Not Extended

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