Evaluate the final merged value of an attribute in the user's actual NixOS/Home Manager configuration via nix eval (e.g. attr='services.openssh.enable'). flake_uri and mode auto-resolve when omitted. Pass a list of attrs to batch several evals into one call, returning per-attr results. Values above the size guard degrade to attr names / length / a head slice (truncated: true).

Validation ladder for the configuration, fast to slow: level='lint' (statix+deadnix; returns a structured findings list — lint only), 'flake' (nix flake check), 'dry-build' (evaluate and plan the closure build), 'dry-activate' (NixOS only, shows what activation would change).

Format .nix files. With paths: nixfmt per file. Without: nix fmt in the flake directory, falling back to nixfmt on all .nix files when the flake defines no formatter.

Build the full system/HM closure without activating it. Returns the output store path.

Build the new closure and diff it against the running system (nvd, falling back to nix store diff-closures): package additions, removals, version changes. Use before switch.

Activate the configuration (sudo nixos-rebuild switch / home-manager switch). Records rollback_generation first. Returns a structured 'summary' (units changed, derivations built) plus gen before/after; on success the raw log is trimmed to a tail (pass full_log=True for all of it). No implicit validation by default; pass validate=True to gate on check('dry-build') first. On a sudo auth failure returns a 'privilege' diagnosis.

action='list': enumerate system/HM generations with dates and current marker. action='rollback': revert to the previous generation. Pass mode='home-manager' for HM generations.

Where does this configuration set an option: files declaring it and every file defining it, with each contributed value (e.g. attr='services.openssh.enable'). The bridge from mcp-nixos discovery to editing the right file. For integrated Home Manager, query the NixOS config with attr='home-manager.users..'.

Structured facts about the config repo in one call: hosts, homeConfigurations, integrated-vs-standalone Home Manager, module dirs, auto-import mechanism, formatter, lint tools, justfile/CI/.mcp.json presence. Run this before onboarding a repo or when orienting in an unfamiliar config; facts that cannot be determined are null/'unknown'.