Not Implemented

5xx Server Error

Last updated: July 19, 2026

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

Description

Different from 405: 405 says 'this URL doesn't support this method'; 501 says 'this server doesn't understand this method at all'.

Return 501 for unsupported HTTP methods like TRACE or CONNECT if you don't implement them.

RFC 9110 §15.6.2

HTTP response

HTTP/1.1 501 Not Implemented
Content-Type: application/json

{"error":"Not Implemented"}

Node.js (Express)

res.status(501).json({ error: "Not Implemented" });

Go (net/http)

w.WriteHeader(http.StatusNotImplemented)

Python (Flask)

return jsonify({"error": "Not Implemented"}), 501

Method Not Allowed

The HTTP method is known but not supported for this specific URL.

Bad Gateway

The server, acting as a gateway or proxy, received an invalid response from the upstream server.