Fix issue where subagent thoughts are appended. (#22975)

packages/core/src/agents/local-invocation.test.ts

@@ -230,7 +230,7 @@ describe('LocalSubagentInvocation', () => {
       expect(display.terminateReason).toBe(AgentTerminateMode.TIMEOUT);
     });
 
-    it('should stream THOUGHT_CHUNK activities from the executor', async () => {
+    it('should stream THOUGHT_CHUNK activities from the executor, replacing the last running thought', async () => {
       mockExecutorInstance.run.mockImplementation(async () => {
         const onActivity = MockLocalAgentExecutor.create.mock.calls[0][2];
 
@@ -245,7 +245,7 @@ describe('LocalSubagentInvocation', () => {
             isSubagentActivityEvent: true,
             agentName: 'MockAgent',
             type: 'THOUGHT_CHUNK',
-            data: { text: ' Still thinking.' },
+            data: { text: 'Thinking about next steps.' },
           } as SubagentActivityEvent);
         }
         return { result: 'Done', terminate_reason: AgentTerminateMode.GOAL };
@@ -258,7 +258,13 @@ describe('LocalSubagentInvocation', () => {
       expect(lastCall.recentActivity).toContainEqual(
         expect.objectContaining({
           type: 'thought',
-          content: 'Analyzing... Still thinking.',
+          content: 'Thinking about next steps.',
+        }),
+      );
+      expect(lastCall.recentActivity).not.toContainEqual(
+        expect.objectContaining({
+          type: 'thought',
+          content: 'Analyzing...',
         }),
       );
     });

packages/core/src/agents/local-invocation.ts

@@ -120,7 +120,7 @@ export class LocalSubagentInvocation extends BaseToolInvocation<
               lastItem.type === 'thought' &&
               lastItem.status === 'running'
             ) {
-              lastItem.content += text;
+              lastItem.content = text;
             } else {
               recentActivity.push({
                 id: randomUUID(),