Formalint

API reference

HTTP Status Codes

Search status codes, filter by response class and read practical debugging notes for the codes developers meet during API reviews and production incidents.

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Common API status codes

Status class
API Debugging ChecklistTrack a failing request HTTP HeadersInspect CORS and cache clues JSON Schema GeneratorReview payload contracts JWT DecoderInspect auth claims Timestamp ConverterCompare log timestamps

Reference notes

How to use status codes during debugging

Do not stop at the number

A status code is a signal, not a full diagnosis. Pair it with the request method, route, request ID, environment, user role and payload shape before deciding what changed.

Keep client and server causes separate

4xx responses usually point to request identity, authorization or payload contract problems. 5xx responses point to server-side handling, dependency failure or unexpected runtime behavior.

Write useful error responses

Good APIs return a stable code, a clear machine-readable error key and a human-readable message that helps the caller fix the next request without exposing secrets.