Nix Agent

Overview

Use this skill for direct NixOS execution tasks through nix-agent and mcp-nixos. Use mcp-nixos for discovery, use nix-agent for local mutation and activation. Do not detour into generic brainstorming for operational tasks.

When to Use

• A user asks to install a package on NixOS.
• A user asks which option to use, then wants it applied.
• A user asks to patch local NixOS config through MCP tools.
• The host exposes both nix-agent and mcp-nixos.

Do not use this skill for writing secret payloads or for broad architecture/design requests.

Tool Surface

nix-agent exposes exactly two tools:

• inspect_state(path) – read a local file.
• apply_patch_set(patch_set, flake_uri=None, mode="nixos") – write each patch, format any .nix files, and (when flake_uri is provided) validate then switch.
  • mode="nixos" (default): sudo nixos-rebuild dry-activate → switch.
  • mode="home-manager": home-manager build → home-manager switch (no sudo).
  • Returns changed_files, rollback_generation, current_generation, command outputs, and status.

mcp-nixos is responsible for package and option discovery.

Required Workflow

1. If the request needs package or option discovery, query mcp-nixos first.
2. Optionally inspect_state(path) on any file you intend to modify.
3. Build a PatchSet of Patch(path, content) entries.
4. Call apply_patch_set(patch_set, flake_uri=..., mode=...) in a single round-trip. Pass mode="home-manager" for user-level Home Manager configs; default mode="nixos" for system configs.
5. Report:
   • the changed_files
   • the final status
   • the rollback_generation (so the user can recover with sudo nixos-rebuild switch --rollback if needed)
   • any non-empty dry_activate_output / switch_output

If status is validation_failed or switch_failed, stop and surface first_error (the extracted first error: line) and the relevant output field instead of retrying blindly.

You can also call apply_patch_set with an empty PatchSet (no patches) plus a flake_uri to trigger a rebuild against the current flake state without writing any files — useful after manual edits.

Common Mistakes

• Starting a generic brainstorming/design workflow for a simple package install.
• Skipping mcp-nixos and guessing package names or option paths.
• Calling apply_patch_set without flake_uri when the user actually wants the change applied.
• Writing secret material through patches.
• Re-running apply_patch_set after a failure without inspecting the dry-activate output.

Example

User: Use mcp-nixos and nix-agent together to install floorp on my NixOS system.

1. Query mcp-nixos for the correct Floorp package attribute.
2. Build a PatchSet that adds pkgs.floorp to the relevant module.
3. Call apply_patch_set(patch_set, flake_uri="/etc/nixos#hostname").
4. Report changed_files, status, and rollback_generation.