The NixPKGs contracts implementation has another PR at https://github.com/NixOS/nixpkgs/pull/506343. If this gets merged, it'd be another big (though completely optional) addition to how services can be defined (not only) in #NixOS, after the introduction of modular services in 25.11. No clue if this is the final PR or when it'll be merged, but it's a good step towards standardizing services #nix