501
Server Error
501 Not Implemented: Unsupported Functionality
✦ The Golden Answer: The 501 Not Implemented status code means the server does not support the functionality required to fulfill the request. This typically occurs when the client uses an HTTP method that the server does not support (e.g., PUT, DELETE, PATCH on a server that only supports GET and POST), or when a server extension or feature is not available (e.g., WebDAV, PROPFIND). Unlike 405 (Method Not Allowed), which says the method is recognized but not allowed for the resource, 501 says the method is not recognized at all.
Common Causes of 501 Errors
| Cause | Example | How to Fix |
|---|---|---|
| Unsupported HTTP method | Client sends PATCH, but server only supports GET and POST. | Use a supported method, or upgrade the server to support the method. |
| Missing server module | WebDAV module not installed; client sends PROPFIND. | Install and enable the required module (e.g., mod_dav for Apache). |
| Unsupported extension | Client requests a feature that the server does not implement (e.g., HTTP/2 or HTTP/3 upgrade). | Upgrade the server or disable the extension on the client. |
| Server misconfiguration | Server is configured to reject certain methods but returns 501 instead of 405. | Correct the configuration to return appropriate status codes. |
Example: 501 Response
A client sends a PATCH request to a server that does not support PATCH:
PATCH /api/users/123 HTTP/1.1
Host: example.com
Content-Type: application/json
{"name": "John"}
HTTP/1.1 501 Not Implemented
Content-Type: text/html
<html>
<body>
<h1>501 Not Implemented</h1>
<p>The PATCH method is not supported by this server.</p>
</body>
</html>
501 vs. 405 Method Not Allowed
| Status | Meaning | Server Knowledge |
|---|---|---|
| 501 Not Implemented | Server does not recognize the method. | Method is unknown or not supported at all. |
| 405 Method Not Allowed | Server recognizes the method but it is not allowed for this specific resource. | Method is known, but not allowed; server sends Allow header. |
How to Fix 501 Errors
- If you're a client developer: Check the API documentation for supported HTTP methods. Use only methods that the server supports. If the server is missing a module, request the server administrator to install it.
- If you're a server administrator: Ensure that required modules are installed and enabled. For Apache, check that
mod_rewrite,mod_dav, or other extensions are loaded. For Nginx, ensure that the necessary modules are compiled in. If the server does not support a method, configure it to return 405 instead of 501 when appropriate. - If you're an end‑user: This error is rare and usually indicates a misconfigured application. Try using a different method or contact the support team.
Preventing 501 Errors
- Document supported methods – clearly state which HTTP methods are supported in your API documentation.
- Implement fallbacks – if a method is not supported, return a clear error message.
- Use OPTIONS – clients can use OPTIONS to discover supported methods.
Frequently Asked Questions
Is 501 a client error or a server error?
It's a server error (5xx) because the server does not support the required functionality. The client's request may be valid, but the server is incapable of processing it.
Can 501 be used for unsupported content types?
No, unsupported content types should return 415 Unsupported Media Type (client error). 501 is for unsupported HTTP methods or server features.
What is the difference between 501 and 503?
501 means the server does not support the method or functionality. 503 means the server is temporarily unable to handle the request due to overload or maintenance.