HTTP 304 Not Modified vs 426 Upgrade Required
HTTP 304 (Not Modified) is a 3xx Redirection response, while 426 (Upgrade Required) is a 4xx Client Error response. 304 indicates that the resource has not been modified since the last request. The client can use its cached copy. 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 resource has not been modified since the last request. The client can use its cached copy.
When You See It
When the browser cache is still valid (If-None-Match / If-Modified-Since headers match).
How to Fix
No fix needed. This saves bandwidth by confirming the cached version is still current.
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
304 is a 3xx Redirection response, while 426 is a 4xx Client Error response.
HTTP 304: The resource has not been modified since the last request. The client can use its cached copy.
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 304 when when the browser cache is still valid (If-None-Match / If-Modified-Since headers match).
You encounter 426 when when a server requires HTTPS or a newer protocol version.
When to Use Which
For 304 (Not Modified): No fix needed. This saves bandwidth by confirming the cached version is still current. For 426 (Upgrade Required): Switch to the protocol specified in the Upgrade response header.