HTTP 303 See Other vs 426 Upgrade Required
HTTP 303 (See Other) is a 3xx Redirection response, while 426 (Upgrade Required) is a 4xx Client 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, 426 means that the server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
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 refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
When You See It
When a server requires HTTPS or a newer protocol version.
How to Fix
Switch to the protocol specified in the Upgrade response header.
Key Differences
303 is a 3xx Redirection response, while 426 is a 4xx Client Error response.
HTTP 303: The server is redirecting to a different resource using GET, typically after a POST operation (Post/Redirect/Get pattern).
HTTP 426: The server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
You encounter 303 when after form submissions to prevent resubmission on browser refresh.
You encounter 426 when when a server requires HTTPS or a newer protocol version.
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 426 (Upgrade Required): Switch to the protocol specified in the Upgrade response header.