HTTP 501 Not Implemented: What It Means and When You See It
Quick Answer
HTTP 501 Not Implemented means the server does not support the functionality required to fulfill the request. Common for unimplemented HTTP methods.
HTTP 501 Not Implemented
HTTP 501 means the server does not support the HTTP method used in the request. This is different from 405 (method not allowed for this resource) - 501 means the server globally does not support the method. In practice, 501 is rare: most servers support GET, POST, PUT, DELETE, PATCH, HEAD, and OPTIONS. You might see 501 if a client sends a non-standard HTTP method the server does not recognize.
When 501 Appears
Developers may return 501 for planned-but-not-yet-built endpoints: res.status(501).json({ error: "Not implemented yet" }). This is more informative than 404 for API endpoints that exist in the spec but have not been built. Proxy servers may return 501 when they cannot forward an unusual HTTP method. In REST APIs, 501 signals "this method is recognized but not yet supported."
Try the interactive tool
Convert any value instantly — no sign-up required
Frequently Asked Questions
Related Values
100
HTTP 100 Continue means the server has received the request headers and the client should proceed to send the request body. It is an interim response used to inform the client to continue.
101
HTTP 101 Switching Protocols indicates the server is switching to the protocol specified in the Upgrade header field. Commonly used when upgrading to WebSocket connections.
200
HTTP 200 OK is the standard success response. The request has succeeded and the server has returned the requested resource in the response body.
201
HTTP 201 Created means the request succeeded and a new resource was created as a result. The Location header typically points to the new resource URL.