510

Not Extended

5xx Server Error

ELI5

Your request needs additional extensions or features that haven't been negotiated or enabled yet.

Server perspective

Use 510 when the request depends on a mandatory HTTP extension or extension declaration that the server cannot satisfy.

When to use

  • Return 510 for explicit extension-policy failures, especially in systems that rely on mandatory HTTP extensions
  • Use it when the request must be extended or renegotiated before the server can process it
  • A mandatory HTTP extension declared or required by the request is not supported
  • The server cannot process the request unless additional extension information is supplied
  • Specialized extension-based protocols built on HTTP require explicit negotiation

How to respond

  • Describe which extension or extension declaration is missing or unsupported so advanced clients can recover
  • Validate that the failure is about extension policy, not a generic unsupported method or malformed request

Headers to consider

  • No status-specific header is required; if your extension model uses hop-by-hop metadata, document the expected Connection behavior clearly

Response body

  • A body is recommended; identify the required or unsupported extension and the expected negotiation contract

Server-side pitfalls

  • Do not use 510 for mainstream API feature gaps that are better represented as 400, 404, 405, or 501
  • Do not assume intermediaries preserved extension-related hop-by-hop headers unless you validated that path
  • Standard HTTP operations (use other appropriate codes)
  • Method not implemented (use 501 Not Implemented)
  • Feature not available for reasons unrelated to HTTP extension policy (use 501 Not Implemented or another more specific status)

Examples

Missing required extension

Request:GET https://api.example.test/resource
Response:510 Not Extended # Headers Content-Type: application/json # Body { "error": "not_extended" }

Server requires specific HTTP extension to be negotiated first

References

Related Status Codes