Not Extended

Overview

510 Not Extended means the server needs an additional extension to process the request, but that extension was not declared in the request. It is defined by the experimental HTTP Extension Framework (Mandatory Extensions) in RFC 2774.

That framework was never widely adopted and RFC 2774 remained Experimental, so 510 is almost never used on the modern web.

When it happens

• The request lacks an HTTP extension declaration the server requires
• In a niche setup using the Mandatory Extensions framework, the extension is missing
• Accessing a server that enforces an experimental/private protocol extension

Request / Response example

GET /resource HTTP/1.1
Host: example.com

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

The request must include a required extension declaration.

In code

app.get('/resource', (req, res) => {
  if (!req.headers['c-ext']) {
    return res.status(510).send('Not Extended: required extension missing');
  }
  // ...
});

Notes

• RFC 2774 is an experimental standard that is effectively unused, so you will rarely encounter 510 in practice.
• Some systems repurpose the code non-standardly, so if you see it, the fastest path is to check that specific service's documentation.

Related status codes

Related headers

Specification