Q: A critical third-party payment API your app relies on starts returning 200 OK, but the JSON payload is silently empty `{}`, causing your app logic to fail downstream. Your standard `HTTP 5xx` alerts didn't fire. How do you monitor for this?

Standard infrastructural monitoring only cares about HTTP status codes. To catch semantic/logic errors from third parties, you must implement Business Metric Alerting or payload validation.

• App-level Metrics: The application code should explicitly parse the payment response. If it's missing expected fields, it should increment a custom Prometheus counter like vendor_payment_payload_errors_total. I can alert when this counter spikes.
• Synthetic Monitoring: Run an automated script every minute that makes a real payment request, explicitly asserts the presence of the expected JSON keys in the response body, and alerts immediately if the assertion fails, regardless of the 200 OK status code.