{
  description = "project-orchestrator — a project that uses the agent-orchestrator harness to manage a fleet of projects";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/50ab793786d9de88ee30ec4e4c24fb4236fc2674";
  };

  outputs = { self, nixpkgs }:
    let
      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
    in
    {
      # Reproducible devShell for the PO. Everything the PO needs to run itself + drive the fleet:
      #   - python311  : stdlib tomllib (>=3.11) — engine/agents.py and scripts/fleet.py import it
      #   - tmux       : the harness runs every agent/watchdog in tmux
      #   - git        : the engine submodule + create/update flows need git (incl. submodule support)
      # The agent CLI the PO agent uses (claude) is an external, non-Nix tool — install it per its
      # own docs and put it on PATH before `engine/agents.py up`.
      devShells = forAllSystems (pkgs: {
        default = pkgs.mkShell {
          packages = [
            pkgs.python311   # tomllib (>=3.11)
            pkgs.tmux        # harness sessions + watchdog
            pkgs.git         # engine submodule + project create/update flows
            pkgs.coreutils
            pkgs.bash
          ];
          shellHook = ''
            echo "project-orchestrator devShell — $(python3 --version), $(tmux -V), $(git --version)"
            echo "try: python3 engine/agents.py status  |  python3 scripts/fleet.py status"
          '';
        };
      });

      # `nix flake check` evaluates this — a cheap smoke that the devShell builds.
      checks = forAllSystems (pkgs: {
        devshell-builds = self.devShells.${pkgs.system}.default;
      });
    };
}