diff --git a/packages/core/src/mcp/oauth-provider-security.test.ts b/packages/core/src/mcp/oauth-provider-security.test.ts
new file mode 100644
index 00000000000..9dca4e70031
--- /dev/null
+++ b/packages/core/src/mcp/oauth-provider-security.test.ts
@@ -0,0 +1,157 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { vi } from 'vitest';
+
+// Mock dependencies AT THE TOP
+const mockOpenBrowserSecurely = vi.hoisted(() => vi.fn());
+vi.mock('../utils/secure-browser-launcher.js', () => ({
+  openBrowserSecurely: mockOpenBrowserSecurely,
+}));
+vi.mock('node:crypto');
+vi.mock('./oauth-token-storage.js');
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as http from 'node:http';
+import * as crypto from 'node:crypto';
+import {
+  MCPOAuthProvider,
+  MCPOAuthConfig,
+  OAuthTokenResponse,
+} from './oauth-provider.js';
+import { MCPOAuthTokenStorage, MCPOAuthToken } from './oauth-token-storage.js';
+
+// Mock fetch globally
+const mockFetch = vi.fn();
+global.fetch = mockFetch;
+
+// Define a reusable mock server with .listen, .close, and .on methods
+const mockHttpServer = vi.hoisted(() => ({
+  listen: vi.fn(),
+  close: vi.fn(),
+  on: vi.fn(),
+}));
+vi.mock('node:http', () => ({
+  createServer: vi.fn(() => mockHttpServer),
+}));
+
+describe('MCPOAuthProvider Security', () => {
+  const mockConfig: MCPOAuthConfig = {
+    enabled: true,
+    clientId: 'test-client-id',
+    clientSecret: 'test-client-secret',
+    authorizationUrl: 'https://auth.example.com/authorize',
+    tokenUrl: 'https://auth.example.com/token',
+    scopes: ['read', 'write'],
+    redirectUri: 'http://localhost:7777/oauth/callback',
+    audiences: ['https://api.example.com'],
+  };
+
+  const mockToken: MCPOAuthToken = {
+    accessToken: 'access_token_1234567890_sensitive_part',
+    refreshToken: 'refresh_token_456',
+    tokenType: 'Bearer',
+    scope: 'read write',
+    expiresAt: Date.now() + 3600000,
+  };
+
+  const mockTokenResponse: OAuthTokenResponse = {
+    access_token: 'access_token_1234567890_sensitive_part',
+    token_type: 'Bearer',
+    expires_in: 3600,
+    refresh_token: 'refresh_token_456',
+    scope: 'read write',
+  };
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockOpenBrowserSecurely.mockClear();
+
+    // Mock console.log to spy on calls
+    vi.spyOn(console, 'log').mockImplementation(() => {});
+    vi.spyOn(console, 'warn').mockImplementation(() => {});
+    vi.spyOn(console, 'error').mockImplementation(() => {});
+
+    // Mock crypto functions
+    vi.mocked(crypto.randomBytes).mockImplementation((size: number) => {
+      if (size === 32) return Buffer.from('code_verifier_mock_32_bytes_long');
+      if (size === 16) return Buffer.from('state_mock_16_by');
+      return Buffer.alloc(size);
+    });
+
+    vi.mocked(crypto.createHash).mockReturnValue({
+      update: vi.fn().mockReturnThis(),
+      digest: vi.fn().mockReturnValue('code_challenge_mock'),
+    } as unknown as crypto.Hash);
+
+    // Mock token storage
+    vi.mocked(MCPOAuthTokenStorage.saveToken).mockResolvedValue(undefined);
+    // Important: we need getToken to return the token so the vulnerable code block executes
+    vi.mocked(MCPOAuthTokenStorage.getToken).mockResolvedValue({
+      serverName: 'test-server',
+      token: mockToken,
+      updatedAt: Date.now(),
+      clientId: mockConfig.clientId,
+      tokenUrl: mockConfig.tokenUrl,
+    });
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('should NOT log sensitive token information during authentication', async () => {
+    // Mock HTTP server callback
+    let callbackHandler: unknown;
+    vi.mocked(http.createServer).mockImplementation((handler) => {
+      callbackHandler = handler;
+      return mockHttpServer as unknown as http.Server;
+    });
+
+    mockHttpServer.listen.mockImplementation((port, callback) => {
+      callback?.();
+      // Simulate OAuth callback
+      setTimeout(() => {
+        const mockReq = {
+          url: '/oauth/callback?code=auth_code_123&state=c3RhdGVfbW9ja18xNl9ieQ', // state_mock_16_by in base64url (approx)
+        };
+        const mockRes = {
+          writeHead: vi.fn(),
+          end: vi.fn(),
+        };
+        (callbackHandler as (req: unknown, res: unknown) => void)(
+          mockReq,
+          mockRes,
+        );
+      }, 10);
+    });
+
+    // Mock token exchange
+    mockFetch.mockResolvedValueOnce({
+      ok: true,
+      json: () => Promise.resolve(mockTokenResponse),
+    });
+
+    await MCPOAuthProvider.authenticate(
+      'test-server',
+      mockConfig,
+    );
+
+    // Get all arguments passed to console.log
+    const logCalls = vi.mocked(console.log).mock.calls.map(args => args.join(' '));
+
+    // Check that none of the log messages contain the access token or its substring
+    const tokenPart = mockToken.accessToken.substring(0, 20);
+
+    const logsWithToken = logCalls.filter(msg => msg.includes(tokenPart));
+
+    // This expectation should fail before the fix
+    expect(logsWithToken, `Found logs containing token substring: ${JSON.stringify(logsWithToken)}`).toHaveLength(0);
+
+    const logsWithFullToken = logCalls.filter(msg => msg.includes(mockToken.accessToken));
+    expect(logsWithFullToken, `Found logs containing full token: ${JSON.stringify(logsWithFullToken)}`).toHaveLength(0);
+  });
+});
diff --git a/packages/core/src/mcp/oauth-provider.ts b/packages/core/src/mcp/oauth-provider.ts
index c86478c6163..3c7fecc824c 100644
--- a/packages/core/src/mcp/oauth-provider.ts
+++ b/packages/core/src/mcp/oauth-provider.ts
@@ -659,9 +659,7 @@ export class MCPOAuthProvider {
       // Verify token was saved
       const savedToken = await MCPOAuthTokenStorage.getToken(serverName);
       if (savedToken) {
-        console.log(
-          `Token verification successful: ${savedToken.token.accessToken.substring(0, 20)}...`,
-        );
+        console.log('Token verification successful');
       } else {
         console.error('Token verification failed: token not found after save');
       }