diff --git a/flake.nix b/flake.nix
index 06cd3fd..505eb2a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -4,19 +4,26 @@
   inputs.flake-utils.url = "github:numtide/flake-utils";
 
   outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system: let
-      pkgs = nixpkgs.legacyPackages.${system};
-    in {
-      packages = {
-        default = pkgs.callPackage ./default.nix { };
-        test-runner = pkgs.callPackage ./run-tests.nix {};
-      };
-      devShells.default = pkgs.callPackage ./shell.nix { };
-      apps.test-runner = {
-        type = "app";
-        program = "${self.packages.${system}.test-runner}";
-      };
-    }) // {
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          packages = {
+            default = pkgs.callPackage ./default.nix { };
+            test-runner = pkgs.callPackage ./test-runner.nix { };
+            test-runner-unstable = pkgs.callPackage ./test-runner.nix { };
+          };
+          devShells.default = pkgs.callPackage ./shell.nix { };
+          apps.test-runner = {
+            type = "app";
+            program = "${self.packages.${system}.test-runner}";
+          };
+          checks = {
+            lint = pkgs.callPackage ./lint.nix { };
+          };
+        }) // {
       overlay = final: prev: {
         nix-direnv = final.callPackage ./default.nix { };
       };
diff --git a/lint.nix b/lint.nix
new file mode 100644
index 0000000..1223cb3
--- /dev/null
+++ b/lint.nix
@@ -0,0 +1,24 @@
+{ shellcheck
+, mypy
+, python3
+, lib
+, ruff
+, runCommand
+}:
+runCommand "lint" {} ''
+  set -e
+  mkdir source
+  cp -r ${./.}/* source
+  chmod +w source
+  cd source
+
+  echo run shellcheck
+  ${shellcheck}/bin/shellcheck direnvrc
+  echo run black
+  ${lib.getExe python3.pkgs.black} --check .
+  echo run ruff
+  ${lib.getExe ruff} tests
+  echo run mypy
+  ${lib.getExe mypy} tests
+  touch $out
+''
diff --git a/run-tests.nix b/run-tests.nix
deleted file mode 100644
index ae908cc..0000000
--- a/run-tests.nix
+++ /dev/null
@@ -1,24 +0,0 @@
-{
-  writeScript,
-  shellcheck,
-  direnv,
-  mypy,
-  python3,
-  lib,
-  ruff
-}:
-writeScript "run-tests" ''
-  set -e
-  PATH="''${PATH}''${PATH:+":"}${direnv}/bin"
-  echo run shellcheck
-  ${shellcheck}/bin/shellcheck direnvrc
-  echo run black
-  LC_ALL=en_US.utf-8 ${lib.getExe python3.pkgs.black} --check .
-  echo run ruff
-  ${lib.getExe ruff} tests
-  echo run mypy
-  ${lib.getExe mypy} tests
-
-  echo run unittest
-  ${lib.getExe python3.pkgs.pytest} .
-''
diff --git a/test-runner.nix b/test-runner.nix
new file mode 100644
index 0000000..53374a6
--- /dev/null
+++ b/test-runner.nix
@@ -0,0 +1,15 @@
+{ writeScript
+, direnv
+, python3
+, lib
+, nix
+, coreutils
+, gnugrep
+}:
+writeScript "test-runner" ''
+  set -e
+  export PATH=${lib.makeBinPath [ direnv nix coreutils gnugrep ]}
+
+  echo run unittest
+  ${lib.getExe python3.pkgs.pytest} .
+''