fix: fall back to entire response body when 'error' field is missing

src/core/error.ts

@@ -66,7 +66,7 @@ export class APIError<
       return new APIConnectionError({ message, cause: castToError(errorResponse) });
     }
 
-    const error = (errorResponse as Record<string, any>)?.['error'];
+    const error = (errorResponse as Record<string, any>)?.['error'] ?? errorResponse;
 
     if (status === 400) {
       return new BadRequestError(status, error, message, headers);

tests/error.test.ts

@@ -0,0 +1,51 @@
+import { APIError, BadRequestError, UnprocessableEntityError } from 'openai/core/error';
+
+describe('APIError.generate', () => {
+  const headers = new Headers({ 'x-request-id': 'req_123' });
+
+  test('extracts error from standard OpenAI error response', () => {
+    const err = APIError.generate(
+      422,
+      { error: { message: 'Invalid model', type: 'invalid_request_error', code: 'model_not_found' } },
+      undefined,
+      headers,
+    );
+    expect(err).toBeInstanceOf(UnprocessableEntityError);
+    expect(err.message).toContain('Invalid model');
+    expect(err.code).toBe('model_not_found');
+    expect(err.type).toBe('invalid_request_error');
+  });
+
+  test('falls back to entire response body when "error" field is missing', () => {
+    const err = APIError.generate(
+      422,
+      { detail: '422: The model gpt-5-gibberish does not exist.' },
+      undefined,
+      headers,
+    );
+    expect(err).toBeInstanceOf(UnprocessableEntityError);
+    expect(err.message).not.toContain('(no body)');
+    expect(err.message).toContain('gpt-5-gibberish');
+  });
+
+  test('falls back to response body with message field (non-standard)', () => {
+    const err = APIError.generate(
+      400,
+      { message: 'Something went wrong', code: 'bad_request' },
+      undefined,
+      headers,
+    );
+    expect(err).toBeInstanceOf(BadRequestError);
+    expect(err.message).toContain('Something went wrong');
+  });
+
+  test('still shows "(no body)" when response is truly empty', () => {
+    const err = APIError.generate(422, undefined, undefined, headers);
+    expect(err.message).toContain('(no body)');
+  });
+
+  test('uses errMessage string fallback when JSON parsing fails', () => {
+    const err = APIError.generate(422, undefined, 'raw error text', headers);
+    expect(err.message).toContain('raw error text');
+  });
+});