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510 Not Extended

Obsolete. The server required further extensions to the request before it could fulfil it.

Status

HTTP/1.1 510 Not Extended

Details

  • Category: 5xx
  • Status: Deprecated. Registered, but the defining specification marks it obsolete.
  • Specification: RFC 2774 §7
  • Cacheable: Not cacheable
  • Response body: Allowed
  • Retry: Retry only after changing the request

510 came from the HTTP Extension Framework, an experimental mechanism for negotiating protocol extensions per request. The server would return 510 with the extensions it required, and the client would resend with them declared.

The framework was never adopted and the code is listed as obsoleted in the IANA registry. Any 510 in modern traffic is a misuse of the number rather than the specified behavior.

Common causes

  • A server implementing the obsolete HTTP Extension Framework.
  • An application repurposing the number for its own signalling.

How to fix it

As the client

  • Read the response body, since the standard meaning almost certainly does not apply.

As the server

  • Use a registered code. 400 or 428 covers the missing-information case in a way clients understand.

Examples

An obsolete extension negotiation

HTTP request

GET /resource HTTP/1.1
Host: legacy.example.com

HTTP response

HTTP/1.1 510 Not Extended

Specifications

See also