HTTP 426 Upgrade Required vs 431 Request Header Fields Too Large
Both HTTP 426 (Upgrade Required) and 431 (Request Header Fields Too Large) belong to the 4xx Client Error category. 426 indicates that the server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol. Meanwhile, 431 means that the server refuses to process the request because an individual header field or all headers collectively are too large.
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.
Description
The server refuses to process the request because an individual header field or all headers collectively are too large.
When You See It
When cookies accumulate to an excessive size, or a single header (like Authorization) is too long.
How to Fix
Reduce header sizes. Clear excessive cookies. Use shorter tokens.
Key Differences
HTTP 426: The server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
HTTP 431: The server refuses to process the request because an individual header field or all headers collectively are too large.
You encounter 426 when when a server requires HTTPS or a newer protocol version.
You encounter 431 when when cookies accumulate to an excessive size, or a single header (like Authorization) is too long.
When to Use Which
For 426 (Upgrade Required): Switch to the protocol specified in the Upgrade response header. For 431 (Request Header Fields Too Large): Reduce header sizes. Clear excessive cookies. Use shorter tokens.