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510 Not Extended

Obsolete: the request needs further extensions (RFC 2774 HTTP Extension Framework).

510 at a glance

Code
510
Name
Not Extended (IANA: Not Extended (OBSOLETED))
Class
5xx Server error
Status
Registered, obsolete or deprecated
Defined in
RFC 2774, Section 7
Cacheable by default
No by default. Caches store it only when the response carries explicit freshness (Cache-Control max-age, s-maxage or Expires).

What 510 means

510 Not Extended belonged to the HTTP Extension Framework (RFC 2774), where a server could demand that the client use a mandatory extension. The framework never caught on and the IETF moved it to Historic status, so IANA marks 510 as obsoleted. Do not use it in new software.

What typically causes a 510

  • Practically never sent; legacy experiments with RFC 2774.

Fix it on the server

  • Use a current mechanism instead, e.g. 426 with Upgrade, or a documented API error.

Fix it as a client or visitor

  • Treat as a generic 500.

Headers that go with 510

HeaderRole
Man / OptRFC 2774 extension declaration headers (historic).

Example response

HTTP/1.1 510 Not Extended
Content-Type: text/plain

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