{
  description = "stable-diffusion.cpp as a NixOS programs.* module (system nixpkgs)";

  outputs = { self, ... }: {
    nixosModules.default = { config, lib, pkgs, ... }:
      let
        cfg = config.programs.stable-diffusion-cpp;

        builtPackage =
          pkgs.stdenv.mkDerivation {
            pname = "stable-diffusion-cpp";
            version = "git";

            src = pkgs.fetchFromGitHub {
              owner = "leejet";
              repo  = "stable-diffusion.cpp";
              rev   = "master-453-4ff2c8c"; # pin to a commit for reproducibility
              hash  = "sha256-0Hl3M6NQK1ZfIH4eIdy/XiPZTeBCnRCtode88NipPp4=";
              fetchSubmodules = true;
            };

            nativeBuildInputs = with pkgs; [ cmake ninja pkg-config ];

            buildInputs =
              (lib.optionals cfg.vulkan.enable (with pkgs; [
                vulkan-headers
                vulkan-loader
                shaderc
              ]));

            cmakeFlags = [
              "-DCMAKE_BUILD_TYPE=Release"
              "-DCMAKE_POLICY_VERSION_MINIMUM=3.5"
              "-DSD_VULKAN=${if cfg.vulkan.enable then "ON" else "OFF"}"
            ];

            installPhase = ''
              runHook preInstall
              mkdir -p $out/bin
              cp -v bin/sd $out/bin/ || cp -v sd $out/bin/
              runHook postInstall
            '';
          };

        selectedPackage = if cfg.package != null then cfg.package else builtPackage;

        extraEnvArgs =
          lib.concatStringsSep " \\\n            "
            (lib.mapAttrsToList (k: v:
              "--set ${lib.escapeShellArg k} ${lib.escapeShellArg v}"
            ) cfg.extraEnv);

        wrappedPackage =
          pkgs.symlinkJoin {
            name = "${selectedPackage.pname or "stable-diffusion-cpp"}";
            paths = [ selectedPackage ];
            nativeBuildInputs = [ pkgs.makeWrapper ];
            postBuild = ''
              if [ -x "$out/bin/sd" ]; then
                wrapProgram "$out/bin/sd" \
                  ${lib.optionalString (cfg.vulkan.enable && cfg.vulkan.icdFile != null)
                    "--set-default VK_ICD_FILENAMES ${lib.escapeShellArg (toString cfg.vulkan.icdFile)}"} \
                  ${lib.optionalString (cfg.vulkan.enable && cfg.vulkan.prefixOpenGLXdgDataDirs)
                    "--prefix XDG_DATA_DIRS : /run/opengl-driver/share"} \
                  ${extraEnvArgs}
              fi
            '';
          };

        finalPackage = if cfg.wrap then wrappedPackage else selectedPackage;
      in {
        options.programs.stable-diffusion-cpp = {
          enable = lib.mkEnableOption "stable-diffusion.cpp";

          package = lib.mkOption {
            type = lib.types.nullOr lib.types.package;
            default = null;
            description = "Override the package used (otherwise build from upstream source).";
          };

          wrap = lib.mkOption {
            type = lib.types.bool;
            default = true;
            description = "Wrap the sd binary to inject environment variables.";
          };

          extraEnv = lib.mkOption {
            type = lib.types.attrsOf lib.types.str;
            default = { };
            description = "Extra environment variables added to the sd wrapper.";
          };

          vulkan = {
            enable = lib.mkOption {
              type = lib.types.bool;
              default = true;
              description = "Build with Vulkan support (toggles -DSD_VULKAN and Vulkan deps).";
            };

            icdFile = lib.mkOption {
              type = lib.types.nullOr lib.types.path;
              default = null;
              example = /run/opengl-driver/share/vulkan/icd.d/intel_icd.x86_64.json;
              description = "Optional VK_ICD_FILENAMES value to select a Vulkan ICD.";
            };

            prefixOpenGLXdgDataDirs = lib.mkOption {
              type = lib.types.bool;
              default = true;
              description = "Prefix XDG_DATA_DIRS with /run/opengl-driver/share (useful for Vulkan layers/ICDs on NixOS).";
            };
          };
        };

        config = lib.mkIf cfg.enable {
          environment.systemPackages = [ finalPackage ];
        };
      };
  };
}