fix(core): prevent unhandled AbortError crash during stream loop detection (#21123)

Co-authored-by: Gaurav <39389231+gsquared94@users.noreply.github.com>
Co-authored-by: ruomeng <ruomeng@google.com>

packages/core/src/core/client.test.ts

@@ -28,6 +28,7 @@ import {
   GeminiEventType,
   Turn,
   type ChatCompressionInfo,
+  type ServerGeminiStreamEvent,
 } from './turn.js';
 import { getCoreSystemPrompt } from './prompts.js';
 import { DEFAULT_GEMINI_MODEL_AUTO } from '../config/models.js';
@@ -1118,6 +1119,54 @@ ${JSON.stringify(
       // The actual token calculation is unit tested in tokenCalculation.test.ts
     });
 
+    it('should cleanly abort and return Turn on LoopDetected without unhandled promise rejections', async () => {
+      // Arrange
+      const mockStream = (async function* () {
+        // Yield an event that will trigger the loop detector
+        yield { type: 'content', value: 'Looping content' };
+      })();
+      mockTurnRunFn.mockReturnValue(mockStream);
+
+      const mockChat: Partial<GeminiChat> = {
+        addHistory: vi.fn(),
+        setTools: vi.fn(),
+        getHistory: vi.fn().mockReturnValue([]),
+        getLastPromptTokenCount: vi.fn(),
+      };
+      client['chat'] = mockChat as GeminiChat;
+
+      // Mock loop detector to return count > 1 on the first event (loop detected)
+      vi.spyOn(client['loopDetector'], 'addAndCheck').mockReturnValue({
+        count: 2,
+      });
+
+      const abortSpy = vi.spyOn(AbortController.prototype, 'abort');
+
+      // Act
+      const stream = client.sendMessageStream(
+        [{ text: 'Hi' }],
+        new AbortController().signal,
+        'prompt-id-1',
+      );
+
+      const events: ServerGeminiStreamEvent[] = [];
+      let finalResult: Turn | undefined;
+
+      while (true) {
+        const result = await stream.next();
+        if (result.done) {
+          finalResult = result.value;
+          break;
+        }
+        events.push(result.value);
+      }
+
+      // Assert
+      expect(events).toContainEqual({ type: GeminiEventType.LoopDetected });
+      expect(abortSpy).toHaveBeenCalled();
+      expect(finalResult).toBeInstanceOf(Turn);
+    });
+
     it('should return the turn instance after the stream is complete', async () => {
       // Arrange
       const mockStream = (async function* () {

packages/core/src/core/client.ts

@@ -708,27 +708,22 @@ export class GeminiClient {
     let isError = false;
     let isInvalidStream = false;
 
+    let loopDetectedAbort = false;
+    let loopRecoverResult: { detail?: string } | undefined;
     for await (const event of resultStream) {
       const loopResult = this.loopDetector.addAndCheck(event);
       if (loopResult.count > 1) {
         yield { type: GeminiEventType.LoopDetected };
-        controller.abort();
+        loopDetectedAbort = true;
-        return turn;
+        break;
       } else if (loopResult.count === 1) {
         if (boundedTurns <= 1) {
           yield { type: GeminiEventType.MaxSessionTurns };
-          controller.abort();
+          loopDetectedAbort = true;
-          return turn;
+          break;
         }
-        return yield* this._recoverFromLoop(
+        loopRecoverResult = loopResult;
-          loopResult,
+        break;
-          signal,
-          prompt_id,
-          boundedTurns,
-          isInvalidStreamRetry,
-          displayContent,
-          controller,
-        );
       }
       yield event;
 
@@ -742,6 +737,23 @@ export class GeminiClient {
       }
     }
 
+    if (loopDetectedAbort) {
+      controller.abort();
+      return turn;
+    }
+
+    if (loopRecoverResult) {
+      return yield* this._recoverFromLoop(
+        loopRecoverResult,
+        signal,
+        prompt_id,
+        boundedTurns,
+        isInvalidStreamRetry,
+        displayContent,
+        controller,
+      );
+    }
+
     if (isError) {
       return turn;
     }