{
  description = "Personal agent skills, packaged through Nix.";

  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  inputs.flake-utils.url = "github:numtide/flake-utils";

  # Present only so `nix flake check` can evaluate `homeModules.default` against
  # real home-manager (the home-manager-module check). Its nixpkgs follows this
  # flake's, so the module is checked against the same nixpkgs-and-home-manager
  # pairing a consumer following this flake would get. It has no bearing on the
  # module a consumer imports — that module is a bare function taking the
  # importing configuration's own home-manager and pkgs.
  inputs.home-manager.url = "github:nix-community/home-manager";
  inputs.home-manager.inputs.nixpkgs.follows = "nixpkgs";

  outputs =
    { self, nixpkgs, flake-utils, home-manager }:
    let
      mkSkill = import ./lib/mk-skill.nix;
      mkSkillsShellHook = import ./lib/mk-skills-shell-hook.nix;
      discoverSkills = import ./lib/discover.nix nixpkgs.lib;

      discoveredSkills = discoverSkills ./skills;

      skillPackages =
        pkgs:
        nixpkgs.lib.listToAttrs (
          map (skill: {
            inherit (skill) name;
            value = mkSkill { inherit pkgs; inherit (skill) name src; };
          }) discoveredSkills
        );
    in
    {
      lib = { inherit mkSkill mkSkillsShellHook; };

      # Global placement: an operator selects skills into enabled agent harnesses
      # via `programs.agents.skills`. Not per-system — it is a module function,
      # and the skills it places come from the importing configuration's own
      # selection rather than this flake's nixpkgs.
      homeModules = rec {
        agents-skills = ./home-manager-module.nix;
        default = agents-skills;
      };
    }
    //
    # An explicit system list rather than `eachDefaultSystem`.
    # That default set includes x86_64-darwin, which current nixpkgs no longer
    # evaluates (its `legacyPackages` throws).
    flake-utils.lib.eachSystem
      [
        "x86_64-linux"
        "aarch64-linux"
        "aarch64-darwin"
      ]
      (
        system:
        let
          pkgs = nixpkgs.legacyPackages.${system};
        in
        {
          # No `default`: a catalog has no single default skill.
          packages = skillPackages pkgs;

          # Each discovered skill is folded in as its own check, so
          # `nix flake check` builds every skill and fails on a malformed one.
          checks = (skillPackages pkgs) // {
            skill-build = import ./checks/skill-build.nix {
              inherit pkgs mkSkill discoverSkills;
            };

            # Proves the global-placement module composes: it builds the
            # home-files derivation `homeModules.default` produces under several
            # operator configurations. Reuses this flake's `mkSkill` so the
            # sample skills carry the same name contract real ones do.
            home-manager-module = import ./checks/home-manager-module.nix {
              inherit pkgs home-manager mkSkill;
              module = self.homeModules.default;
            };

            # Proves the per-project dev-shell helper: it runs the produced hook
            # against a fixture project and asserts placement, stateless
            # stale-removal, and that hand-authored skills stay untouched.
            shell-hook = import ./checks/shell-hook.nix {
              inherit pkgs mkSkill mkSkillsShellHook;
            };

            # Proves the benchmark skill's deterministic core at its single seam:
            # it feeds committed fixtures to the core and asserts the metrics, the
            # Efficacy verdict, and the rendered report.
            benchmark-core = import ./checks/benchmark-core.nix {
              inherit pkgs;
            };
          };

          # A working shell for developing skills in this repo.
          # python3 runs the benchmark skill's deterministic core.
          # The benchmark-skill is placed into ./.agents/skills/ on entry, with
          # ./.claude/skills as a compatibility symlink.
          devShells.default = pkgs.mkShell {
            packages = [ pkgs.python3 ];
            shellHook = mkSkillsShellHook [ (skillPackages pkgs).benchmark-skill ];
          };
        }
      );
}