510 Not Extended

Further extensions to the request are required for the server to fulfill it, and the response describes which extensions are needed.

Defined in RFC 2774 §7

What 510 means

HTTP 510 Not Extended, defined in RFC 2774, means the server requires further extensions to the request before it can fulfill it, and the response body is expected to describe what extension is needed and how to apply it. It belongs to the HTTP Extension Framework, a mechanism from the late 1990s that let clients and servers negotiate optional, mandatory-to-understand extensions to standard HTTP methods using a mandatory extension header, layered on top of ordinary requests without requiring a whole new protocol version.

The HTTP Extension Framework never achieved meaningful adoption, and as a result 510 is one of the most obscure codes in the registry, essentially unseen in production traffic on the modern web. Some hosting providers and older Apache modules historically repurposed 510 informally to signal that a resource requires a paid plan, quota increase, or additional account extension before continuing, but this is a nonstandard reuse of the code rather than its literal RFC 2774 meaning, and should not be relied upon by API consumers expecting the original semantics.

Common causes

A client sends a request using the HTTP Extension Framework's mandatory extension header that the server does not support or requires additional parameters for.
A hosting platform has repurposed 510 nonstandardly to indicate an account, plan, or quota extension is needed to continue.
Legacy middleware built around RFC 2774 extension negotiation is still active and rejecting requests that omit a now-required extension.

How to fix a 510

If you are the client (browser user or API caller)

Check the response body for the specific extension the server says is required, since RFC 2774 expects the server to describe it explicitly.
If the 510 comes from a hosting provider using it nonstandardly, check account or billing settings rather than looking for a literal HTTP extension.

If you run the server

If genuinely using the HTTP Extension Framework, document the required extension clearly in the 510 response body so clients can act on it.
If 510 is being reused nonstandardly for account or quota messaging, prefer a clearer, more conventional status such as 402 or 403 with a descriptive body instead.

Example

POST /resource HTTP/1.1
Host: legacy.example.com
Man: "http://example.com/ext/audit"; ns=1

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

This request requires the "audit" extension, which was not applied.

A server built around the HTTP Extension Framework rejects a request missing a mandatory extension it requires.

Try it live

Our free status responder returns a real HTTP 510 you can point tests, monitors or a browser at.

GET https://mcp.httpstatus.com/status/510