HTTP 422 Unprocessable Entity means the request body is syntactically valid but semantically incorrect — the server understands the content type and syntax but cannot process the contained instructions. Originally a WebDAV status (RFC 4918), it's been adopted widely in REST APIs: valid JSON that fails business validation returns 422. FastAPI and many modern frameworks default to 422 for validation errors instead of 400.
Response includes the status code, standard headers (including Content-Type), and a small diagnostic JSON body describing the request and returned status.
Simulator URL (copy in the app after load — not a normal link):
https://httpstatus.com/api/status/422
Example request:
curl -i "https://httpstatus.com/api/status/422"