5 files changed, 51 insertions, 25 deletions
diff --git a/packages/core/src/code_assist/errors.ts b/packages/core/src/code_assist/errors.ts
deleted file mode 100644
index f870ab5c..00000000
--- a/packages/core/src/code_assist/errors.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-/**
- * @license
- * Copyright 2025 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-import { GaxiosError } from 'gaxios';
-
-export function isAuthError(error: unknown): boolean {
-  return (
-    error instanceof GaxiosError && error.response?.data?.error?.code === 401
-  );
-}
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index e6d59db9..36acb3e8 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -292,8 +292,7 @@ export class GeminiClient {
         throw error;
       }
       try {
-        const parsedJson = JSON.parse(text);
-        return parsedJson;
+        return JSON.parse(text);
       } catch (parseError) {
         await reportError(
           parseError,
diff --git a/packages/core/src/core/turn.ts b/packages/core/src/core/turn.ts
index 85fffd93..cf5e0620 100644
--- a/packages/core/src/core/turn.ts
+++ b/packages/core/src/core/turn.ts
@@ -20,7 +20,7 @@ import { getResponseText } from '../utils/generateContentResponseUtilities.js';
 import { reportError } from '../utils/errorReporting.js';
 import { getErrorMessage } from '../utils/errors.js';
 import { GeminiChat } from './geminiChat.js';
-import { isAuthError } from '../code_assist/errors.js';
+import { UnauthorizedError, toFriendlyError } from '../utils/errors.js';
 
 // Define a structure for tools passed to the server
 export interface ServerTool {
@@ -224,8 +224,9 @@ export class Turn {
           value: { ...this.lastUsageMetadata, apiTimeMs: durationMs },
         };
       }
-    } catch (error) {
-      if (isAuthError(error)) {
+    } catch (e) {
+      const error = toFriendlyError(e);
+      if (error instanceof UnauthorizedError) {
         throw error;
       }
       if (signal.aborted) {
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 163a4819..3a123452 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -21,7 +21,6 @@ export * from './core/nonInteractiveToolExecutor.js';
 
 export * from './code_assist/codeAssist.js';
 export * from './code_assist/oauth2.js';
-export * from './code_assist/errors.js';
 
 // Export utilities
 export * from './utils/paths.js';
diff --git a/packages/core/src/utils/errors.ts b/packages/core/src/utils/errors.ts
index 32139c1a..4787c439 100644
--- a/packages/core/src/utils/errors.ts
+++ b/packages/core/src/utils/errors.ts
@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import { GaxiosError } from 'gaxios';
+
 export function isNodeError(error: unknown): error is NodeJS.ErrnoException {
   return error instanceof Error && 'code' in error;
 }
@@ -11,12 +13,50 @@ export function isNodeError(error: unknown): error is NodeJS.ErrnoException {
 export function getErrorMessage(error: unknown): string {
   if (error instanceof Error) {
     return error.message;
-  } else {
-    try {
-      const errorMessage = String(error);
-      return errorMessage;
-    } catch {
-      return 'Failed to get error details';
+  }
+  try {
+    return String(error);
+  } catch {
+    return 'Failed to get error details';
+  }
+}
+
+export class ForbiddenError extends Error {}
+export class UnauthorizedError extends Error {}
+export class BadRequestError extends Error {}
+
+interface ResponseData {
+  error?: {
+    code?: number;
+    message?: string;
+  };
+}
+
+export function toFriendlyError(error: unknown): unknown {
+  if (error instanceof GaxiosError) {
+    const data = parseResponseData(error);
+    if (data.error && data.error.message && data.error.code) {
+      switch (data.error.code) {
+        case 400:
+          return new BadRequestError(data.error.message);
+        case 401:
+          return new UnauthorizedError(data.error.message);
+        case 403:
+          // It's import to pass the message here since it might
+          // explain the cause like "the cloud project you're
+          // using doesn't have code assist enabled".
+          return new ForbiddenError(data.error.message);
+        default:
+      }
     }
   }
+  return error;
+}
+
+function parseResponseData(error: GaxiosError): ResponseData {
+  // Inexplicably, Gaxios sometimes doesn't JSONify the response data.
+  if (typeof error.response?.data === 'string') {
+    return JSON.parse(error.response?.data) as ResponseData;
+  }
+  return typeof error.response?.data as ResponseData;
 }