HTTP 501 Not Implemented means the server does not recognize or support the HTTP method used in the request. This is different from 405 Method Not Allowed — 501 means the server does not support the method at all, while 405 means the method is not allowed for that specific resource.
Status
501 Not ImplementedExample
PATCH /contact HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/htmlHTTP/1.1 501 Not Implemented
Content-Type: text/html; charset=utf-8
Allow: GET, POST
Content-Length: 176
<!doctype html>
<html lang="en">
<head>
<title>501 Not Implemented</title>
</head>
<body>
<h1>Not Implemented</h1>
<p>The server does not support the request method used.</p>
</body>
</html>How to Troubleshoot
Use a supported HTTP method
Try using GET or POST instead of the method that caused the error.
Check server capabilities
Send an OPTIONS request to discover which methods the server supports.
curl -X OPTIONS -v https://example.com/api/resource
Common Causes
Unsupported HTTP method
highThe server received a request with an HTTP method it does not implement (e.g., PATCH on a server that only supports GET and POST).
Feature not yet implemented
mediumThe endpoint exists but the functionality behind it has not been built yet.
Related Status Codes
References
| Specification | Section |
|---|---|
| HTTP Semantics | RFC 9110 §15.6.2 |
Frequently Asked Questions
This reference was compiled from official RFCs, protocol specifications, and hands-on troubleshooting experience. AI tools were used primarily for formatting and organizing the content on the page.