An HTTP 501 Not Implemented response indicates that the server does not support the functionality required to fulfill the request.

HTTP 501 at a glance

  • Status: 501
  • Phrase: Not Implemented
  • Class: 5xx Server Error
  • Specification: RFC 9110 Section 15.6.2
  • Origin: Web server or proxy protocol parser
  • Typical context: Unrecognized HTTP methods or unsupported protocol extensions
  • Client retry: Unsafe without changing the method or protocol mechanism

What a 501 response tells you

When a client receives an HTTP 501 status code, specific protocol facts are established:

  1. The server software does not support the requested method: The server cannot recognize or handle the method across any route on the server.
  2. The error is at the server capability layer: It is not a syntax error or a resource lookup failure; the server fundamentally lacks the software capability to process the action.

The critical difference: 501 vs 405

  • HTTP 405 Method Not Allowed: The server recognizes the method globally, but the specific target resource URL does not permit it (e.g., sending POST to a read-only endpoint). The server must return an Allow header.
  • HTTP 501 Not Implemented: The server does not recognize or support the method anywhere on the server.

For a direct comparison, see our guide on 405 vs. 501.

Similar status codes

405 Method Not Allowed

The method is recognized by the server software but not supported on the target resource.

500 Internal Server Error

A generic server-side error indicating an unhandled code crash.

For more details, see our reference on HTTP 405 Method Not Allowed.

Diagnostic scenarios

Scenario 1: Using proprietary proxy cache purge methods

A developer attempts to purge an edge cache entry using PURGE /api/resource. When sent directly to an origin server that lacks a custom purge handler, the web server returns 501.

Scenario 2: WebDAV methods sent to standard HTTP server

A client issues WebDAV commands (PROPFIND, MKCOL, LOCK) against a standard NGINX or Apache server that has not enabled WebDAV modules.

Common causes of HTTP 501

Unrecognized or custom HTTP method sent to server

The client transmitted an esoteric, non-standard, or custom HTTP method (such as PURGE, PROPFIND, or LINK) that the web server software does not recognize.

Proxy or gateway unable to support requested protocol feature

An intermediary proxy or load balancer does not implement the protocol features required to forward the request to the upstream origin.

Web server missing required extension module

The requested operation requires a server module (e.g., WebDAV module for Apache/NGINX) that is not installed or enabled in the server configuration.

Stubbed endpoint in development environment

A newly designed API route has been scaffolded but the underlying implementation has not yet been written, returning 501 as a placeholder.

Headers you may encounter

  • Content-Type: Describes the problem details (application/problem+json).
  • Date: Timestamp of the response.

Troubleshooting flow

HTTP 501 Not Implemented
  |
  +-- What HTTP method was used in the request?
        |
        +-- Non-standard verb (PURGE, PROPFIND, etc.):
        |     -> Switch to standard verbs or enable server extensions.
        |
        +-- Standard verb (GET, POST, etc.):
              -> Check reverse proxy capabilities or stubbed API endpoints.

What to check before changing backend code

  1. Verify the exact HTTP verb sent by the client.
  2. Check if a standard REST method (POST, DELETE, PATCH) should be used instead.
  3. Review web server module configurations for WebDAV or custom method support.
  4. Confirm whether the endpoint is an unfinished development stub.

How to verify the fix

Test the endpoint with curl using standard HTTP verbs:

curl -i -X POST https://api.example.test/cache/users/123 \
  -H "Authorization: Bearer test-token" \
  -H "Content-Type: application/json" \
  -d '{"action": "invalidate"}'

Ensure the server returns 200 OK or 204 No Content when standard verbs are used.

Remediation paths to evaluate

If using custom HTTP methods

Refactor the API to use standard HTTP methods with explicit payload actions (e.g., POST /api/cache/purge).

If enabling WebDAV

Install and enable the http_dav_module in NGINX or mod_dav in Apache.

FAQ

Can a client retry a 501 response?

No. Retrying without changing the method will result in the same 501 error because server software capabilities remain unchanged.

Does 501 require an Allow header?

No. Only 405 Method Not Allowed requires an Allow header.

Key takeaway

HTTP 501 Not Implemented indicates that the server software does not recognize or support the requested HTTP method globally. Switch to standard HTTP methods or install required protocol extensions.