HTTP 510 Not Extended (RFC 2774) indicates that further extensions to the request are required for the server to fulfill it. The server is saying 'I understand your request but I need additional protocol extensions to handle it.' This is extremely rare — RFC 2774 (HTTP Extension Framework) was experimental and never widely adopted. You're unlikely to encounter 510 in production systems.
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/510
Example request:
curl -i "https://httpstatus.com/api/status/510"