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5015xx
5xx — Server Error

Not Implemented

The server does not support the functionality required to fulfill the request.

What it means

The HTTP 501 Not Implemented status code indicates that the server does not recognize the request method or lacks the capacity to fulfill it. This is not a temporary failure, but an explicit statement by the server that it fundamentally does not support the action requested by the client.

How it works

  • 1The client sends a request using an uncommon or advanced HTTP method (e.g., `PATCH`, `OPTIONS`, or a custom WebDAV method like `LOCK`).
  • 2The server parses the method but realizes its routing engine or protocol handler has no logic built to handle it.
  • 3The server returns a 501 status code, letting the client know it should not try this method again.

Why you're seeing this

  • • The client used a rare or invalid HTTP method.
  • • The server is a proxy or CDN that does not pass through specific HTTP extensions.

How to fix it

  • • If you are a developer, check if your routing framework is automatically blocking unhandled HTTP methods.
  • • Change the client-side code to use a widely supported alternative like `POST` or `PUT`.

Framework Implementations

Go (net/http)

func handler(w http.ResponseWriter, r *http.Request) {
    if r.Method != "GET" && r.Method != "POST" {
        http.Error(w, "Method not supported yet", http.StatusNotImplemented)
        return
    }
}

Real-Life Scenarios

Use this code when designing an API that intends to support certain HTTP methods or features in the future, but currently lacks the code implementation to do so.

Unimplemented HTTP Methods

A client attempts to send a `PATCH` request to an old legacy API that only supports `GET` and `POST`. The server returns a 501.

Experimental Feature Flags

An API client attempts to call a new GraphQL mutation or endpoint on a server variant that hasn't had that module compiled or enabled yet.

HTTP Transaction

1. Client Request

LOCK /documents/file.txt HTTP/1.1
Host: api.example.com

2. Server Response

HTTP/1.1 501 Not Implemented
Content-Type: application/json

{"error": "NotImplemented", "message": "This server does not support WebDAV locking methods."}

SEO Implications

This code rarely affects public SEO because standard search bots only ever use `GET` and sometimes `HEAD` requests. If a `GET` request returns a 501, bots will treat it exactly like a 500 error and de-index the page.

Related Codes

405500