What it means
Further extensions are required for the server to fulfill the request (HTTP Extension Framework). The framework is now historic.
Common causes
- Server requires a mandatory extension declaration the client did not send
How to fix it
As a client or visitor
Nothing practical to fix; the extension framework is obsolete.
As the site or API owner
Do not use 510 in new systems.
Example response
HTTP/1.1
HTTP/1.1 510 Not Extended
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
Content-Length: 0Specification
FAQ
› Is 510 still used?
Practically never; RFC 2774 has been moved to historic status.
› What replaced it?
Nothing specific; extensions are negotiated with ordinary headers.