fix: GCA creds loading order (#6498)

packages/core/src/code_assist/oauth2.test.ts

@@ -324,6 +324,70 @@ describe('oauth2', () => {
     });
   });
 
+  describe('credential loading order', () => {
+    it('should prioritize default cached credentials over GOOGLE_APPLICATION_CREDENTIALS', async () => {
+      // Setup default cached credentials
+      const defaultCreds = { refresh_token: 'default-cached-token' };
+      const defaultCredsPath = path.join(
+        tempHomeDir,
+        '.gemini',
+        'oauth_creds.json',
+      );
+      await fs.promises.mkdir(path.dirname(defaultCredsPath), {
+        recursive: true,
+      });
+      await fs.promises.writeFile(
+        defaultCredsPath,
+        JSON.stringify(defaultCreds),
+      );
+
+      // Setup credentials via environment variable
+      const envCreds = { refresh_token: 'env-var-token' };
+      const envCredsPath = path.join(tempHomeDir, 'env_creds.json');
+      await fs.promises.writeFile(envCredsPath, JSON.stringify(envCreds));
+      vi.stubEnv('GOOGLE_APPLICATION_CREDENTIALS', envCredsPath);
+
+      const mockClient = {
+        setCredentials: vi.fn(),
+        getAccessToken: vi.fn().mockResolvedValue({ token: 'test-token' }),
+        getTokenInfo: vi.fn().mockResolvedValue({}),
+        on: vi.fn(),
+      };
+      (OAuth2Client as unknown as Mock).mockImplementation(
+        () => mockClient as unknown as OAuth2Client,
+      );
+
+      await getOauthClient(AuthType.LOGIN_WITH_GOOGLE, mockConfig);
+
+      // Assert the correct credentials were used
+      expect(mockClient.setCredentials).toHaveBeenCalledWith(defaultCreds);
+      expect(mockClient.setCredentials).not.toHaveBeenCalledWith(envCreds);
+    });
+
+    it('should fall back to GOOGLE_APPLICATION_CREDENTIALS if default cache is missing', async () => {
+      // Setup credentials via environment variable
+      const envCreds = { refresh_token: 'env-var-token' };
+      const envCredsPath = path.join(tempHomeDir, 'env_creds.json');
+      await fs.promises.writeFile(envCredsPath, JSON.stringify(envCreds));
+      vi.stubEnv('GOOGLE_APPLICATION_CREDENTIALS', envCredsPath);
+
+      const mockClient = {
+        setCredentials: vi.fn(),
+        getAccessToken: vi.fn().mockResolvedValue({ token: 'test-token' }),
+        getTokenInfo: vi.fn().mockResolvedValue({}),
+        on: vi.fn(),
+      };
+      (OAuth2Client as unknown as Mock).mockImplementation(
+        () => mockClient as unknown as OAuth2Client,
+      );
+
+      await getOauthClient(AuthType.LOGIN_WITH_GOOGLE, mockConfig);
+
+      // Assert the correct credentials were used
+      expect(mockClient.setCredentials).toHaveBeenCalledWith(envCreds);
+    });
+  });
+
   describe('with GCP environment variables', () => {
     it('should use GOOGLE_CLOUD_ACCESS_TOKEN when GOOGLE_GENAI_USE_GCA is true', async () => {
       vi.stubEnv('GOOGLE_GENAI_USE_GCA', 'true');

packages/core/src/code_assist/oauth2.ts

@@ -351,27 +351,32 @@ export function getAvailablePort(): Promise<number> {
 }
 
 async function loadCachedCredentials(client: OAuth2Client): Promise<boolean> {
-  try {
-    const keyFile =
-      process.env['GOOGLE_APPLICATION_CREDENTIALS'] ||
-      getCachedCredentialPath();
+  const pathsToTry = [
+    getCachedCredentialPath(),
+    process.env['GOOGLE_APPLICATION_CREDENTIALS'],
+  ].filter((p): p is string => !!p);
 
-    const creds = await fs.readFile(keyFile, 'utf-8');
-    client.setCredentials(JSON.parse(creds));
+  for (const keyFile of pathsToTry) {
+    try {
+      const creds = await fs.readFile(keyFile, 'utf-8');
+      client.setCredentials(JSON.parse(creds));
 
-    // This will verify locally that the credentials look good.
-    const { token } = await client.getAccessToken();
-    if (!token) {
-      return false;
+      // This will verify locally that the credentials look good.
+      const { token } = await client.getAccessToken();
+      if (!token) {
+        continue;
+      }
+
+      // This will check with the server to see if it hasn't been revoked.
+      await client.getTokenInfo(token);
+
+      return true;
+    } catch (_) {
+      // Ignore and try next path.
     }
-
-    // This will check with the server to see if it hasn't been revoked.
-    await client.getTokenInfo(token);
-
-    return true;
-  } catch (_) {
-    return false;
   }
+
+  return false;
 }
 
 async function cacheCredentials(credentials: Credentials) {