HTTP Status Codes
HTTP 501 Not Implemented
Not Implemented — the server does not support the HTTP method used in the request
What 501 Means
The 501 error on the HTTP Status Codes indicates not implemented — the server does not support the http method used in the request. This typically occurs due to client sends an http method such as patch or trace that the server does not implement.
501 indicates a permanent limitation of the server — the feature or method requested is not supported. It differs from 405 which means the method is recognized but not permitted for that specific resource.
Technical Background
A 501 response means the server either does not recognize the request method or lacks the ability to fulfill it. This is distinct from 405 Method Not Allowed, which means the method is recognized but not permitted for that specific resource.
Unlike 404 or 500, a 501 indicates a fundamental server capability gap. Clients should consult server documentation and switch to a supported method.
Common Causes
- Client sends an HTTP method such as PATCH or TRACE that the server does not implement
- Attempting to use a newer HTTP feature against a legacy server
- Incorrect HTTP method in a programmatic API call
Typical Scenarios
- API client sending PATCH requests to a server that only supports GET and POST
- Webhooks using HTTP methods not implemented by the receiving endpoint
- Legacy application server receiving modern HTTP method extensions
What to Know
A 501 is a permanent server-side capability limitation, not a transient error. Unlike 405, which restricts a method on a specific resource, 501 signals the server has no implementation for the method at all.
Frequently Asked Questions
Common questions about HTTP 501 error
501 means the server does not implement the method at all. 405 means the server understands the method but that specific resource does not allow it.