422 Unprocessable Entity

HTTP 422 Unprocessable Entity (originally WebDAV, now widely used in REST APIs) indicates the server understood the request and the syntax is correct, but the contained instructions cannot be processed due to validation errors.

REST API validation failures where 400 (syntactic) is too generic — e.g., email format valid but already taken, password too short, age out of range.

• Email already registered
• Password fails complexity rules
• Date in the past for a future-only field
• Foreign-key reference to non-existent record
• Business-rule violation

• Inspect the response body for detailed error fields
• Display field-level validation errors to the user
• Pre-validate on the client to reduce round-trips

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/json

{"errors":{"email":"already taken"}}