diff --git a/src/services/acp/__tests__/agent.test.ts b/src/services/acp/__tests__/agent.test.ts
index 3aee6c52a..be1e87b5f 100644
--- a/src/services/acp/__tests__/agent.test.ts
+++ b/src/services/acp/__tests__/agent.test.ts
@@ -1226,19 +1226,20 @@ describe('AcpAgent', () => {
       )
     })
 
-    test('prompt does not trigger additional switchSession for multi-session', async () => {
+    test('prompt switches global sessionId to the correct session', async () => {
       const agent = new AcpAgent(makeConn())
       await agent.newSession({ cwd: '/tmp' } as any)
       await agent.newSession({ cwd: '/tmp' } as any)
       mockSwitchSession.mockClear()
 
-      // Prompts should not call switchSession — alignment happens at session creation
+      // Prompts must switch global state so recordTranscript writes to
+      // the correct session file in multi-session scenarios.
       const s1 = agent.sessions.keys().next().value
       await agent.prompt({
         sessionId: s1,
         prompt: [{ type: 'text', text: 'hello' }],
       } as any)
-      expect(mockSwitchSession).not.toHaveBeenCalled()
+      expect(mockSwitchSession).toHaveBeenCalledWith(s1, null)
     })
   })
 })
diff --git a/src/services/acp/agent.ts b/src/services/acp/agent.ts
index 617879103..8868b7367 100644
--- a/src/services/acp/agent.ts
+++ b/src/services/acp/agent.ts
@@ -300,6 +300,10 @@ export class AcpAgent implements Agent {
       // After a previous interrupt(), the internal controller is stuck in
       // aborted state — without this, submitMessage() fails immediately.
       session.queryEngine.resetAbortController()
+      // Switch global session state so recordTranscript writes to the correct
+      // session file. Without this, multi-session scenarios (or creating a new
+      // session after another) write transcript data to the wrong file.
+      switchSession(params.sessionId as SessionId, getSessionProjectDir())
 
       const sdkMessages = session.queryEngine.submitMessage(promptInput)