Skip to content

501 Not Implemented

Description

The 501 Not Implemented status code indicates that the server does not support the functionality required to fulfill the request. This status code is appropriate when the server does not recognize the request method and is incapable of supporting it for any resource.

This response is commonly encountered when:

  • A client attempts to use an HTTP method that the server doesn't support
  • A client requests a feature or capability that hasn't been implemented yet
  • The server recognizes the request but cannot fulfill it due to missing functionality

A 501 response is different from a 405 Method Not Allowed status. A 405 indicates that the server knows the method but has chosen not to support it for the specific resource requested, while a 501 indicates that the server doesn't support the method at all for any resource.

Syntax

HTTP/1.1 501 Not Implemented
Content-Type: application/json
Content-Length: 172

{
  "error": {
    "code": 501,
    "message": "Not Implemented",
    "details": "The server does not support the functionality required to fulfill the request."
  }
}

Examples

Example 1: Unsupported HTTP Method

A client attempts to use the PATCH method on a server that only supports GET, POST, PUT, and DELETE.

Request: 

PATCH /api/resources/123 HTTP/1.1
Host: example.com
Content-Type: application/json
Content-Length: 42

{
  "partial_update": "some new value"
}

Response: 

HTTP/1.1 501 Not Implemented
Content-Type: application/json
Content-Length: 161

{
  "error": {
    "code": 501,
    "message": "Not Implemented",
    "details": "The PATCH method is not supported by this server."
  }
}

Example 2: Feature Not Yet Implemented

A client attempts to use a feature that is planned but not yet implemented in the API.

Request: 

GET /api/resources/export?format=pdf HTTP/1.1
Host: example.com
Accept: application/pdf

Response: 

HTTP/1.1 501 Not Implemented
Content-Type: application/json
Content-Length: 183

{
  "error": {
    "code": 501,
    "message": "Not Implemented",
    "details": "PDF export functionality is not currently implemented but is planned for a future release."
  }
}

Example 3: Protocol Extension Not Supported

A client attempts to use a protocol extension that the server doesn't support.

Request: 

GET /api/data HTTP/1.1
Host: example.com
Accept: application/json
X-Custom-Protocol-Extension: version=2.0

Response: 

HTTP/1.1 501 Not Implemented
Content-Type: application/json
Content-Length: 189

{
  "error": {
    "code": 501,
    "message": "Not Implemented",
    "details": "The requested protocol extension 'X-Custom-Protocol-Extension' is not supported by this server."
  }
}

Summary

The 501 Not Implemented status code indicates that the server does not support the functionality required to fulfill the request. It signals to the client that the server lacks the capability to process the request, not because of a temporary condition but due to a fundamental limitation in the server's implementation.

Key points about 501 Not Implemented:

  1. It indicates a permanent condition (unlike 503 Service Unavailable which is temporary)
  2. It's used when the server doesn't recognize or support the HTTP method for any resource
  3. It differs from 405 Method Not Allowed, which is used when a method is not allowed for a specific resource
  4. It can indicate functionality that is planned but not yet implemented
  5. The response should include information about which functionality is not implemented
  6. Servers should include an Allow header listing the supported methods when responding with 501
  7. Clients receiving this status should not retry the same request without modifications