From ccd810f84d74ce452d858b553eb0cbdb282d715c Mon Sep 17 00:00:00 2001
From: Tianyi Cui <53024+tianyicui@users.noreply.github.com>
Date: Tue, 14 Jul 2026 09:36:22 +0800
Subject: [PATCH] fix: normalize empty stdio resume identity

---
 packages/ui/stdio-agent/src/stdio-chat.ts        | 5 +++--
 packages/ui/stdio-agent/tests/stdio-chat.spec.ts | 9 +++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/packages/ui/stdio-agent/src/stdio-chat.ts b/packages/ui/stdio-agent/src/stdio-chat.ts
index 76347c91e4..17a11168db 100644
--- a/packages/ui/stdio-agent/src/stdio-chat.ts
+++ b/packages/ui/stdio-agent/src/stdio-chat.ts
@@ -104,9 +104,10 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
   // with durable parentSession lineage. Keeping the matching candidates also
   // covers HMR's publish-new-before-dispose-old ordering without ever falling
   // through to an unrelated root owned by another app or test fixture.
-  const matchesConfiguredIdentity = (agent: Agent): boolean => config.resumeSessionId === undefined
+  const resumeSessionId = config.resumeSessionId === '' ? undefined : config.resumeSessionId
+  const matchesConfiguredIdentity = (agent: Agent): boolean => resumeSessionId === undefined
     ? agent.id.startsWith('main-session-')
-    : agent.id === config.resumeSessionId
+    : agent.id === resumeSessionId
   const configuredRoots = new Set(ctx.agents.roots().filter(matchesConfiguredIdentity))
   let target: Agent | undefined = [...configuredRoots].at(-1)
   ctx.on('agent/created', (agent) => {
diff --git a/packages/ui/stdio-agent/tests/stdio-chat.spec.ts b/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
index 668068a72d..2ebc51e14e 100644
--- a/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
+++ b/packages/ui/stdio-agent/tests/stdio-chat.spec.ts
@@ -747,6 +747,15 @@ describe('createStdioChat input', () => {
     await new Promise(r => setImmediate(r))
     expect(agent.sent).toHaveLength(1)
   })
+
+  it('treats an empty resume session id as a fresh configured identity', async () => {
+    const { ctx, input } = await setup({ welcome: 'w', resumeSessionId: '' })
+    const agent = makeAgent('main-session-fresh')
+    ctx.agents.register(agent)
+    input.feed('hi')
+    await new Promise(r => setImmediate(r))
+    expect(agent.sent).toHaveLength(1)
+  })
 })
 
 describe('createStdioChat EOF exit', () => {