Introduce loop detection service that breaks simple loop (#3919)

Co-authored-by: Scott Densmore <scottdensmore@mac.com>
Co-authored-by: N. Taylor Mullen <ntaylormullen@google.com>

packages/cli/src/ui/hooks/useGeminiStream.ts

@@ -141,6 +141,8 @@ export const useGeminiStream = (
     [toolCalls],
   );
 
+  const loopDetectedRef = useRef(false);
+
   const onExec = useCallback(async (done: Promise<void>) => {
     setIsResponding(true);
     await done;
@@ -450,6 +452,16 @@ export const useGeminiStream = (
     [addItem, config],
   );
 
+  const handleLoopDetectedEvent = useCallback(() => {
+    addItem(
+      {
+        type: 'info',
+        text: `A potential loop was detected. This can happen due to repetitive tool calls or other model behavior. The request has been halted.`,
+      },
+      Date.now(),
+    );
+  }, [addItem]);
+
   const processGeminiStreamEvents = useCallback(
     async (
       stream: AsyncIterable<GeminiEvent>,
@@ -489,6 +501,11 @@ export const useGeminiStream = (
           case ServerGeminiEventType.MaxSessionTurns:
             handleMaxSessionTurnsEvent();
             break;
+          case ServerGeminiEventType.LoopDetected:
+            // handle later because we want to move pending history to history
+            // before we add loop detected message to history
+            loopDetectedRef.current = true;
+            break;
           default: {
             // enforces exhaustive switch-case
             const unreachable: never = event;
@@ -579,6 +596,10 @@ export const useGeminiStream = (
           addItem(pendingHistoryItemRef.current, userMessageTimestamp);
           setPendingHistoryItem(null);
         }
+        if (loopDetectedRef.current) {
+          loopDetectedRef.current = false;
+          handleLoopDetectedEvent();
+        }
       } catch (error: unknown) {
         if (error instanceof UnauthorizedError) {
           onAuthError();
@@ -616,6 +637,7 @@ export const useGeminiStream = (
       config,
       startNewPrompt,
       getPromptCount,
+      handleLoopDetectedEvent,
     ],
   );
 

packages/core/src/core/client.ts

@@ -40,6 +40,7 @@ import {
 } from './contentGenerator.js';
 import { ProxyAgent, setGlobalDispatcher } from 'undici';
 import { DEFAULT_GEMINI_FLASH_MODEL } from '../config/models.js';
+import { LoopDetectionService } from '../services/loopDetectionService.js';
 
 function isThinkingSupported(model: string) {
   if (model.startsWith('gemini-2.5')) return true;
@@ -100,6 +101,9 @@ export class GeminiClient {
    */
   private readonly COMPRESSION_PRESERVE_THRESHOLD = 0.3;
 
+  private readonly loopDetector = new LoopDetectionService();
+  private lastPromptId?: string;
+
   constructor(private config: Config) {
     if (config.getProxy()) {
       setGlobalDispatcher(new ProxyAgent(config.getProxy() as string));
@@ -272,6 +276,10 @@ export class GeminiClient {
     turns: number = this.MAX_TURNS,
     originalModel?: string,
   ): AsyncGenerator<ServerGeminiStreamEvent, Turn> {
+    if (this.lastPromptId !== prompt_id) {
+      this.loopDetector.reset();
+      this.lastPromptId = prompt_id;
+    }
     this.sessionTurnCount++;
     if (
       this.config.getMaxSessionTurns() > 0 &&
@@ -297,6 +305,10 @@ export class GeminiClient {
     const turn = new Turn(this.getChat(), prompt_id);
     const resultStream = turn.run(request, signal);
     for await (const event of resultStream) {
+      if (this.loopDetector.addAndCheck(event)) {
+        yield { type: GeminiEventType.LoopDetected };
+        return turn;
+      }
       yield event;
     }
     if (!turn.pendingToolCalls.length && signal && !signal.aborted) {

packages/core/src/core/turn.ts

@@ -49,6 +49,7 @@ export enum GeminiEventType {
   ChatCompressed = 'chat_compressed',
   Thought = 'thought',
   MaxSessionTurns = 'max_session_turns',
+  LoopDetected = 'loop_detected',
 }
 
 export interface StructuredError {
@@ -133,6 +134,10 @@ export type ServerGeminiMaxSessionTurnsEvent = {
   type: GeminiEventType.MaxSessionTurns;
 };
 
+export type ServerGeminiLoopDetectedEvent = {
+  type: GeminiEventType.LoopDetected;
+};
+
 // The original union type, now composed of the individual types
 export type ServerGeminiStreamEvent =
   | ServerGeminiContentEvent
@@ -143,7 +148,8 @@ export type ServerGeminiStreamEvent =
   | ServerGeminiErrorEvent
   | ServerGeminiChatCompressedEvent
   | ServerGeminiThoughtEvent
-  | ServerGeminiMaxSessionTurnsEvent;
+  | ServerGeminiMaxSessionTurnsEvent
+  | ServerGeminiLoopDetectedEvent;
 
 // A turn manages the agentic loop turn within the server context.
 export class Turn {

packages/core/src/services/loopDetectionService.test.ts

@@ -0,0 +1,294 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { LoopDetectionService } from './loopDetectionService.js';
+import {
+  GeminiEventType,
+  ServerGeminiContentEvent,
+  ServerGeminiToolCallRequestEvent,
+} from '../core/turn.js';
+import { ServerGeminiStreamEvent } from '../core/turn.js';
+
+const TOOL_CALL_LOOP_THRESHOLD = 5;
+const CONTENT_LOOP_THRESHOLD = 10;
+
+describe('LoopDetectionService', () => {
+  let service: LoopDetectionService;
+
+  beforeEach(() => {
+    service = new LoopDetectionService();
+  });
+
+  const createToolCallRequestEvent = (
+    name: string,
+    args: Record<string, unknown>,
+  ): ServerGeminiToolCallRequestEvent => ({
+    type: GeminiEventType.ToolCallRequest,
+    value: {
+      name,
+      args,
+      callId: 'test-id',
+      isClientInitiated: false,
+      prompt_id: 'test-prompt-id',
+    },
+  });
+
+  const createContentEvent = (content: string): ServerGeminiContentEvent => ({
+    type: GeminiEventType.Content,
+    value: content,
+  });
+
+  describe('Tool Call Loop Detection', () => {
+    it(`should not detect a loop for fewer than TOOL_CALL_LOOP_THRESHOLD identical calls`, () => {
+      const event = createToolCallRequestEvent('testTool', { param: 'value' });
+      for (let i = 0; i < TOOL_CALL_LOOP_THRESHOLD - 1; i++) {
+        expect(service.addAndCheck(event)).toBe(false);
+      }
+    });
+
+    it(`should detect a loop on the TOOL_CALL_LOOP_THRESHOLD-th identical call`, () => {
+      const event = createToolCallRequestEvent('testTool', { param: 'value' });
+      for (let i = 0; i < TOOL_CALL_LOOP_THRESHOLD - 1; i++) {
+        service.addAndCheck(event);
+      }
+      expect(service.addAndCheck(event)).toBe(true);
+    });
+
+    it('should detect a loop on subsequent identical calls', () => {
+      const event = createToolCallRequestEvent('testTool', { param: 'value' });
+      for (let i = 0; i < TOOL_CALL_LOOP_THRESHOLD; i++) {
+        service.addAndCheck(event);
+      }
+      expect(service.addAndCheck(event)).toBe(true);
+    });
+
+    it('should not detect a loop for different tool calls', () => {
+      const event1 = createToolCallRequestEvent('testTool', {
+        param: 'value1',
+      });
+      const event2 = createToolCallRequestEvent('testTool', {
+        param: 'value2',
+      });
+      const event3 = createToolCallRequestEvent('anotherTool', {
+        param: 'value1',
+      });
+
+      for (let i = 0; i < TOOL_CALL_LOOP_THRESHOLD - 2; i++) {
+        expect(service.addAndCheck(event1)).toBe(false);
+        expect(service.addAndCheck(event2)).toBe(false);
+        expect(service.addAndCheck(event3)).toBe(false);
+      }
+    });
+  });
+
+  describe('Content Loop Detection', () => {
+    it(`should not detect a loop for fewer than CONTENT_LOOP_THRESHOLD identical content strings`, () => {
+      const event = createContentEvent('This is a test sentence.');
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        expect(service.addAndCheck(event)).toBe(false);
+      }
+    });
+
+    it(`should detect a loop on the CONTENT_LOOP_THRESHOLD-th identical content string`, () => {
+      const event = createContentEvent('This is a test sentence.');
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        service.addAndCheck(event);
+      }
+      expect(service.addAndCheck(event)).toBe(true);
+    });
+
+    it('should not detect a loop for different content strings', () => {
+      const event1 = createContentEvent('Sentence A');
+      const event2 = createContentEvent('Sentence B');
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 2; i++) {
+        expect(service.addAndCheck(event1)).toBe(false);
+        expect(service.addAndCheck(event2)).toBe(false);
+      }
+    });
+  });
+
+  describe('Sentence Extraction and Punctuation', () => {
+    it('should not check for loops when content has no sentence-ending punctuation', () => {
+      const eventNoPunct = createContentEvent('This has no punctuation');
+      expect(service.addAndCheck(eventNoPunct)).toBe(false);
+
+      const eventWithPunct = createContentEvent('This has punctuation!');
+      expect(service.addAndCheck(eventWithPunct)).toBe(false);
+    });
+
+    it('should not treat function calls or method calls as sentence endings', () => {
+      // These should not trigger sentence detection, so repeating them many times should never cause a loop
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD + 2; i++) {
+        expect(service.addAndCheck(createContentEvent('console.log()'))).toBe(
+          false,
+        );
+      }
+
+      service.reset();
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD + 2; i++) {
+        expect(service.addAndCheck(createContentEvent('obj.method()'))).toBe(
+          false,
+        );
+      }
+
+      service.reset();
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD + 2; i++) {
+        expect(
+          service.addAndCheck(createContentEvent('arr.filter().map()')),
+        ).toBe(false);
+      }
+
+      service.reset();
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD + 2; i++) {
+        expect(
+          service.addAndCheck(
+            createContentEvent('if (condition) { return true; }'),
+          ),
+        ).toBe(false);
+      }
+    });
+
+    it('should correctly identify actual sentence endings and trigger loop detection', () => {
+      // These should trigger sentence detection, so repeating them should eventually cause a loop
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        expect(
+          service.addAndCheck(createContentEvent('This is a sentence.')),
+        ).toBe(false);
+      }
+      expect(
+        service.addAndCheck(createContentEvent('This is a sentence.')),
+      ).toBe(true);
+
+      service.reset();
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        expect(
+          service.addAndCheck(createContentEvent('Is this a question? ')),
+        ).toBe(false);
+      }
+      expect(
+        service.addAndCheck(createContentEvent('Is this a question? ')),
+      ).toBe(true);
+
+      service.reset();
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        expect(
+          service.addAndCheck(createContentEvent('What excitement!\n')),
+        ).toBe(false);
+      }
+      expect(
+        service.addAndCheck(createContentEvent('What excitement!\n')),
+      ).toBe(true);
+    });
+
+    it('should handle content with mixed punctuation', () => {
+      service.addAndCheck(createContentEvent('Question?'));
+      service.addAndCheck(createContentEvent('Exclamation!'));
+      service.addAndCheck(createContentEvent('Period.'));
+
+      // Repeat one of them multiple times
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        service.addAndCheck(createContentEvent('Period.'));
+      }
+      expect(service.addAndCheck(createContentEvent('Period.'))).toBe(true);
+    });
+
+    it('should handle empty sentences after trimming', () => {
+      service.addAndCheck(createContentEvent('   .'));
+      expect(service.addAndCheck(createContentEvent('Normal sentence.'))).toBe(
+        false,
+      );
+    });
+
+    it('should require at least two sentences for loop detection', () => {
+      const event = createContentEvent('Only one sentence.');
+      expect(service.addAndCheck(event)).toBe(false);
+
+      // Even repeating the same single sentence shouldn't trigger detection
+      for (let i = 0; i < 5; i++) {
+        expect(service.addAndCheck(event)).toBe(false);
+      }
+    });
+  });
+
+  describe('Performance Optimizations', () => {
+    it('should cache sentence extraction and only re-extract when content grows significantly', () => {
+      // Add initial content
+      service.addAndCheck(createContentEvent('First sentence.'));
+      service.addAndCheck(createContentEvent('Second sentence.'));
+
+      // Add small amounts of content (shouldn't trigger re-extraction)
+      for (let i = 0; i < 10; i++) {
+        service.addAndCheck(createContentEvent('X'));
+      }
+      service.addAndCheck(createContentEvent('.'));
+
+      // Should still work correctly
+      expect(service.addAndCheck(createContentEvent('Test.'))).toBe(false);
+    });
+
+    it('should re-extract sentences when content grows by more than 100 characters', () => {
+      service.addAndCheck(createContentEvent('Initial sentence.'));
+
+      // Add enough content to trigger re-extraction
+      const longContent = 'X'.repeat(101);
+      service.addAndCheck(createContentEvent(longContent + '.'));
+
+      // Should work correctly after re-extraction
+      expect(service.addAndCheck(createContentEvent('Test.'))).toBe(false);
+    });
+
+    it('should use indexOf for efficient counting instead of regex', () => {
+      const repeatedSentence = 'This is a repeated sentence.';
+
+      // Build up content with the sentence repeated
+      for (let i = 0; i < CONTENT_LOOP_THRESHOLD - 1; i++) {
+        service.addAndCheck(createContentEvent(repeatedSentence));
+      }
+
+      // The threshold should be reached
+      expect(service.addAndCheck(createContentEvent(repeatedSentence))).toBe(
+        true,
+      );
+    });
+  });
+
+  describe('Edge Cases', () => {
+    it('should handle empty content', () => {
+      const event = createContentEvent('');
+      expect(service.addAndCheck(event)).toBe(false);
+    });
+  });
+
+  describe('Reset Functionality', () => {
+    it('tool call should reset content count', () => {
+      const contentEvent = createContentEvent('Some content.');
+      const toolEvent = createToolCallRequestEvent('testTool', {
+        param: 'value',
+      });
+      for (let i = 0; i < 9; i++) {
+        service.addAndCheck(contentEvent);
+      }
+
+      service.addAndCheck(toolEvent);
+
+      // Should start fresh
+      expect(service.addAndCheck(createContentEvent('Fresh content.'))).toBe(
+        false,
+      );
+    });
+  });
+
+  describe('General Behavior', () => {
+    it('should return false for unhandled event types', () => {
+      const otherEvent = {
+        type: 'unhandled_event',
+      } as unknown as ServerGeminiStreamEvent;
+      expect(service.addAndCheck(otherEvent)).toBe(false);
+      expect(service.addAndCheck(otherEvent)).toBe(false);
+    });
+  });
+});

packages/core/src/services/loopDetectionService.ts

@@ -0,0 +1,121 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { createHash } from 'crypto';
+import { GeminiEventType, ServerGeminiStreamEvent } from '../core/turn.js';
+
+const TOOL_CALL_LOOP_THRESHOLD = 5;
+const CONTENT_LOOP_THRESHOLD = 10;
+const SENTENCE_ENDING_PUNCTUATION_REGEX = /[.!?]+(?=\s|$)/;
+
+/**
+ * Service for detecting and preventing infinite loops in AI responses.
+ * Monitors tool call repetitions and content sentence repetitions.
+ */
+export class LoopDetectionService {
+  // Tool call tracking
+  private lastToolCallKey: string | null = null;
+  private toolCallRepetitionCount: number = 0;
+
+  // Content streaming tracking
+  private lastRepeatedSentence: string = '';
+  private sentenceRepetitionCount: number = 0;
+  private partialContent: string = '';
+
+  private getToolCallKey(toolCall: { name: string; args: object }): string {
+    const argsString = JSON.stringify(toolCall.args);
+    const keyString = `${toolCall.name}:${argsString}`;
+    return createHash('sha256').update(keyString).digest('hex');
+  }
+
+  /**
+   * Processes a stream event and checks for loop conditions.
+   * @param event - The stream event to process
+   * @returns true if a loop is detected, false otherwise
+   */
+  addAndCheck(event: ServerGeminiStreamEvent): boolean {
+    switch (event.type) {
+      case GeminiEventType.ToolCallRequest:
+        // content chanting only happens in one single stream, reset if there
+        // is a tool call in between
+        this.resetSentenceCount();
+        return this.checkToolCallLoop(event.value);
+      case GeminiEventType.Content:
+        return this.checkContentLoop(event.value);
+      default:
+        this.reset();
+        return false;
+    }
+  }
+
+  private checkToolCallLoop(toolCall: { name: string; args: object }): boolean {
+    const key = this.getToolCallKey(toolCall);
+    if (this.lastToolCallKey === key) {
+      this.toolCallRepetitionCount++;
+    } else {
+      this.lastToolCallKey = key;
+      this.toolCallRepetitionCount = 1;
+    }
+    return this.toolCallRepetitionCount >= TOOL_CALL_LOOP_THRESHOLD;
+  }
+
+  private checkContentLoop(content: string): boolean {
+    this.partialContent += content;
+
+    if (!SENTENCE_ENDING_PUNCTUATION_REGEX.test(this.partialContent)) {
+      return false;
+    }
+
+    const completeSentences =
+      this.partialContent.match(/[^.!?]+[.!?]+(?=\s|$)/g) || [];
+    if (completeSentences.length === 0) {
+      return false;
+    }
+
+    const lastSentence = completeSentences[completeSentences.length - 1];
+    const lastCompleteIndex = this.partialContent.lastIndexOf(lastSentence);
+    const endOfLastSentence = lastCompleteIndex + lastSentence.length;
+    this.partialContent = this.partialContent.slice(endOfLastSentence);
+
+    for (const sentence of completeSentences) {
+      const trimmedSentence = sentence.trim();
+      if (trimmedSentence === '') {
+        continue;
+      }
+
+      if (this.lastRepeatedSentence === trimmedSentence) {
+        this.sentenceRepetitionCount++;
+      } else {
+        this.lastRepeatedSentence = trimmedSentence;
+        this.sentenceRepetitionCount = 1;
+      }
+
+      if (this.sentenceRepetitionCount >= CONTENT_LOOP_THRESHOLD) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Resets all loop detection state.
+   */
+  reset(): void {
+    this.resetToolCallCount();
+    this.resetSentenceCount();
+  }
+
+  private resetToolCallCount(): void {
+    this.lastToolCallKey = null;
+    this.toolCallRepetitionCount = 0;
+  }
+
+  private resetSentenceCount(): void {
+    this.lastRepeatedSentence = '';
+    this.sentenceRepetitionCount = 0;
+    this.partialContent = '';
+  }
+}