Rack updated its status codes to be in sync with IANA HTTP Status Code Registry : rack/rack#2137

The 422 status code is now Unprocessable Content instead of Unprocessable Entity. While it still works, it will be deprecated at some point. This currently results in a deprecation message when using the default config :

Status code :unprocessable_entity is deprecated and will be removed in a future version of Rack. Please use :unprocessable_content instead.

To reproduce this, try to log in with an invalid email/password on a Rails 8 app with Turbo.

This is easily fixable by devise user by setting config.responder.error_status = :unprocessable_content in the devise config.

However I think it would be nice to reflect this in the docs at least, and in the default generator when officially supporting Rails 8.