HTTP 428 Precondition Required (RFC 6585) indicates the server requires the request to include conditional headers (If-Match, If-Unmodified-Since). Unlike 412 Precondition Failed (where a condition was present but failed), 428 means no condition was provided at all. This enforces optimistic concurrency — the server demands the client prove it has the latest version before making changes, preventing accidental overwrites.
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/428
Example request:
curl -i "https://httpstatus.com/api/status/428"