The HTTP Status Code 501 means that the server either does not recognize the request method, or it lacks the ability to fulfill the request method made by the client.
The 501 status code will typically imply a future availability, such as a new service that will be available in the future.
Frequently Asked Questions
What does the HTTP 501 status code mean?
HTTP 501 Not Implemented is a server-side error that means the server either does not recognize the request method or lacks the ability to fulfill it. Unlike client errors, the problem lies entirely with the server's capabilities, not the request itself.
When does a server return a 501 status code?
A server returns 501 when it receives an HTTP method it has no handler for, such as a PATCH or PROPFIND request sent to a server that was only built to handle GET and POST. It can also occur when a server lacks the required modules or configuration to support a particular method on any resource.
What is the difference between HTTP 501 and HTTP 405?
A 405 Method Not Allowed means the server recognizes the HTTP method but does not permit it for the specific resource being requested. A 501 Not Implemented means the server does not recognize or support the method at all, regardless of the resource.
Is a 501 error temporary or permanent?
A 501 error typically indicates a permanent limitation — the server does not support the requested functionality. RFC 9110 does not characterize 501 as a temporary condition, and there is no standard basis for including a Retry-After header in a 501 response.
Can a 501 response be cached?
Yes, a 501 response is cacheable by default unless caching headers in the response instruct otherwise. This means intermediary caches and clients may store and reuse the response without re-contacting the server.
How do I fix an HTTP 501 error?
If you are a developer calling an API, verify that you are using an HTTP method the server explicitly supports and that your request is correctly formed. If you operate the server, check that the required server modules are installed and enabled, and that handler mappings are correctly configured for the methods your application needs to support.
