nix-dev

Develop, debug, and iterate on Nix expressions.

Capabilities

This skill provides access to Nix tooling for development work:

• nix eval - Evaluate Nix expressions and inspect values
• nix build - Build derivations
• nix flake check - Validate flake
• nix flake show - Show flake outputs
• nix repl - Interactive exploration (use --expr for non-interactive)
• nix-instantiate - Debug derivations

Common Workflows

Evaluate an expression:

nix eval .#nixosConfigurations.hostname.config.services.nginx.enable

Evaluate with JSON output:

nix eval --json .#nixosConfigurations.hostname.config.networking.firewall.allowedTCPPorts

Debug a module option:

nix eval .#nixosConfigurations.hostname.config.homelab --apply builtins.attrNames

Check derivation without building:

nix-instantiate --eval -E 'with import <nixpkgs> {}; lib.version'

Build and check:

nix build .#nixosConfigurations.hostname.config.system.build.toplevel --dry-run

Explore flake outputs:

nix flake show --json | jq '.nixosConfigurations | keys'

Debugging Tips

Type errors:

nix eval --show-trace .#problematic.expression

Infinite recursion:

nix eval --show-trace --option eval-max-statements 100000 .#expression

Check option types:

nix eval .#nixosConfigurations.host.options.services.nginx.enable.type.name

Rules

Follow Nix debugging rules from CLAUDE.md. Run nix flake check to validate after changes.