HTTP 303 See Other vs 501 Not Implemented
HTTP 303 (See Other) is a 3xx Redirection response, while 501 (Not Implemented) is a 5xx Server Error response. 303 indicates that the server is redirecting to a different resource using GET, typically after a POST operation (Post/Redirect/Get pattern). In contrast, 501 means that the server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
Description
The server is redirecting to a different resource using GET, typically after a POST operation (Post/Redirect/Get pattern).
When You See It
After form submissions to prevent resubmission on browser refresh.
How to Fix
Follow the Location header with a GET request. This is intentional — part of the PRG pattern.
Description
The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
When You See It
When using an HTTP method (like PATCH) that the server doesn't support.
How to Fix
Use a different HTTP method that the server supports. Check server documentation.
Key Differences
303 is a 3xx Redirection response, while 501 is a 5xx Server Error response.
HTTP 303: The server is redirecting to a different resource using GET, typically after a POST operation (Post/Redirect/Get pattern).
HTTP 501: The server does not support the functionality required to fulfill the request. The server either does not recognize the request method or lacks the ability to fulfill it.
You encounter 303 when after form submissions to prevent resubmission on browser refresh.
You encounter 501 when when using an HTTP method (like PATCH) that the server doesn't support.
When to Use Which
For 303 (See Other): Follow the Location header with a GET request. This is intentional — part of the PRG pattern. For 501 (Not Implemented): Use a different HTTP method that the server supports. Check server documentation.