diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index 5e8e7674ff..90ebb26d54 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -649,6 +649,9 @@ export class GeminiChat {
 
       if (this.onModelChanged) {
         this.tools = await this.onModelChanged(modelToUse);
+        // CRITICAL: Update the request config with the fresh tools
+        // to ensure mode-switches (like exit_plan_mode) are reflected immediately.
+        config.tools = this.tools;
       }
 
       // Track final request parameters for AfterModel hooks
diff --git a/packages/core/src/core/geminiChat_sync.test.ts b/packages/core/src/core/geminiChat_sync.test.ts
new file mode 100644
index 0000000000..1be1c0356b
--- /dev/null
+++ b/packages/core/src/core/geminiChat_sync.test.ts
@@ -0,0 +1,127 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { GeminiChat } from './geminiChat.js';
+import type { Config } from '../config/config.js';
+import type { ContentGenerator } from './contentGenerator.js';
+import { createMockMessageBus } from '../test-utils/mock-message-bus.js';
+import { LlmRole } from '../telemetry/types.js';
+import type { AgentLoopContext } from '../config/agent-loop-context.js';
+import type { Tool } from '@google/genai';
+import type { ToolRegistry } from '../tools/tool-registry.js';
+
+// Mock retryWithBackoff
+vi.mock('../utils/retry.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../utils/retry.js')>();
+  return {
+    ...actual,
+    retryWithBackoff: vi.fn().mockImplementation(async (apiCall) => apiCall()),
+  };
+});
+
+describe('GeminiChat Tool Synchronization', () => {
+  let mockContentGenerator: ContentGenerator;
+  let mockConfig: Config;
+
+  beforeEach(() => {
+    mockContentGenerator = {
+      generateContent: vi.fn().mockResolvedValue({
+        candidates: [
+          {
+            content: { parts: [{ text: 'response' }] },
+            finishReason: 'STOP',
+          },
+        ],
+      }),
+      generateContentStream: vi.fn(),
+    } as unknown as ContentGenerator;
+
+    mockConfig = {
+      getDisableStreaming: vi.fn().mockReturnValue(true),
+      getContentGenerator: vi.fn().mockReturnValue(mockContentGenerator),
+      getActiveModel: vi.fn().mockReturnValue('gemini-pro'),
+      getModel: vi.fn().mockReturnValue('gemini-pro'),
+      getGemini31Launched: vi.fn().mockResolvedValue(false),
+      getGemini31FlashLiteLaunched: vi.fn().mockResolvedValue(false),
+      getHasAccessToPreviewModel: vi.fn().mockReturnValue(false),
+      getSessionId: vi.fn().mockReturnValue('test-session-id'),
+      getMaxAttempts: vi.fn().mockReturnValue(1),
+      getRetryFetchErrors: vi.fn().mockReturnValue(false),
+      getHookSystem: vi.fn().mockReturnValue(undefined),
+      isInteractive: vi.fn().mockReturnValue(true),
+      getExperiments: vi.fn().mockReturnValue(undefined),
+      getContentGeneratorConfig: vi
+        .fn()
+        .mockReturnValue({ model: 'gemini-pro' }),
+      getProjectRoot: vi.fn().mockReturnValue('/test/project/root'),
+      getTelemetryLogPromptsEnabled: vi.fn().mockReturnValue(true),
+      getUsageStatisticsEnabled: vi.fn().mockReturnValue(true),
+      getDebugMode: vi.fn().mockReturnValue(false),
+      getValidationHandler: vi.fn().mockReturnValue(undefined),
+      getModelAvailabilityService: vi.fn().mockReturnValue({
+        selectFirstAvailable: vi.fn().mockImplementation((models) => ({
+          model: models[0],
+          config: {},
+        })),
+        markHealthy: vi.fn(),
+      }),
+      modelConfigService: {
+        getResolvedConfig: vi.fn().mockReturnValue({
+          model: 'gemini-pro',
+          generateContentConfig: {},
+        }),
+      },
+    } as unknown as Config;
+  });
+
+  it('should update config.tools when this.tools is updated via onModelChanged', async () => {
+    const initialTools = [{ functionDeclarations: [{ name: 'tool1' }] }];
+    const updatedTools = [{ functionDeclarations: [{ name: 'tool2' }] }];
+
+    const onModelChanged = vi.fn().mockResolvedValue(updatedTools);
+
+    const chat = new GeminiChat(
+      {
+        config: mockConfig,
+        toolRegistry: {
+          getMessageBus: () => createMockMessageBus(),
+        } as unknown as ToolRegistry,
+      } as unknown as AgentLoopContext,
+      'system instruction',
+      initialTools as unknown as Tool[],
+      [], // history
+      undefined, // resumedSessionData
+      onModelChanged,
+    );
+
+    const stream = await chat.sendMessageStream(
+      { model: 'gemini-pro' },
+      [{ text: 'user prompt' }],
+      'prompt-id',
+      new AbortController().signal,
+      LlmRole.UTILITY_TOOL,
+    );
+     
+    for await (const _ of stream) {
+      // consume stream
+    }
+
+    // Verify onModelChanged was called
+    expect(onModelChanged).toHaveBeenCalled();
+
+    // Verify generateContent was called with updated tools
+    expect(mockContentGenerator.generateContent).toHaveBeenCalledWith(
+      expect.objectContaining({
+        config: expect.objectContaining({
+          tools: updatedTools,
+        }),
+      }),
+      expect.any(String),
+      LlmRole.UTILITY_TOOL,
+    );
+  });
+});