API Validation and Error Responses

API validation should distinguish malformed syntax, unsupported media types, invalid fields, and conflicts with current resource state.

Use 400 for malformed request syntax, 415 for an unsupported representation format, 422 for semantically invalid content, and 409 for state conflicts. Return stable Problem Details with field-level extensions rather than leaking parser internals.

HTTP/1.1 422 Unprocessable Content
Content-Type: application/problem+json

{"type":"https://api.example.com/problems/validation","status":422,"invalidParams":[{"name":"email","reason":"invalid format"}]}