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5xx Server Error
What it means
The server doesn't support the functionality required to fulfill the request.
| Property | Detail |
|---|---|
| Status class | 5xx Server Error |
| Defined in | RFC 9110, Section 15.6.2 |
| Cacheable by default | Yes. RFC 9110 lists it as heuristically cacheable, so a cache may reuse it without explicit freshness headers unless Cache-Control says otherwise. That suits a code that describes a lasting limitation rather than a momentary failure. |
| Who needs to act | The client should use a supported method; the server owner can add support. |
What does HTTP 501 Not Implemented mean?
HTTP 501 Not Implemented means the server does not support the functionality needed to fulfil the request, typically because it does not recognise the HTTP method at all. Unlike 405, which says a method is not allowed on one particular URL, 501 means the server cannot handle that method for any resource. Retrying will not help.
How 501 Not Implemented works
A 501 sits in the 5xx class because it describes a limitation of the server, but in practice it is a lasting answer to one kind of request rather than a crash. RFC 9112 adds two specific uses in HTTP/1.1: a server that receives a method longer than any it implements should answer 501, and so should a server that receives a Transfer-Encoding it does not understand. Because RFC 9110 marks 501 as heuristically cacheable, clients and caches may remember it, which is reasonable for an answer that will not change until the server is upgraded.
Example 501 request and response
PURGE /index.html HTTP/1.1
Host: www.example.com
HTTP/1.1 501 Not Implemented
Content-Type: text/htmlPURGE is a non-standard method that cache servers such as Varnish are often configured to use for evicting content. A web server that has never heard of it answers 501, whatever the URL.
What causes a 501 Not Implemented
- A request using a method the server does not implement, such as a custom or non-standard method.
- A Transfer-Encoding the server cannot decode.
- Minimal servers and embedded devices with a small HTTP feature set.
- Placeholder API endpoints that deliberately return 501 until a feature ships.
How to fix a 501 Not Implemented
If you sent the request
- Check which methods the server supports, in the API documentation or with an OPTIONS request.
- Remove unusual Transfer-Encoding headers added by custom client code.
- If a proxy sits in between, confirm it passes the method through instead of rejecting it.
If you run the server
- Return 405 with an Allow header when the method exists but not for this URL, and keep 501 for methods you do not support anywhere.
- Enable the module or handler the method needs, such as WebDAV, if you mean to support it.
- Do not use 501 for features that are simply broken; that is a 500.
Related status codes
- 405 Method Not Allowed: The server knows the method, but this URL does not accept it.
- 500 Internal Server Error: An unexpected failure, not a missing feature.
- 505 HTTP Version Not Supported: The protocol version, rather than the method, is not supported.
Frequently asked questions
The server doesn't support the functionality required to fulfill the request.
The client requested an HTTP method or feature the server hasn't implemented.
Check the API documentation to confirm the endpoint and method are actually supported.
Because the limitation belongs to the server: the method is valid HTTP, but this server has not implemented it. A different server could accept the same request, which is why it is not classed as a client error like 405.
Sources
Other 5xx Server Error codes
Reviewed by Raja Jahangir · Last reviewed: October 7, 2026