diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 03316ce..d04ec48 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -21,4 +21,4 @@ jobs:
           keep-derivations = true
           experimental-features = nix-command flakes
     - run:
-        nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz ci.nix --run 'true'
+        "nix run .#test-runner"
diff --git a/ci.nix b/ci.nix
deleted file mode 100644
index 310774b..0000000
--- a/ci.nix
+++ /dev/null
@@ -1,21 +0,0 @@
-with import <nixpkgs> {};
-mkShell {
-  nativeBuildInputs = [
-    shellcheck direnv mypy python3.pkgs.black python3.pkgs.flake8
-  ];
-
-  shellHook = ''
-    set -e
-    echo -e "\x1b[32m## run shellcheck\x1b[0m"
-    shellcheck direnvrc
-    echo -e "\x1b[32m## run black\x1b[0m"
-    LC_ALL=en_US.utf-8 black --check .
-    echo -e "\x1b[32m## run flake8\x1b[0m"
-    flake8 --ignore E501 tests
-    echo -e "\x1b[32m## run mypy\x1b[0m"
-    mypy tests
-
-    echo -e "\x1b[32m## run unittest\x1b[0m"
-    ${pkgs.python3.interpreter} -m unittest discover tests
-  '';
-}
diff --git a/flake.nix b/flake.nix
index 23cd2d5..a71e47a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -7,8 +7,16 @@
     flake-utils.lib.eachDefaultSystem (system: let
       pkgs = nixpkgs.legacyPackages.${system};
     in {
-      defaultPackage = pkgs.callPackage ./default.nix { };
+      packages = {
+        default = pkgs.callPackage ./default.nix { };
+        test-runner = pkgs.callPackage ./run-tests.nix {};
+      };
+      defaultPackage = self.packages.${system}.default;
       devShell = pkgs.callPackage ./shell.nix { };
+      apps.test-runner = {
+        type = "app";
+        program = "${self.packages.${system}.test-runner}";
+      };
     }) // {
       overlay = final: prev: {
         nix-direnv = final.callPackage ./default.nix { };
diff --git a/run-tests.nix b/run-tests.nix
new file mode 100644
index 0000000..5ba3e20
--- /dev/null
+++ b/run-tests.nix
@@ -0,0 +1,22 @@
+{
+  writeScript,
+  shellcheck,
+  direnv,
+  mypy,
+  python3,
+}:
+writeScript "run-tests" ''
+  set -e
+  PATH="''${PATH}''${PATH:+":"}${direnv}/bin"
+  echo -e "\x1b[32m## run shellcheck\x1b[0m"
+  ${shellcheck}/bin/shellcheck direnvrc
+  echo -e "\x1b[32m## run black\x1b[0m"
+  LC_ALL=en_US.utf-8 ${python3.pkgs.black}/bin/black --check .
+  echo -e "\x1b[32m## run flake8\x1b[0m"
+  ${python3.pkgs.flake8}/bin/flake8 --ignore E501 tests
+  echo -e "\x1b[32m## run mypy\x1b[0m"
+  ${mypy}/bin/mypy tests
+
+  echo -e "\x1b[32m## run unittest\x1b[0m"
+  ${python3.interpreter} -m unittest discover tests
+''