diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5c2fefc..51b2213 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,7 +12,7 @@ jobs:
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: actions/checkout@v2.4.0
     - name: Check format
-      run: ./check-fmt
+      run: nix-shell --arg packages 'pkgs:[ pkgs.nixpkgs-fmt ]' --run 'nixpkgs-fmt --check .'
 
   py2-compat:
     runs-on: ubuntu-latest
@@ -22,7 +22,7 @@ jobs:
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: actions/checkout@v2.4.0
     - name: Check format
-      run: nix-shell --run 'git ls-files | grep -Pv "^tools" | grep -P "\.py$" | xargs py2-astparse'
+      run: nix-shell --arg packages 'pkgs:[ pkgs.p2nix-tools.py2-astparse ]' --run 'git ls-files | grep -Pv "^tools" | grep -P "\.py$" | xargs py2-astparse'
 
   black-fmt:
     runs-on: ubuntu-latest
@@ -32,7 +32,7 @@ jobs:
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: actions/checkout@v2.4.0
     - name: Check format
-      run: nix-shell --run 'black --check .'
+      run: nix-shell --arg packages 'pkgs:[ pkgs.p2nix-tools.env ]' --run 'black --check .'
 
   matrix_generate:
     runs-on: ubuntu-latest
@@ -64,4 +64,4 @@ jobs:
         with:
           name: poetry2nix
           signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
-      - run: nix-shell --run 'nix-build-uncached -build-flags "-L" --keep-going --show-trace tests/default.nix -A ${{ matrix.attr }}'
+      - run: nix-shell --arg packages 'pkgs:[ pkgs.nix-build-uncached ]' --run 'nix-build-uncached -build-flags "-L" --keep-going --show-trace tests/default.nix -A ${{ matrix.attr }}'
diff --git a/shell.nix b/shell.nix
index dcf8c18..689571f 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,21 +1,7 @@
-let
-  sources = import ./nix/sources.nix;
-  pkgs = import sources.nixpkgs {
-    overlays = [
-      (import ./overlay.nix)
-    ];
-  };
-  tools = pkgs.callPackage ./tools { };
-
-in
-pkgs.mkShell {
-
-  NIX_PATH = "nixpkgs=${sources.nixpkgs}";
-
-  buildInputs = [
-    tools.env
-    tools.py2-astparse
-    tools.flamegraph
+{ packages ? pkgs: [
+    pkgs.p2nix-tools.env
+    pkgs.p2nix-tools.py2-astparse
+    pkgs.p2nix-tools.flamegraph
     pkgs.nixpkgs-fmt
     pkgs.poetry
     pkgs.niv
@@ -23,5 +9,22 @@ pkgs.mkShell {
     pkgs.nix-prefetch-git
     pkgs.nix-eval-jobs
     pkgs.nix-build-uncached
-  ];
+  ]
+}:
+
+let
+  sources = import ./nix/sources.nix;
+  pkgs = import sources.nixpkgs {
+    overlays = [
+      (import ./overlay.nix)
+      (self: super: {
+        p2nix-tools = self.callPackage ./tools { };
+      })
+    ];
+  };
+
+in
+pkgs.mkShell {
+  NIX_PATH = "nixpkgs=${sources.nixpkgs}";
+  packages = packages pkgs;
 }