I use Nix and Devenv to manage dependencies and build environments for all of my hobby projects. While I got the tutorial server up and running in a terminal relatively quickly, I had some trouble figuring out how to invoke it as a CLI from Claude Desktop’s MCP configuration. During development, I typically run nix develop --no-pure-eval to enter a devShell and then have access to all the packages I’ve declared a dependency on. In order to run node build/index.js from Claude Desktop, I needed the right config to get a nix run invocation to work. With some help from Claude itself, here’s the flake I ended up with:
{
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
systems.url = "github:nix-systems/default";
devenv.url = "github:cachix/devenv";
devenv.inputs.nixpkgs.follows = "nixpkgs";
};
nixConfig = {
extra-trusted-public-keys = "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw=";
extra-substituters = "https://devenv.cachix.org";
};
outputs = {
self,
nixpkgs,
devenv,
systems,
...
} @ inputs: let
forEachSystem = nixpkgs.lib.genAttrs (import systems);
in {
packages = forEachSystem (system: let
pkgs = nixpkgs.legacyPackages.${system};
in {
devenv-up = self.devShells.${system}.default.config.procfileScript;
weather-server = pkgs.stdenv.mkDerivation {
name = "weather-server";
src = ./.;
installPhase = ''
mkdir -p $out/bin $out/lib
cp -r build $out/lib/
cp -r node_modules $out/lib/
# Create a simple wrapper script
cat > $out/bin/weather-server <<EOF
#!${pkgs.runtimeShell}
exec ${pkgs.nodejs_22}/bin/node $out/lib/build/index.js "\$@"
EOF
chmod +x $out/bin/weather-server
'';
};
default = self.packages.${system}.weather-server;
});
devShells =
forEachSystem
(system: let
pkgs = nixpkgs.legacyPackages.${system};
in {
default = devenv.lib.mkShell {
inherit inputs pkgs;
modules = [
{
# https://devenv.sh/reference/options/
packages = with pkgs; [
pkgs.nodejs_22
];
languages.javascript = {
enable = true;
package = pkgs.nodejs_22;
pnpm.enable = true;
};
processes = {
app.exec = "node build/index.js";
};
}
];
};
});
};
}
The corresponding entry in Claude Desktop’s config is:
{
"mcpServers": {
"weather": {
"command": "/nix/var/nix/profiles/default/bin/nix",
"args": [
"run",
"/Users/amey/Developer/mcp-server-tutorial#weather-server"
]
}
}
}