API Response Errors in Vibecoded Apps

Open the browser Network tab and monitor API calls as you use the app. Look for responses with unexpected status codes — a 200 OK that contains an error message, a 500 that returns no body, or a 404 for a resource that should exist. AI backends often return 200 for everything and embed error information in the response body in inconsistent formats.

Check response shapes for consistency. The same endpoint might return an array when data exists but null or an empty object when it does not, causing the frontend to crash when it calls .map() on a non-array value. Vibecoded APIs frequently have these shape inconsistencies.

Trigger every API endpoint through normal app usage and record the responses. Then try edge cases: request a resource with an ID that does not exist, send a request with missing required fields, or submit data that exceeds expected limits. AI-generated backends rarely handle these scenarios gracefully.

Test concurrent requests. Submit a form while another request is in flight. Make rapid sequential requests to the same endpoint. These patterns expose race conditions and inconsistent responses that only occur under realistic usage patterns.

Define a consistent response schema and use it across all endpoints. Every response should have a predictable structure with proper HTTP status codes. Validate request inputs with a schema library like Zod on the backend before processing them.

Add response validation on the frontend as well. Parse API responses through a schema validator before using the data. This catches shape mismatches early and provides clear error messages instead of cryptic runtime crashes. Never trust that the API response will match the expected shape in a vibecoded app.