# AGENTS.md

## Purpose

This repo uses the Dendritic Pattern with `flake-parts`.

Design and change the configuration as a composition of **features**, not as a host-first tree.

For deeper design rationale and pattern descriptions, refer to `.agents/dendritic-design-with-flake-parts.wiki`.

## Core Terms

- **Feature**: a flake-parts module under `modules/` that defines one coherent concern.
- **Aspect**: a reusable module published at `flake.modules.<module class>.<aspect name>`.
- **Module class**: the configuration context of an aspect. This repo primarily uses `nixos` and `homeManager`.
- **Feature module**: the flake-parts module that defines aspects, flake outputs, options, or shared helpers.

In this repo, `flake.nix` imports `./modules` recursively via `inputs.import-tree`. Any non-private `.nix` file under `modules/` is therefore treated as a feature module.

## Design Principles

- Work bottom-up. Define features first; assemble hosts from features.
- Keep semantic ownership local. A feature should contain the configuration for that concern across all relevant module classes.
- Name aspects semantically. The aspect name should usually match the file or directory name that defines it.
- Prefer small, composable aspects. Build larger configurations with `imports`.
- Import aspects unconditionally and only within the same module class.
- Put conditions inside module content with `lib.mkIf` or `lib.mkMerge`, never around `imports`.
- Avoid importing the same aspect multiple times along one import path.
- Keep private helper files next to the feature that uses them and prefix them with `_` so `import-tree` does not import them as feature modules.
- Put shared schemas and constructors in dedicated shared modules, not ad hoc host files.

## Repo Structure

- `modules/features/`: reusable features and most aspect definitions.
- `modules/hosts/<name>/default.nix`: host features that assemble NixOS aspects into `flake.modules.nixos.<name>`.
- `modules/secrets/`: secret-related features shared by hosts.
- `modules/flake-parts.nix`: flake-parts entrypoint; defines systems, formatter, and `flake.nixosConfigurations`.
- `modules/lib.nix`: shared constructors in `config.meta.lib`, especially `mkHost`, `mkHostUser`, and `mkCaddyReverseProxy`.
- `modules/users.nix`: canonical user attrsets exposed through `meta.lib.users`.
- `modules/features/meta.nix`: shared metadata schema for `meta.host` and `meta.user`.

## How Features Are Applied Here

- Reusable NixOS concerns are published as `flake.modules.nixos.<name>`.
- Reusable Home Manager concerns are published as `flake.modules.homeManager.<name>`.
- Hosts are aspects too. `orion`, `polaris`, and `zenith` are `nixos` aspects assembled from smaller aspects.
- Host modules should use `config.meta.lib.mkHost` to define `meta.host`, base imports, hostname, and state version.
- Per-user Home Manager composition should use `config.meta.lib.mkHostUser` so `meta.host` and `meta.user` stay available to Home Manager features.
- Features may rely on the `meta` contract. Existing modules already read `config.meta.host`, `config.meta.user`, and `config.meta.lib`.

## Preferred Aspect Patterns

- **Simple Aspect**: use for one self-contained concern in one or more module classes.
- **Multi Context Aspect**: use when one concern must configure both `nixos` and `homeManager`.
- **Inheritance Aspect**: use by importing a parent aspect and extending it.
- **Conditional Aspect**: use `lib.mkMerge` plus `lib.mkIf` for conditional content.

Use **Collector Aspect** only when composition through imports or shared library helpers is insufficient.

## Change Rules

- When adding a feature, add or extend aspects under `modules/features/` and let hosts opt into them explicitly.
- When adding a host, create `modules/hosts/<name>/default.nix` and keep host-local generated files private as `_hardware.nix`, `_disk.nix`, or similar.
- When a feature needs local data or helper code, keep it inside that feature directory and prefix non-feature files with `_` when they live under `modules/`.
- Do not place arbitrary non-feature `.nix` files under `modules/` unless they are intentionally private and excluded from recursive import.
- If a concern is shared across hosts, it belongs in a reusable feature, not inline in one host unless it is truly host-specific.

## Practical Heuristics

- If you are about to edit a host because of a reusable concern, that concern probably wants its own feature.
- If a Home Manager module needs host or user facts, prefer reading `config.meta.host` or `config.meta.user` instead of duplicating literals.
- If a concern spans system and user space, keep both aspects in one feature so the behavior stays coherent.
- If imports would need to be conditional, redesign the aspect boundary instead.