feat(ID token support): Add ID token support for authenticating to MC… (#12031)

Co-authored-by: Adam Weidman <adamfweidman@google.com>

docs/tools/mcp-server.md

@@ -150,6 +150,11 @@ Each server configuration supports the following properties:
   server. Tools listed here will not be available to the model, even if they are
   exposed by the server. **Note:** `excludeTools` takes precedence over
   `includeTools` - if a tool is in both lists, it will be excluded.
+- **`allow_unscoped_id_tokens_cloud_run`** (boolean): When `true` and the MCP
+  server host is a Cloud Run service (`*.run.app`), the CLI will use Google
+  Application Default Credentials (ADC) to generate an unscoped ID token and
+  send it as `Authorization: Bearer <token>`. When using this flag, do not set
+  OAuth scopes; they are not needed.
 - **`targetAudience`** (string): The OAuth Client ID allowlisted on the
   IAP-protected application you are trying to access. Used with
   `authProviderType: 'service_account_impersonation'`.
@@ -281,6 +286,26 @@ property:
 }
 ```
 
+#### Google Credential with Cloud Run ID tokens
+
+When connecting to a Cloud Run service endpoint (`*.run.app`), you must opt into
+ID token based authentication using ADC. Note that the generated ID token is
+unscoped.
+
+```json
+{
+  "mcpServers": {
+    "googleCloudServer": {
+      "url": "https://my-gcp-service.run.app/sse",
+      "authProviderType": "google_credentials",
+      "allow_unscoped_id_tokens_cloud_run": true
+    }
+  }
+}
+```
+
+Note: Only `*.run.app` hosts are supported for this flag.
+
 #### Service Account Impersonation
 
 To authenticate with a server using Service Account Impersonation, you must set

packages/core/src/config/config.ts

@@ -193,6 +193,8 @@ export class MCPServerConfig {
     // OAuth configuration
     readonly oauth?: MCPOAuthConfig,
     readonly authProviderType?: AuthProviderType,
+    // When true, use Google ADC to fetch ID tokens for Cloud Run
+    readonly allow_unscoped_id_tokens_cloud_run?: boolean,
     // Service Account Configuration
     /* targetAudience format: CLIENT_ID.apps.googleusercontent.com */
     readonly targetAudience?: string,

packages/core/src/mcp/google-auth-provider.test.ts

@@ -20,12 +20,16 @@ describe('GoogleCredentialProvider', () => {
     },
   } as MCPServerConfig;
 
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
   it('should throw an error if no scopes are provided', () => {
     const config = {
       url: 'https://test.googleapis.com',
     } as MCPServerConfig;
     expect(() => new GoogleCredentialProvider(config)).toThrow(
-      'Scopes must be provided in the oauth config for Google Credentials provider',
+      'Scopes must be provided in the oauth config for Google Credentials provider (or enable allow_unscoped_id_tokens_for_cloud_run to use ID tokens for Cloud Run endpoints)',
     );
   });
 
@@ -80,7 +84,19 @@ describe('GoogleCredentialProvider', () => {
     );
   });
 
-  describe('with provider instance', () => {
+  it('should not allow run.app host even when unscoped ID token flag is not present', () => {
+    const config = {
+      url: 'https://test.run.app',
+      oauth: {
+        scopes: ['scope1', 'scope2'],
+      },
+    } as MCPServerConfig;
+    expect(() => new GoogleCredentialProvider(config)).toThrow(
+      'To enable the Cloud Run MCP Server at https://test.run.app please set allow_unscoped_id_tokens_cloud_run:true in the MCP Server config.',
+    );
+  });
+
+  describe('with provider instance (Access Tokens)', () => {
     let provider: GoogleCredentialProvider;
     let mockGetAccessToken: Mock;
     let mockClient: {
@@ -154,4 +170,72 @@ describe('GoogleCredentialProvider', () => {
       vi.useRealTimers();
     });
   });
+
+  describe('ID token flow (allow_unscoped_id_tokens_cloud_run)', () => {
+    let mockFetchIdToken: Mock;
+    let mockIdClient: {
+      idTokenProvider: {
+        fetchIdToken: Mock;
+      };
+    };
+
+    beforeEach(() => {
+      mockFetchIdToken = vi.fn();
+      mockIdClient = {
+        idTokenProvider: {
+          fetchIdToken: mockFetchIdToken,
+        },
+      };
+      (GoogleAuth.prototype.getIdTokenClient as Mock).mockResolvedValue(
+        mockIdClient,
+      );
+    });
+
+    it('should return ID token when flag is enabled and derive audience from hostname', async () => {
+      const config = {
+        url: 'https://test.run.app/path',
+        allow_unscoped_id_tokens_cloud_run: true,
+      } as MCPServerConfig;
+      const payload = { exp: Math.floor(Date.now() / 1000) + 3600 };
+      const validToken = `header.${Buffer.from(JSON.stringify(payload)).toString('base64')}.signature`;
+      mockFetchIdToken.mockResolvedValue(validToken);
+
+      const provider = new GoogleCredentialProvider(config);
+      const tokens = await provider.tokens();
+      expect(tokens?.access_token).toBe(validToken);
+      expect(GoogleAuth.prototype.getIdTokenClient).toHaveBeenCalledWith(
+        'test.run.app',
+      );
+      expect(mockFetchIdToken).toHaveBeenCalledWith('test.run.app');
+    });
+
+    it('should return undefined and log error when fetching ID token fails', async () => {
+      const config = {
+        url: 'https://test.run.app/path',
+        allow_unscoped_id_tokens_cloud_run: true,
+      } as MCPServerConfig;
+      const consoleErrorSpy = vi
+        .spyOn(console, 'error')
+        .mockImplementation(() => {});
+      mockFetchIdToken.mockRejectedValue(new Error('Fetch failed'));
+
+      const provider = new GoogleCredentialProvider(config);
+      const tokens = await provider.tokens();
+      expect(tokens).toBeUndefined();
+      expect(consoleErrorSpy).toHaveBeenCalledWith(
+        'Failed to get ID token from Google ADC',
+        expect.any(Error),
+      );
+      consoleErrorSpy.mockRestore();
+    });
+
+    it('should not require scopes when flag allow_unscoped_id_tokens_cloud_run is true', () => {
+      const config = {
+        url: 'https://test.run.app',
+        allow_unscoped_id_tokens_cloud_run: true,
+      } as MCPServerConfig;
+
+      expect(() => new GoogleCredentialProvider(config)).not.toThrow();
+    });
+  });
 });

packages/core/src/mcp/google-auth-provider.ts

@@ -13,12 +13,17 @@ import type {
 } from '@modelcontextprotocol/sdk/shared/auth.js';
 import { GoogleAuth } from 'google-auth-library';
 import type { MCPServerConfig } from '../config/config.js';
-import { FIVE_MIN_BUFFER_MS } from './oauth-utils.js';
+import { OAuthUtils, FIVE_MIN_BUFFER_MS } from './oauth-utils.js';
 
+const CLOUD_RUN_HOST_REGEX = /^(.*\.)?run\.app$/;
+
+// An array of hosts that are allowed to use the Google Credential provider.
 const ALLOWED_HOSTS = [/^.+\.googleapis\.com$/, /^(.*\.)?luci\.app$/];
 
 export class GoogleCredentialProvider implements OAuthClientProvider {
   private readonly auth: GoogleAuth;
+  private readonly useIdToken: boolean = false;
+  private readonly audience?: string;
   private cachedToken?: OAuthTokens;
   private tokenExpiryTime?: number;
 
@@ -42,20 +47,35 @@ export class GoogleCredentialProvider implements OAuthClientProvider {
     }
 
     const hostname = new URL(url).hostname;
-    if (!ALLOWED_HOSTS.some((pattern) => pattern.test(hostname))) {
+    const isRunAppHost = CLOUD_RUN_HOST_REGEX.test(hostname);
+    if (!this.config?.allow_unscoped_id_tokens_cloud_run && isRunAppHost) {
+      throw new Error(
+        `To enable the Cloud Run MCP Server at ${url} please set allow_unscoped_id_tokens_cloud_run:true in the MCP Server config.`,
+      );
+    }
+    if (this.config?.allow_unscoped_id_tokens_cloud_run && isRunAppHost) {
+      this.useIdToken = true;
+    }
+    this.audience = hostname;
+
+    if (
+      !this.useIdToken &&
+      !ALLOWED_HOSTS.some((pattern) => pattern.test(hostname))
+    ) {
       throw new Error(
         `Host "${hostname}" is not an allowed host for Google Credential provider.`,
       );
     }
 
-    const scopes = this.config?.oauth?.scopes;
+    // If we are using the access token flow, we MUST have scopes.
-    if (!scopes || scopes.length === 0) {
+    if (!this.useIdToken && !this.config?.oauth?.scopes) {
       throw new Error(
-        'Scopes must be provided in the oauth config for Google Credentials provider',
+        'Scopes must be provided in the oauth config for Google Credentials provider (or enable allow_unscoped_id_tokens_for_cloud_run to use ID tokens for Cloud Run endpoints)',
       );
     }
+
     this.auth = new GoogleAuth({
-      scopes,
+      scopes: this.config?.oauth?.scopes,
     });
   }
 
@@ -81,6 +101,31 @@ export class GoogleCredentialProvider implements OAuthClientProvider {
     this.cachedToken = undefined;
     this.tokenExpiryTime = undefined;
 
+    // If allow_unscoped_id_tokens_for_cloud_run is configured, use ID tokens.
+    if (this.useIdToken) {
+      try {
+        const idClient = await this.auth.getIdTokenClient(this.audience!);
+        const idToken = await idClient.idTokenProvider.fetchIdToken(
+          this.audience!,
+        );
+
+        const newToken: OAuthTokens = {
+          access_token: idToken,
+          token_type: 'Bearer',
+        };
+
+        const expiryTime = OAuthUtils.parseTokenExpiry(idToken);
+        if (expiryTime) {
+          this.tokenExpiryTime = expiryTime;
+          this.cachedToken = newToken;
+        }
+        return newToken;
+      } catch (e) {
+        console.error('Failed to get ID token from Google ADC', e);
+        return undefined;
+      }
+    }
+
     const client = await this.auth.getClient();
     const accessTokenResponse = await client.getAccessToken();