diff --git a/flake.nix b/flake.nix
index aebd6d8..fb5faf0 100644
--- a/flake.nix
+++ b/flake.nix
@@ -13,9 +13,12 @@
       ...
     }:
     let
-      forEachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
+      forEachSystem =
+        f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
     in
     {
+      formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
+
       devShells = forEachSystem (pkgs: {
         default = pkgs.mkShell {
           packages = [
diff --git a/scripts/ci.sh b/scripts/ci.sh
index 3e9356d..7714909 100755
--- a/scripts/ci.sh
+++ b/scripts/ci.sh
@@ -4,4 +4,6 @@ set -eu
 nix develop --command stylua --check lua spec
 git ls-files '*.lua' | xargs nix develop --command selene --display-style quiet
 nix develop --command prettier --check .
+nix fmt
+git diff --exit-code -- '*.nix'
 nix develop --command busted