diff --git a/packages/core/src/telemetry/activity-detector.test.ts b/packages/core/src/telemetry/activity-detector.test.ts
new file mode 100644
index 0000000000..dd5d56e5a9
--- /dev/null
+++ b/packages/core/src/telemetry/activity-detector.test.ts
@@ -0,0 +1,164 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import {
+  ActivityDetector,
+  getActivityDetector,
+  recordUserActivity,
+  isUserActive,
+} from './activity-detector.js';
+
+describe('ActivityDetector', () => {
+  let detector: ActivityDetector;
+
+  beforeEach(() => {
+    detector = new ActivityDetector(1000); // 1 second idle threshold for testing
+  });
+
+  describe('constructor', () => {
+    it('should initialize with default idle threshold', () => {
+      const defaultDetector = new ActivityDetector();
+      expect(defaultDetector).toBeInstanceOf(ActivityDetector);
+    });
+
+    it('should initialize with custom idle threshold', () => {
+      const customDetector = new ActivityDetector(5000);
+      expect(customDetector).toBeInstanceOf(ActivityDetector);
+    });
+  });
+
+  describe('recordActivity', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+    it('should update last activity time', () => {
+      const beforeTime = detector.getLastActivityTime();
+      vi.advanceTimersByTime(100);
+
+      detector.recordActivity();
+      const afterTime = detector.getLastActivityTime();
+
+      expect(afterTime).toBeGreaterThan(beforeTime);
+    });
+  });
+
+  describe('isUserActive', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+    it('should return true immediately after construction', () => {
+      expect(detector.isUserActive()).toBe(true);
+    });
+
+    it('should return true within idle threshold', () => {
+      detector.recordActivity();
+      expect(detector.isUserActive()).toBe(true);
+    });
+
+    it('should return false after idle threshold', () => {
+      // Advance time beyond idle threshold
+      vi.advanceTimersByTime(2000); // 2 seconds, threshold is 1 second
+
+      expect(detector.isUserActive()).toBe(false);
+    });
+
+    it('should return true again after recording new activity', () => {
+      // Go idle
+      vi.advanceTimersByTime(2000);
+      expect(detector.isUserActive()).toBe(false);
+
+      // Record new activity
+      detector.recordActivity();
+      expect(detector.isUserActive()).toBe(true);
+    });
+  });
+
+  describe('getTimeSinceLastActivity', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+    it('should return time elapsed since last activity', () => {
+      detector.recordActivity();
+      vi.advanceTimersByTime(500);
+
+      const timeSince = detector.getTimeSinceLastActivity();
+      expect(timeSince).toBe(500);
+    });
+  });
+
+  describe('getLastActivityTime', () => {
+    it('should return the timestamp of last activity', () => {
+      const before = Date.now();
+      detector.recordActivity();
+      const activityTime = detector.getLastActivityTime();
+      const after = Date.now();
+
+      expect(activityTime).toBeGreaterThanOrEqual(before);
+      expect(activityTime).toBeLessThanOrEqual(after);
+    });
+  });
+});
+
+describe('Global Activity Detector Functions', () => {
+  describe('global instance', () => {
+    it('should expose a global ActivityDetector via getActivityDetector', () => {
+      const detector = getActivityDetector();
+      expect(detector).toBeInstanceOf(ActivityDetector);
+    });
+  });
+
+  describe('getActivityDetector', () => {
+    it('should always return the global instance', () => {
+      const detector = getActivityDetector();
+      const detectorAgain = getActivityDetector();
+      expect(detectorAgain).toBe(detector);
+    });
+  });
+
+  describe('recordUserActivity', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+    it('should record activity on existing detector', () => {
+      const detector = getActivityDetector()!;
+      const beforeTime = detector.getLastActivityTime();
+      vi.advanceTimersByTime(100);
+
+      recordUserActivity();
+
+      const afterTime = detector.getLastActivityTime();
+      expect(afterTime).toBeGreaterThan(beforeTime);
+    });
+  });
+
+  describe('isUserActive', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+    it('should reflect global detector state', () => {
+      expect(isUserActive()).toBe(true);
+      // Default idle threshold is 30s; advance beyond it
+      vi.advanceTimersByTime(31000);
+      expect(isUserActive()).toBe(false);
+    });
+  });
+});
diff --git a/packages/core/src/telemetry/activity-detector.ts b/packages/core/src/telemetry/activity-detector.ts
new file mode 100644
index 0000000000..84a03629f1
--- /dev/null
+++ b/packages/core/src/telemetry/activity-detector.ts
@@ -0,0 +1,70 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Tracks user activity state to determine when memory monitoring should be active
+ */
+export class ActivityDetector {
+  private lastActivityTime: number = Date.now();
+  private readonly idleThresholdMs: number;
+
+  constructor(idleThresholdMs: number = 30000) {
+    this.idleThresholdMs = idleThresholdMs;
+  }
+
+  /**
+   * Record user activity (called by CLI when user types, adds messages, etc.)
+   */
+  recordActivity(): void {
+    this.lastActivityTime = Date.now();
+  }
+
+  /**
+   * Check if user is currently active (activity within idle threshold)
+   */
+  isUserActive(): boolean {
+    const timeSinceActivity = Date.now() - this.lastActivityTime;
+    return timeSinceActivity < this.idleThresholdMs;
+  }
+
+  /**
+   * Get time since last activity in milliseconds
+   */
+  getTimeSinceLastActivity(): number {
+    return Date.now() - this.lastActivityTime;
+  }
+
+  /**
+   * Get last activity timestamp
+   */
+  getLastActivityTime(): number {
+    return this.lastActivityTime;
+  }
+}
+
+// Global activity detector instance (eagerly created with default threshold)
+const globalActivityDetector: ActivityDetector = new ActivityDetector();
+
+/**
+ * Get global activity detector instance
+ */
+export function getActivityDetector(): ActivityDetector {
+  return globalActivityDetector;
+}
+
+/**
+ * Record user activity (convenience function for CLI to call)
+ */
+export function recordUserActivity(): void {
+  globalActivityDetector.recordActivity();
+}
+
+/**
+ * Check if user is currently active (convenience function)
+ */
+export function isUserActive(): boolean {
+  return globalActivityDetector.isUserActive();
+}
diff --git a/packages/core/src/telemetry/activity-types.ts b/packages/core/src/telemetry/activity-types.ts
new file mode 100644
index 0000000000..c970a725c7
--- /dev/null
+++ b/packages/core/src/telemetry/activity-types.ts
@@ -0,0 +1,20 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Types of user activities that can be tracked
+ */
+export enum ActivityType {
+  USER_INPUT_START = 'user_input_start',
+  USER_INPUT_END = 'user_input_end',
+  MESSAGE_ADDED = 'message_added',
+  TOOL_CALL_SCHEDULED = 'tool_call_scheduled',
+  TOOL_CALL_COMPLETED = 'tool_call_completed',
+  STREAM_START = 'stream_start',
+  STREAM_END = 'stream_end',
+  HISTORY_UPDATED = 'history_updated',
+  MANUAL_TRIGGER = 'manual_trigger',
+}
diff --git a/packages/core/src/telemetry/index.ts b/packages/core/src/telemetry/index.ts
index e713593cf8..9560269b7a 100644
--- a/packages/core/src/telemetry/index.ts
+++ b/packages/core/src/telemetry/index.ts
@@ -59,3 +59,10 @@ export { SemanticAttributes } from '@opentelemetry/semantic-conventions';
 export * from './uiTelemetry.js';
 export { HighWaterMarkTracker } from './high-water-mark-tracker.js';
 export { RateLimiter } from './rate-limiter.js';
+export { ActivityType } from './activity-types.js';
+export {
+  ActivityDetector,
+  getActivityDetector,
+  recordUserActivity,
+  isUserActive,
+} from './activity-detector.js';