diff --git a/.luarc.json b/.luarc.json
index 23646d3..676c096 100644
--- a/.luarc.json
+++ b/.luarc.json
@@ -2,7 +2,8 @@
   "runtime.version": "LuaJIT",
   "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
   "diagnostics.globals": ["vim", "jit"],
-  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
+  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library", "${3rd}/busted/library"],
   "workspace.checkThirdParty": false,
+  "workspace.ignoreDir": [".direnv"],
   "completion.callSnippet": "Replace"
 }
diff --git a/flake.nix b/flake.nix
index 0c5f5d8..38e98cd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -16,15 +16,26 @@
       forEachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
     in
     {
+      formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
+
       devShells = forEachSystem (pkgs: {
-        default = pkgs.mkShell {
+        default =
+          let
+            ts-plugin = pkgs.vimPlugins.nvim-treesitter.withPlugins (p: [ p.diff ]);
+            diff-grammar = pkgs.vimPlugins.nvim-treesitter-parsers.diff;
+            luaEnv = pkgs.luajit.withPackages (ps: with ps; [ busted nlua ]);
+            busted-with-grammar = pkgs.writeShellScriptBin "busted" ''
+              nvim_bin=$(which nvim)
+              tmpdir=$(mktemp -d)
+              trap 'rm -rf "$tmpdir"' EXIT
+              printf '#!/bin/sh\nexec "%s" --cmd "set rtp+=${ts-plugin}/runtime" --cmd "set rtp+=${diff-grammar}" "$@"\n' "$nvim_bin" > "$tmpdir/nvim"
+              chmod +x "$tmpdir/nvim"
+              PATH="$tmpdir:$PATH" exec ${luaEnv}/bin/busted "$@"
+            '';
+          in
+          pkgs.mkShell {
           packages = [
-            (pkgs.luajit.withPackages (
-              ps: with ps; [
-                busted
-                nlua
-              ]
-            ))
+            busted-with-grammar
             pkgs.prettier
             pkgs.stylua
             pkgs.selene
diff --git a/scripts/ci.sh b/scripts/ci.sh
new file mode 100755
index 0000000..e06bf09
--- /dev/null
+++ b/scripts/ci.sh
@@ -0,0 +1,10 @@
+#!/bin/sh
+set -eu
+
+nix develop --command stylua --check .
+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 lua-language-server --check . --checklevel=Warning
+nix develop --command busted