We run these end-to-end tests:

• candidate -> candidate: Current compatibility of the release candidate
• candidate -> release: Backward compatibility
• release -> candidate: Forward compatibility

The Haskell code for these tests can be found in smos-server-gen/app. The NixOS module for running end-to-end tests can be found in nix/end-to-end-test-nixos-module.nix. You can run each one via the corresponding check in flake.nix.