diff --git a/packages/core/src/agents/agentLoader.ts b/packages/core/src/agents/agentLoader.ts
index e0ccba0782..c867a1c9a3 100644
--- a/packages/core/src/agents/agentLoader.ts
+++ b/packages/core/src/agents/agentLoader.ts
@@ -44,7 +44,7 @@ interface FrontmatterLocalAgentDefinition
  * Authentication configuration for remote agents in frontmatter format.
  */
 interface FrontmatterAuthConfig {
-  type: 'apiKey' | 'http' | 'oauth2';
+  type: 'apiKey' | 'http' | 'google-credentials' | 'oauth2';
   // API Key
   key?: string;
   name?: string;
@@ -54,10 +54,11 @@ interface FrontmatterAuthConfig {
   username?: string;
   password?: string;
   value?: string;
+  // Google Credentials
+  scopes?: string[];
   // OAuth2
   client_id?: string;
   client_secret?: string;
-  scopes?: string[];
   authorization_url?: string;
   token_url?: string;
 }
@@ -152,6 +153,15 @@ const httpAuthSchema = z.object({
   value: z.string().min(1).optional(),
 });
 
+/**
+ * Google Credentials auth schema.
+ */
+const googleCredentialsAuthSchema = z.object({
+  ...baseAuthFields,
+  type: z.literal('google-credentials'),
+  scopes: z.array(z.string()).optional(),
+});
+
 /**
  * OAuth2 auth schema.
  * authorization_url and token_url can be discovered from the agent card if omitted.
@@ -170,6 +180,7 @@ const authConfigSchema = z
   .discriminatedUnion('type', [
     apiKeyAuthSchema,
     httpAuthSchema,
+    googleCredentialsAuthSchema,
     oauth2AuthSchema,
   ])
   .superRefine((data, ctx) => {
@@ -369,6 +380,13 @@ function convertFrontmatterAuthToConfig(
         name: frontmatter.name,
       };
 
+    case 'google-credentials':
+      return {
+        ...base,
+        type: 'google-credentials',
+        scopes: frontmatter.scopes,
+      };
+
     case 'http': {
       if (!frontmatter.scheme) {
         throw new Error(
diff --git a/packages/core/src/agents/auth-provider/factory.ts b/packages/core/src/agents/auth-provider/factory.ts
index 7ec067ff59..1d08d99b77 100644
--- a/packages/core/src/agents/auth-provider/factory.ts
+++ b/packages/core/src/agents/auth-provider/factory.ts
@@ -12,12 +12,15 @@ import type {
 } from './types.js';
 import { ApiKeyAuthProvider } from './api-key-provider.js';
 import { HttpAuthProvider } from './http-provider.js';
+import { GoogleCredentialsAuthProvider } from './google-credentials-provider.js';
 
 export interface CreateAuthProviderOptions {
   /** Required for OAuth/OIDC token storage. */
   agentName?: string;
   authConfig?: A2AAuthConfig;
   agentCard?: AgentCard;
+  /** Required by some providers (like google-credentials) to determine token audience. */
+  targetUrl?: string;
   /** URL to fetch the agent card from, used for OAuth2 URL discovery. */
   agentCardUrl?: string;
 }
@@ -43,9 +46,14 @@ export class A2AAuthProviderFactory {
     }
 
     switch (authConfig.type) {
-      case 'google-credentials':
-        // TODO: Implement
-        throw new Error('google-credentials auth provider not yet implemented');
+      case 'google-credentials': {
+        const provider = new GoogleCredentialsAuthProvider(
+          authConfig,
+          options.targetUrl,
+        );
+        await provider.initialize();
+        return provider;
+      }
 
       case 'apiKey': {
         const provider = new ApiKeyAuthProvider(authConfig);
diff --git a/packages/core/src/agents/auth-provider/google-credentials-provider.test.ts b/packages/core/src/agents/auth-provider/google-credentials-provider.test.ts
new file mode 100644
index 0000000000..f9d6ab18b7
--- /dev/null
+++ b/packages/core/src/agents/auth-provider/google-credentials-provider.test.ts
@@ -0,0 +1,205 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, type Mock } from 'vitest';
+import { GoogleCredentialsAuthProvider } from './google-credentials-provider.js';
+import type { GoogleCredentialsAuthConfig } from './types.js';
+import { GoogleAuth } from 'google-auth-library';
+import { OAuthUtils } from '../../mcp/oauth-utils.js';
+
+// Mock the external dependencies
+vi.mock('google-auth-library', () => ({
+  GoogleAuth: vi.fn(),
+}));
+
+describe('GoogleCredentialsAuthProvider', () => {
+  const mockConfig: GoogleCredentialsAuthConfig = {
+    type: 'google-credentials',
+  };
+
+  let mockGetClient: Mock;
+  let mockGetAccessToken: Mock;
+  let mockGetIdTokenClient: Mock;
+  let mockFetchIdToken: Mock;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+
+    mockGetAccessToken = vi
+      .fn()
+      .mockResolvedValue({ token: 'mock-access-token' });
+    mockGetClient = vi.fn().mockResolvedValue({
+      getAccessToken: mockGetAccessToken,
+      credentials: { expiry_date: Date.now() + 3600 * 1000 },
+    });
+
+    mockFetchIdToken = vi.fn().mockResolvedValue('mock-id-token');
+    mockGetIdTokenClient = vi.fn().mockResolvedValue({
+      idTokenProvider: {
+        fetchIdToken: mockFetchIdToken,
+      },
+    });
+
+    (GoogleAuth as unknown as Mock).mockImplementation(() => ({
+      getClient: mockGetClient,
+      getIdTokenClient: mockGetIdTokenClient,
+    }));
+  });
+
+  describe('Initialization', () => {
+    it('throws if no targetUrl is provided', () => {
+      expect(() => new GoogleCredentialsAuthProvider(mockConfig)).toThrow(
+        /targetUrl must be provided/,
+      );
+    });
+
+    it('throws if targetHost is not allowed', () => {
+      expect(
+        () =>
+          new GoogleCredentialsAuthProvider(mockConfig, 'https://example.com'),
+      ).toThrow(/is not an allowed host/);
+    });
+
+    it('initializes seamlessly with .googleapis.com', () => {
+      expect(
+        () =>
+          new GoogleCredentialsAuthProvider(
+            mockConfig,
+            'https://language.googleapis.com/v1/models',
+          ),
+      ).not.toThrow();
+    });
+
+    it('initializes seamlessly with .run.app', () => {
+      expect(
+        () =>
+          new GoogleCredentialsAuthProvider(
+            mockConfig,
+            'https://my-cloud-run-service.run.app',
+          ),
+      ).not.toThrow();
+    });
+  });
+
+  describe('Token Fetching', () => {
+    it('fetches an access token for googleapis.com endpoint', async () => {
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://language.googleapis.com',
+      );
+      const headers = await provider.headers();
+
+      expect(headers).toEqual({ Authorization: 'Bearer mock-access-token' });
+      expect(mockGetClient).toHaveBeenCalled();
+      expect(mockGetAccessToken).toHaveBeenCalled();
+      expect(mockGetIdTokenClient).not.toHaveBeenCalled();
+    });
+
+    it('fetches an identity token for run.app endpoint', async () => {
+      // Mock OAuthUtils.parseTokenExpiry to avoid Base64 decoding issues in tests
+      vi.spyOn(OAuthUtils, 'parseTokenExpiry').mockReturnValue(
+        Date.now() + 1000000,
+      );
+
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://my-service.run.app/some-path',
+      );
+      const headers = await provider.headers();
+
+      expect(headers).toEqual({ Authorization: 'Bearer mock-id-token' });
+      expect(mockGetIdTokenClient).toHaveBeenCalledWith('my-service.run.app');
+      expect(mockFetchIdToken).toHaveBeenCalledWith('my-service.run.app');
+      expect(mockGetClient).not.toHaveBeenCalled();
+    });
+
+    it('returns cached access token on subsequent calls', async () => {
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://language.googleapis.com',
+      );
+
+      await provider.headers();
+      await provider.headers();
+
+      // Should only call getClient/getAccessToken once due to caching
+      expect(mockGetClient).toHaveBeenCalledTimes(1);
+      expect(mockGetAccessToken).toHaveBeenCalledTimes(1);
+    });
+
+    it('returns cached id token on subsequent calls', async () => {
+      vi.spyOn(OAuthUtils, 'parseTokenExpiry').mockReturnValue(
+        Date.now() + 1000000,
+      );
+
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://my-service.run.app',
+      );
+
+      await provider.headers();
+      await provider.headers();
+
+      expect(mockGetIdTokenClient).toHaveBeenCalledTimes(1);
+      expect(mockFetchIdToken).toHaveBeenCalledTimes(1);
+    });
+
+    it('re-fetches access token on 401 (shouldRetryWithHeaders)', async () => {
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://language.googleapis.com',
+      );
+
+      // Prime the cache
+      await provider.headers();
+      expect(mockGetAccessToken).toHaveBeenCalledTimes(1);
+
+      const req = {} as RequestInit;
+      const res = { status: 401 } as Response;
+
+      const retryHeaders = await provider.shouldRetryWithHeaders(req, res);
+
+      expect(retryHeaders).toEqual({
+        Authorization: 'Bearer mock-access-token',
+      });
+      // Cache was cleared, so getAccessToken was called again
+      expect(mockGetAccessToken).toHaveBeenCalledTimes(2);
+    });
+
+    it('re-fetches token on 403', async () => {
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://language.googleapis.com',
+      );
+
+      const req = {} as RequestInit;
+      const res = { status: 403 } as Response;
+
+      const retryHeaders = await provider.shouldRetryWithHeaders(req, res);
+
+      expect(retryHeaders).toEqual({
+        Authorization: 'Bearer mock-access-token',
+      });
+    });
+
+    it('stops retrying after MAX_AUTH_RETRIES', async () => {
+      const provider = new GoogleCredentialsAuthProvider(
+        mockConfig,
+        'https://language.googleapis.com',
+      );
+
+      const req = {} as RequestInit;
+      const res = { status: 401 } as Response;
+
+      // First two retries should succeed (MAX_AUTH_RETRIES = 2)
+      expect(await provider.shouldRetryWithHeaders(req, res)).toBeDefined();
+      expect(await provider.shouldRetryWithHeaders(req, res)).toBeDefined();
+
+      // Third should return undefined (exhausted)
+      expect(await provider.shouldRetryWithHeaders(req, res)).toBeUndefined();
+    });
+  });
+});
diff --git a/packages/core/src/agents/auth-provider/google-credentials-provider.ts b/packages/core/src/agents/auth-provider/google-credentials-provider.ts
new file mode 100644
index 0000000000..30729c064b
--- /dev/null
+++ b/packages/core/src/agents/auth-provider/google-credentials-provider.ts
@@ -0,0 +1,161 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { HttpHeaders } from '@a2a-js/sdk/client';
+import { BaseA2AAuthProvider } from './base-provider.js';
+import type { GoogleCredentialsAuthConfig } from './types.js';
+import { GoogleAuth } from 'google-auth-library';
+import { debugLogger } from '../../utils/debugLogger.js';
+import { OAuthUtils, FIVE_MIN_BUFFER_MS } from '../../mcp/oauth-utils.js';
+
+const CLOUD_RUN_HOST_REGEX = /^(.*\.)?run\.app$/;
+const ALLOWED_HOSTS = [/^.+\.googleapis\.com$/, CLOUD_RUN_HOST_REGEX];
+
+/**
+ * Authentication provider for Google ADC (Application Default Credentials).
+ * Automatically decides whether to use identity tokens or access tokens
+ * based on the target endpoint URL.
+ */
+export class GoogleCredentialsAuthProvider extends BaseA2AAuthProvider {
+  readonly type = 'google-credentials' as const;
+
+  private readonly auth: GoogleAuth;
+  private readonly useIdToken: boolean = false;
+  private readonly audience?: string;
+  private cachedToken?: string;
+  private tokenExpiryTime?: number;
+
+  constructor(
+    private readonly config: GoogleCredentialsAuthConfig,
+    targetUrl?: string,
+  ) {
+    super();
+
+    if (!targetUrl) {
+      throw new Error(
+        'targetUrl must be provided to GoogleCredentialsAuthProvider to determine token audience.',
+      );
+    }
+
+    const hostname = new URL(targetUrl).hostname;
+    const isRunAppHost = CLOUD_RUN_HOST_REGEX.test(hostname);
+
+    if (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.`,
+      );
+    }
+
+    // A2A spec requires scopes if configured, otherwise use default cloud-platform
+    const scopes =
+      this.config.scopes && this.config.scopes.length > 0
+        ? this.config.scopes
+        : ['https://www.googleapis.com/auth/cloud-platform'];
+
+    this.auth = new GoogleAuth({
+      scopes,
+    });
+  }
+
+  override async initialize(): Promise<void> {
+    // We can pre-fetch or validate if necessary here,
+    // but deferred fetching is usually better for auth tokens.
+  }
+
+  async headers(): Promise<HttpHeaders> {
+    // Check cache
+    if (
+      this.cachedToken &&
+      this.tokenExpiryTime &&
+      Date.now() < this.tokenExpiryTime - FIVE_MIN_BUFFER_MS
+    ) {
+      return { Authorization: `Bearer ${this.cachedToken}` };
+    }
+
+    // Clear expired cache
+    this.cachedToken = undefined;
+    this.tokenExpiryTime = undefined;
+
+    if (this.useIdToken) {
+      try {
+        const idClient = await this.auth.getIdTokenClient(this.audience!);
+        const idToken = await idClient.idTokenProvider.fetchIdToken(
+          this.audience!,
+        );
+
+        const expiryTime = OAuthUtils.parseTokenExpiry(idToken);
+        if (expiryTime) {
+          this.tokenExpiryTime = expiryTime;
+          this.cachedToken = idToken;
+        }
+
+        return { Authorization: `Bearer ${idToken}` };
+      } catch (e) {
+        const errorMessage = `Failed to get ADC ID token: ${
+          e instanceof Error ? e.message : String(e)
+        }`;
+        debugLogger.error(errorMessage, e);
+        throw new Error(errorMessage);
+      }
+    }
+
+    // Otherwise, access token
+    try {
+      const client = await this.auth.getClient();
+      const token = await client.getAccessToken();
+
+      if (token.token) {
+        this.cachedToken = token.token;
+        // Use expiry_date from the underlying credentials if available.
+        const creds = client.credentials;
+        if (creds.expiry_date) {
+          this.tokenExpiryTime = creds.expiry_date;
+        }
+        return { Authorization: `Bearer ${token.token}` };
+      }
+      throw new Error('Failed to retrieve ADC access token.');
+    } catch (e) {
+      const errorMessage = `Failed to get ADC access token: ${
+        e instanceof Error ? e.message : String(e)
+      }`;
+      debugLogger.error(errorMessage, e);
+      throw new Error(errorMessage);
+    }
+  }
+
+  override async shouldRetryWithHeaders(
+    _req: RequestInit,
+    res: Response,
+  ): Promise<HttpHeaders | undefined> {
+    if (res.status !== 401 && res.status !== 403) {
+      this.authRetryCount = 0;
+      return undefined;
+    }
+
+    if (this.authRetryCount >= BaseA2AAuthProvider.MAX_AUTH_RETRIES) {
+      return undefined;
+    }
+    this.authRetryCount++;
+
+    debugLogger.debug(
+      '[GoogleCredentialsAuthProvider] Re-fetching token after auth failure',
+    );
+
+    // Clear cache to force a re-fetch
+    this.cachedToken = undefined;
+    this.tokenExpiryTime = undefined;
+
+    return this.headers();
+  }
+}
diff --git a/packages/core/src/agents/registry.test.ts b/packages/core/src/agents/registry.test.ts
index 9ac2ec0cf9..49786de4b0 100644
--- a/packages/core/src/agents/registry.test.ts
+++ b/packages/core/src/agents/registry.test.ts
@@ -593,6 +593,7 @@ describe('AgentRegistry', () => {
       expect(A2AAuthProviderFactory.create).toHaveBeenCalledWith({
         authConfig: mockAuth,
         agentName: 'RemoteAgentWithAuth',
+        targetUrl: 'https://example.com/card',
         agentCardUrl: 'https://example.com/card',
       });
       expect(loadAgentSpy).toHaveBeenCalledWith(
diff --git a/packages/core/src/agents/registry.ts b/packages/core/src/agents/registry.ts
index c4b08eba22..b91fcad3ed 100644
--- a/packages/core/src/agents/registry.ts
+++ b/packages/core/src/agents/registry.ts
@@ -420,6 +420,7 @@ export class AgentRegistry {
         const provider = await A2AAuthProviderFactory.create({
           authConfig: definition.auth,
           agentName: definition.name,
+          targetUrl: definition.agentCardUrl,
           agentCardUrl: remoteDef.agentCardUrl,
         });
         if (!provider) {
diff --git a/packages/core/src/agents/remote-invocation.test.ts b/packages/core/src/agents/remote-invocation.test.ts
index e870090a31..e186cc7aa9 100644
--- a/packages/core/src/agents/remote-invocation.test.ts
+++ b/packages/core/src/agents/remote-invocation.test.ts
@@ -195,6 +195,7 @@ describe('RemoteAgentInvocation', () => {
       expect(A2AAuthProviderFactory.create).toHaveBeenCalledWith({
         authConfig: mockAuth,
         agentName: 'test-agent',
+        targetUrl: 'http://test-agent/card',
         agentCardUrl: 'http://test-agent/card',
       });
       expect(mockClientManager.loadAgent).toHaveBeenCalledWith(
diff --git a/packages/core/src/agents/remote-invocation.ts b/packages/core/src/agents/remote-invocation.ts
index fe1e3cd077..489f0f91cc 100644
--- a/packages/core/src/agents/remote-invocation.ts
+++ b/packages/core/src/agents/remote-invocation.ts
@@ -22,7 +22,6 @@ import {
   type SendMessageResult,
 } from './a2a-client-manager.js';
 import { extractIdsFromResponse, A2AResultReassembler } from './a2aUtils.js';
-import { GoogleAuth } from 'google-auth-library';
 import type { AuthenticationHandler } from '@a2a-js/sdk/client';
 import { debugLogger } from '../utils/debugLogger.js';
 import { safeJsonToMarkdown } from '../utils/markdownUtils.js';
@@ -30,39 +29,6 @@ import type { AnsiOutput } from '../utils/terminalSerializer.js';
 import { A2AAuthProviderFactory } from './auth-provider/factory.js';
 import { A2AAgentError } from './a2a-errors.js';
 
-/**
- * Authentication handler implementation using Google Application Default Credentials (ADC).
- */
-export class ADCHandler implements AuthenticationHandler {
-  private auth = new GoogleAuth({
-    scopes: ['https://www.googleapis.com/auth/cloud-platform'],
-  });
-
-  async headers(): Promise<Record<string, string>> {
-    try {
-      const client = await this.auth.getClient();
-      const token = await client.getAccessToken();
-      if (token.token) {
-        return { Authorization: `Bearer ${token.token}` };
-      }
-      throw new Error('Failed to retrieve ADC access token.');
-    } catch (e) {
-      const errorMessage = `Failed to get ADC token: ${
-        e instanceof Error ? e.message : String(e)
-      }`;
-      debugLogger.log('ERROR', errorMessage);
-      throw new Error(errorMessage);
-    }
-  }
-
-  async shouldRetryWithHeaders(
-    _response: unknown,
-  ): Promise<Record<string, string> | undefined> {
-    // For ADC, we usually just re-fetch the token if needed.
-    return this.headers();
-  }
-}
-
 /**
  * A tool invocation that proxies to a remote A2A agent.
  *
@@ -121,6 +87,7 @@ export class RemoteAgentInvocation extends BaseToolInvocation<
       const provider = await A2AAuthProviderFactory.create({
         authConfig: this.definition.auth,
         agentName: this.definition.name,
+        targetUrl: this.definition.agentCardUrl,
         agentCardUrl: this.definition.agentCardUrl,
       });
       if (!provider) {