{
  config,
  lib,
  pkgs,
  ...
}:
{

  # dix: https://github.com/faukah/dix
  # rust based
  # module from https://github.com/getchoo/borealis/blob/a35749aa6280fbe4cbe72291bb9c4cc643780c76/modules/nixos/custom/activation-diff.nix

  # nvd: https://discourse.nixos.org/t/nvd-simple-nix-nixos-version-diff-tool/12397
  # python based
  # module from https://github.com/luishfonseca/dotfiles/blob/ab7625ec406b48493eda701911ad1cd017ce5bc1/modules/upgrade-diff.nix

  # gated on formatAttr which is set by nixos-generators
  system.activationScripts.show-nixos-closure-diff = lib.mkIf ((config.formatAttr or null) == null) {
    supportsDryActivation = true;
    text =
      if pkgs ? dix then
        ''
          ${pkgs.dix}/bin/dix /run/current-system "$systemConfig"
        ''
      else
        ''
          ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff /run/current-system "$systemConfig"
        '';
  };

}