HTTP 501 Not Implemented

What It Means

The HTTP 501 Not Implemented status code indicates that the server does not support the functionality required to fulfill the request. This is typically used when the server doesn't recognize the request method at all.

501 vs 405

405: "I know this method, but it's not allowed on this resource"
501: "I don't support this method at all on any resource"

When to Use 501

Unrecognized HTTP methods: Custom method server doesn't handle
WebDAV methods: PROPFIND, MKCOL, COPY, MOVE on non-WebDAV server
Future HTTP methods: New methods not yet implemented
Proxy limitations: Proxy can't handle certain methods

Example Response

PROPFIND /resource HTTP/1.1
Host: example.com

---

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

{
  "error": "Not Implemented",
  "message": "The PROPFIND method is not supported"
}

Comparison Table

Scenario	Status
DELETE on /users/:id (not allowed)	405
PROPFIND on any endpoint	501
Custom "PURGE" method	501
PUT on read-only endpoint	405

Implementation

Express.js

const supportedMethods = ['GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS'];

app.use((req, res, next) => {
  if (!supportedMethods.includes(req.method)) {
    return res.status(501).json({
      error: 'Not Implemented',
      message: `The ${req.method} method is not supported`
    });
  }
  next();
});

Nginx

# Nginx returns 501 for methods it doesn't understand by default
# For custom handling:
if ($request_method !~ ^(GET|POST|PUT|DELETE|OPTIONS)$ ) {
    return 501;
}

Common HTTP Methods

Standard Methods (should NOT return 501):

GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS

WebDAV Methods (may return 501):

PROPFIND, PROPPATCH, MKCOL, COPY, MOVE, LOCK, UNLOCK

Best Practices

Most REST APIs should handle standard HTTP methods (use 405, not 501)
Return 501 for truly unsupported methods like WebDAV
Include a clear message about what is not supported
Consider if you actually mean 405 instead