diff --git a/packages/core/src/utils/errors.test.ts b/packages/core/src/utils/errors.test.ts
index c7f8c5f287..8ee1bcfd7a 100644
--- a/packages/core/src/utils/errors.test.ts
+++ b/packages/core/src/utils/errors.test.ts
@@ -5,7 +5,13 @@
  */
 
 import { describe, it, expect } from 'vitest';
-import { isAuthenticationError, UnauthorizedError } from './errors.js';
+import {
+  isAuthenticationError,
+  UnauthorizedError,
+  toFriendlyError,
+  BadRequestError,
+  ForbiddenError,
+} from './errors.js';
 
 describe('isAuthenticationError', () => {
   it('should detect error with code: 401 property (MCP SDK style)', () => {
@@ -40,3 +46,132 @@ describe('isAuthenticationError', () => {
     expect(isAuthenticationError(new Error('Status code: 401'))).toBe(true);
   });
 });
+
+describe('toFriendlyError', () => {
+  it('should return BadRequestError for 400', () => {
+    const error = {
+      response: {
+        data: {
+          error: {
+            code: 400,
+            message: 'Bad Request',
+          },
+        },
+      },
+    };
+    const result = toFriendlyError(error);
+    expect(result).toBeInstanceOf(BadRequestError);
+    expect((result as BadRequestError).message).toBe('Bad Request');
+  });
+
+  it('should return UnauthorizedError for 401', () => {
+    const error = {
+      response: {
+        data: {
+          error: {
+            code: 401,
+            message: 'Unauthorized',
+          },
+        },
+      },
+    };
+    const result = toFriendlyError(error);
+    expect(result).toBeInstanceOf(UnauthorizedError);
+    expect((result as UnauthorizedError).message).toBe('Unauthorized');
+  });
+
+  it('should return ForbiddenError for 403', () => {
+    const error = {
+      response: {
+        data: {
+          error: {
+            code: 403,
+            message: 'Forbidden',
+          },
+        },
+      },
+    };
+    const result = toFriendlyError(error);
+    expect(result).toBeInstanceOf(ForbiddenError);
+    expect((result as ForbiddenError).message).toBe('Forbidden');
+  });
+
+  it('should parse stringified JSON data', () => {
+    const error = {
+      response: {
+        data: JSON.stringify({
+          error: {
+            code: 400,
+            message: 'Parsed Message',
+          },
+        }),
+      },
+    };
+    const result = toFriendlyError(error);
+    expect(result).toBeInstanceOf(BadRequestError);
+    expect((result as BadRequestError).message).toBe('Parsed Message');
+  });
+
+  it('should return original error if response data is undefined', () => {
+    const error = {
+      response: {
+        data: undefined,
+      },
+    };
+    expect(toFriendlyError(error)).toBe(error);
+  });
+
+  it('should return original error if error object is missing in data', () => {
+    const error = {
+      response: {
+        data: {
+          somethingElse: 'value',
+        },
+      },
+    };
+    expect(toFriendlyError(error)).toBe(error);
+  });
+
+  it('should return original error if error code or message is missing', () => {
+    const errorNoCode = {
+      response: {
+        data: {
+          error: {
+            message: 'No Code',
+          },
+        },
+      },
+    };
+    expect(toFriendlyError(errorNoCode)).toBe(errorNoCode);
+
+    const errorNoMessage = {
+      response: {
+        data: {
+          error: {
+            code: 400,
+          },
+        },
+      },
+    };
+    expect(toFriendlyError(errorNoMessage)).toBe(errorNoMessage);
+  });
+
+  it('should return original error for unknown codes', () => {
+    const error = {
+      response: {
+        data: {
+          error: {
+            code: 500,
+            message: 'Internal Server Error',
+          },
+        },
+      },
+    };
+    expect(toFriendlyError(error)).toBe(error);
+  });
+
+  it('should return original error if not a Gaxios error object', () => {
+    const error = new Error('Regular Error');
+    expect(toFriendlyError(error)).toBe(error);
+  });
+});
diff --git a/packages/core/src/utils/errors.ts b/packages/core/src/utils/errors.ts
index a91fafb922..8db1153d92 100644
--- a/packages/core/src/utils/errors.ts
+++ b/packages/core/src/utils/errors.ts
@@ -92,7 +92,7 @@ export function toFriendlyError(error: unknown): unknown {
   if (error && typeof error === 'object' && 'response' in error) {
     const gaxiosError = error as GaxiosError;
     const data = parseResponseData(gaxiosError);
-    if (data.error && data.error.message && data.error.code) {
+    if (data && data.error && data.error.message && data.error.code) {
       switch (data.error.code) {
         case 400:
           return new BadRequestError(data.error.message);
@@ -110,12 +110,16 @@ export function toFriendlyError(error: unknown): unknown {
   return error;
 }
 
-function parseResponseData(error: GaxiosError): ResponseData {
+function parseResponseData(error: GaxiosError): ResponseData | undefined {
   // Inexplicably, Gaxios sometimes doesn't JSONify the response data.
   if (typeof error.response?.data === 'string') {
-    return JSON.parse(error.response?.data) as ResponseData;
+    try {
+      return JSON.parse(error.response?.data) as ResponseData;
+    } catch {
+      return undefined;
+    }
   }
-  return error.response?.data as ResponseData;
+  return error.response?.data as ResponseData | undefined;
 }
 
 /**