fix: improved ax generate error

Author: dosco

package-lock.json

@@ -634,6 +634,31 @@ describe('Error handling in AxGen', () => {
     }
   });
 
+  it('should serialize cause properly in JSON', async () => {
+    const originalError = new Error('AI service failed');
+    const ai = new AxMockAIService({
+      features: { functions: false, streaming: false },
+      chatResponse: () => Promise.reject(originalError),
+    });
+
+    const gen = new AxGen<{ userQuestion: string }, { modelAnswer: string }>(
+      signature
+    );
+    try {
+      await gen.forward(ai, { userQuestion: 'test' });
+      throw new Error('Test should have failed but did not.');
+    } catch (e) {
+      const error = e as Error;
+      const serialized = JSON.parse(JSON.stringify(error));
+      expect(serialized.name).toBe('AxGenerateError');
+      expect(serialized.message).toBe('Generate failed: AI service failed');
+      expect(serialized.cause).toBeDefined();
+      expect(serialized.cause.name).toBe('Error');
+      expect(serialized.cause.message).toBe('AI service failed');
+      expect(serialized.details).toBeDefined();
+    }
+  });
+
   it('should handle streaming errors gracefully', async () => {
     const originalError = new Error('Streaming failed mid-stream');
     // Create a stream that errors after first chunk

src/ax/dsp/generate.test.ts

@@ -1492,6 +1492,23 @@ export class AxGenerateError extends Error {
       (this as ErrorOptions).cause = options.cause;
     }
   }
+
+  toJSON(): Record<string, unknown> {
+    const cause = (this as ErrorOptions).cause;
+    return {
+      name: this.name,
+      message: this.message,
+      details: this.details,
+      cause: cause
+        ? {
+            name: cause.name,
+            message: cause.message,
+            stack: cause.stack,
+          }
+        : undefined,
+      stack: this.stack,
+    };
+  }
 }
 
 function enhanceError(
@@ -1537,7 +1554,11 @@ function enhanceError(
   };
 
   // Return custom error with short message and details as object property
-  return new AxGenerateError('Generate failed', details, {
-    cause: originalError,
-  });
+  return new AxGenerateError(
+    `Generate failed: ${originalError.message}`,
+    details,
+    {
+      cause: originalError,
+    }
+  );
 }