diff --git a/dev/flake-module.nix b/dev/flake-module.nix
index ae227e5..40fe958 100644
--- a/dev/flake-module.nix
+++ b/dev/flake-module.nix
@@ -12,6 +12,7 @@
       nativeBuildInputs = [
         pkgs.nixpkgs-fmt
         pkgs.pre-commit
+        pkgs.hci
       ];
       shellHook = ''
         ${config.pre-commit.installationScript}
@@ -30,5 +31,10 @@
     # for repl exploration / debug
     config.config = config;
     options.mySystem = lib.mkOption { default = config.allSystems.${builtins.currentSystem}; };
+    config.effects = withSystem "x86_64-linux" ({ config, pkgs, hci-effects, ... }: {
+      tests = {
+        template = pkgs.callPackage ./tests/template.nix { inherit hci-effects; };
+      };
+    });
   };
 }
diff --git a/dev/tests/README.md b/dev/tests/README.md
new file mode 100644
index 0000000..73c85f6
--- /dev/null
+++ b/dev/tests/README.md
@@ -0,0 +1,15 @@
+
+# Running the tests
+
+These tests can be run locally with the `hci effect run` command. This gives
+the tests access to a proper nix daemon and the network.
+
+Designed for convenient deployments, it needs some information from git. You
+may use `--no-token` to disable this functionality if you're getting errors, or
+if you're asked to log in.
+
+Example:
+
+```console
+hci effect run --no-token default.effects.tests.template
+```
diff --git a/dev/tests/template.nix b/dev/tests/template.nix
new file mode 100644
index 0000000..04671fa
--- /dev/null
+++ b/dev/tests/template.nix
@@ -0,0 +1,37 @@
+{ hci-effects, nix, git, path }:
+
+hci-effects.mkEffect {
+  inputs = [ nix git ];
+  effectScript = ''
+    ann() { # announce
+      printf '\n\e[34;1m%s\e[0m\n' "$*"
+    }
+    mkdir -p ~/.config/nix
+    echo 'experimental-features = nix-command flakes' >>~/.config/nix/nix.conf
+    mkdir clean
+    cd clean
+
+    ann nix flake init...
+    nix -v flake init -t ${../..}
+
+    ann pointing to local sources...
+    sed -i flake.nix -e 's^nixpkgs.url = ".*";^nixpkgs.url = "${path}"; flake-parts.url = "${../..}";^'
+    # head flake.nix
+    grep -F ${path} flake.nix >/dev/null
+
+    ann nix flake lock...
+    nix flake lock
+
+    ann nix flake show...
+    nix -v flake show
+
+    ann nix build...
+    nix build .
+
+    ann checking result...
+    readlink ./result | grep hello
+
+    echo
+    printf '\n\e[32;1m%s\e[0m\n' 'All good!'
+  '';
+}
diff --git a/modules/formatter.nix b/modules/formatter.nix
index b6098db..975e664 100644
--- a/modules/formatter.nix
+++ b/modules/formatter.nix
@@ -1,21 +1,55 @@
 { config, lib, flake-parts-lib, ... }:
 let
   inherit (lib)
+    filterAttrs
+    mapAttrs
     mkOption
+    optionalAttrs
     types
     ;
   inherit (flake-parts-lib)
-    mkTransposedPerSystemModule
+    mkSubmoduleOptions
+    mkPerSystemOption
     ;
 in
-mkTransposedPerSystemModule {
-  name = "formatter";
-  option = mkOption {
-    type = types.nullOr types.package;
-    default = null;
-    description = ''
-      A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
-    '';
+{
+  options = {
+    flake = mkSubmoduleOptions {
+      formatter = mkOption {
+        type = types.lazyAttrsOf types.package;
+        default = { };
+        description = ''
+          An attribute set of per system a package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
+        '';
+      };
+    };
+
+    perSystem = mkPerSystemOption ({ config, ... }: {
+      _file = ./formatter.nix;
+      options = {
+        formatter = mkOption {
+          type = types.nullOr types.package;
+          default = null;
+          description = ''
+            A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
+          '';
+        };
+      };
+    });
+  };
+  config = {
+    flake.formatter =
+      mapAttrs
+        (k: v: v.formatter)
+        (filterAttrs
+          (k: v: v.formatter != null)
+          config.allSystems
+        );
+
+    perInput = system: flake:
+      optionalAttrs (flake?formatter.${system}) {
+        formatter = flake.formatter.${system};
+      };
+
   };
-  file = ./formatter.nix;
 }