API request/response mismatch: when one side changes first
An API request/response mismatch occurs when the payload a client sends or receives no longer matches what the server expects. The JSON is syntactically valid, but fields, types, or structures have changed.
Typical mismatch scenarios
- A mobile app keeps sending a deprecated field that the backend no longer reads.
- The server starts returning additional properties that older clients ignore or mishandle.
- Response schemas change shape (arrays, nested objects) without updating contract documentation.
Diagnosing mismatches with Inspectly
Paste the contract example and the real payload into the HTTP Contract Drift Inspector. It shows you where the shapes disagree: missing fields, extra fields, type mismatches, and structural differences.
For issues rooted in outdated or overly-permissive OpenAPI specs, pair this with the OpenAPI Silent Failure Validator to find silent failures.