510

Not Extended

5xx Server Error

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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

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

Examples

Server rejects mandatory extension request

Request:M-GET /document with C-Man extension metadata
Response:510 Not Extended with error: required_extension_not_supported

The server cannot honor the declared mandatory extension and tells the client the extension contract must change.

References

Related 5xx Server Error Codes