mcp dev environment

modules/dev/mcp.nix

@@ -0,0 +1,53 @@
+{
+  config,
+  inputs,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  python = pkgs.python3.withPackages (
+    ps:
+    builtins.attrValues {
+      inherit (ps)
+        click
+        pytest
+        black
+        ruff
+        ;
+    }
+  );
+  packages = builtins.attrValues {
+    inherit python;
+    inherit (pkgs) codex; # codex-cli from openai
+  };
+in
+{
+  options = {
+    my.dev.mcp = {
+      enable = lib.mkEnableOption "Install MCP tooling globally";
+      users = lib.mkOption {
+        type = inputs.self.lib.usersOptionType lib;
+        default = config.my.toggleUsers.dev;
+        description = "Users to install MCP packages for";
+      };
+    };
+    devShells.mcp = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.mkShell {
+        inherit packages;
+        name = "mcp-dev-shell";
+        shellHook = ''
+          export CODEX_HOME=$PWD/.codex
+          export PYTHONPATH=$PWD/scripts/mcp-server/src
+          alias mcp-run="python -m mcp_server.server"
+          echo "MCP shell ready: codex + python + PYTHONPATH set"
+        '';
+      };
+      description = "MCP + Codex shell for this repo";
+    };
+  };
+  config = lib.mkIf config.my.dev.mcp.enable {
+    users.users = inputs.self.lib.mkUserAttrs lib config.my.dev.mcp.users { inherit packages; };
+  };
+}