register(agent) // registered BEFORE the UI plugin below
     const { runtime, out } = makeRuntime()
     await ctx.plugin(Object.assign((inner: Context) => {
@@ -196,6 +199,18 @@ describe('createStdioChat rendering', () => {
     expect(out.text()).toContain('[main turn 5] ')
   })
 
+  it('accepts a lineage-bearing configured agent created after the UI installs', async () => {
+    const { ctx, input } = await setup()
+    const resumed = makeAgent('resumed')
+    ;(resumed.session.header as { parentSession?: string }).parentSession = 'persisted-parent'
+    ctx.emit('agent/created', resumed)
+
+    input.feed('continue')
+    await new Promise(resolve => setImmediate(resolve))
+
+    expect(resumed.sent).toEqual([[{ type: 'text', text: 'continue' }]])
+  })
+
   it('resets dim styling at turn/end if a turn ends mid-reasoning', async () => {
     const { ctx, out } = await setup()
     const session = makeSession('main')
@@ -242,7 +257,7 @@ describe('createStdioChat rendering', () => {
 
     // 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.
+    // most recently published survivor is the HMR replacement.
     disposeOld()
     input.feed('after hmr')
     await new Promise(resolve => setImmediate(resolve))