Not Implemented

Overview

501 Not Implemented means the server does not support the functionality needed to fulfill the request. It is returned for a request method the server does not recognize, or for a feature that simply has not been built yet.

It looks similar to 405 Method Not Allowed but differs: 405 means "the method is known but not allowed for this resource," while 501 means "the server does not implement that functionality/method at all" — a server-side limitation.

When it happens

• Using an HTTP method the server does not support (e.g. a rare extension method)
• Calling an endpoint or feature that has not been developed yet
• A proxy or gateway cannot relay a particular capability

Request / Response example

PATCH /api/resource HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"x":1}

HTTP/1.1 501 Not Implemented
Content-Type: text/plain

The server does not support the functionality required to fulfill this request.

In code

app.all('/api/experimental', (req, res) => {
  res.status(501).send('Not Implemented');
});

# Reject methods the server does not implement
if ($request_method !~ ^(GET|POST|HEAD)$) {
  return 501;
}

Notes

• 501 means the server does not support it at all — it is not a transient outage, so retrying fails the same way.
• By spec a 501 response is cacheable by default, so mind your cache headers if you plan to implement the feature later.

Related status codes

Related headers

Specification