510 Not Extended was defined by the HTTP Extension Framework, RFC 2774, an experimental RFC from 2000. The framework let clients declare mandatory extensions with headers such as Man and Opt. A server answered 510 when the policy for accessing a resource required an extension that the request did not declare, and the response was supposed to contain the information needed to send an extended request.
The framework was never widely implemented. The IETF has since moved RFC 2774 to Historic status, and the IANA registry marks 510 as obsoleted. Do not use it in new systems.
Example response
HTTP
HTTP/1.1 510 Not Extended
Date: Fri, 06 Nov 2026 10:15:32 GMT
Content-Type: text/plain
Content-Length: 70
This resource requires a mandatory extension declaration (Man header).SEO and redirect checking
510 has no current use, and a page that returns it will not be indexed. If you see it in a check, the server runs very old or custom software.
Common causes and use cases
- Legacy software that implements the HTTP Extension Framework.
- A custom application that misuses 510 for its own purposes.
- A test endpoint that returns every registered status code.