? HTTP 501 – Not Implemented

? Quick Overview

The HTTP 501 Not Implemented status code means the server does not support the functionality required to fulfill the request. This usually indicates that the server recognizes the request method but has not implemented it.

? Key Concepts

It is a server-side error (5xx category)
Indicates missing or unsupported HTTP method
Different from 405 (Method Not Allowed)
Often returned by proxies or incomplete APIs

? Syntax / Theory

The server understands the request but cannot process it because the required method or feature is not implemented.

Typical response line:

HTTP/1.1 501 Not Implemented

? Code Example(s)

? View Code Example

// Example of an HTTP response with 501 status
HTTP/1.1 501 Not Implemented
Content-Type: application/json

{
  "error": "This HTTP method is not supported"
}

? View Code Example

// Fetch API example handling a 501 response
fetch("/api/legacy-endpoint", { method: "PATCH" })
.then(res => {
if (res.status === 501) {
console.log("Feature not implemented on server");
}
});

? Live Output / Explanation

What Happens?

If a client sends a request using an unsupported HTTP method (like PATCH or PROPFIND), the server may return a 501 Not Implemented response indicating the feature does not exist on the server.

? Diagram

?️ Interactive Simulation

Simulate a server that only understands GET and POST.

Method:

Waiting for request...

? Use Cases

API endpoints under development
Reverse proxies rejecting unsupported methods
Legacy servers with limited HTTP support
Incorrect client-side request configuration

✅ Tips & Best Practices

Ensure the HTTP method is supported by the server
Check API documentation before using advanced methods
Differentiate between 400, 405, and 501 errors
Log unimplemented requests on the server

? Try It Yourself

1. Send a request using an uncommon HTTP method to a public API.

2. Observe the status code returned.

3. Update the request to a supported method and retry.