/gat-design

name: gat-design
description: "Design one system within a specific milestone's scope: write the milestone-scoped system GDD, content data, and system art doc, and update the milestone brief's progress. Requires a mandatory milestone argument."
argument-hint: "{milestone} {system | hint}"
user-invocable: true
allowed-tools: Read, Glob, Grep, Write, Edit, Agent, AskUserQuestion

Design

This skill designs a single system within a specific milestone, after `/gat-brainstorm` (overview) and `/gat-milestone` (milestone plan) are done. For story, worldbuilding, characters, quests, dialogue, or authored narrative content, delegate to `/gat-story`. For the initial concept interview, delegate to `/gat-brainstorm`.

The `milestone` argument is **mandatory**. If it is omitted, the skill stops and asks the user to specify a milestone — no inference, default, or fallback.

Phase 1: Resolve Mode

Check existing files:

• If `gat/overview/game.md` or `gat/overview/systems-index.md` missing → hand off: tell the user to run `/gat-brainstorm [hint]` first. Stop.
• If `gat/overview/art-direction.md` missing → hand off: tell the user to run `/gat-brainstorm [hint]` to establish global art direction. Stop.
• If `gat/milestone/milestone.md` missing → hand off: tell the user to run `/gat-milestone` first. Stop.
• If no `milestone` argument provided → STOP. Tell the user a milestone is required and show the milestones listed in `gat/milestone/milestone.md`. Do NOT infer or default one. Do NOT write any files.
• If the `milestone` argument does not match any milestone directory under `gat/milestone/` (i.e., no `m{N}-<name>/m{N}-brief.md` exists) → STOP. Tell the user the milestone was not found and list the valid milestones.

Then resolve sub-mode:

• If a `system` (or hint) argument is also provided → Mode: `system` (design that one system in the milestone)
• If only a `milestone` argument is provided → Mode: `continue` (auto-fill all systems in that milestone's brief that are not yet designed). Continue is scoped to that milestone only — it does NOT cross into other milestones.

Phase 2: Execute

Read templates:

• `.claude/docs/templates/design/system-gdd.md`
• `.claude/docs/templates/design/system-art.md`
• `.claude/docs/templates/design/content-data.md`

Read context:

• `gat/overview/game.md`, `gat/overview/systems-index.md`, `gat/overview/art-direction.md`
• the target milestone's `m{N}-brief.md` (for scope, in-scope systems, and the progress tracker)
• relevant `gat/narrative/*.md` when narrative context affects the system
• existing system docs under `gat/milestone/m{N}-<name>/<system>/` if present

Also read existing `gat/narrative/*.md` when present and pass relevant narrative context to spawned agents. Do not create or rewrite narrative docs here; use `/gat-story` for that.

Continue mode (`/gat-design {milestone}`)

For each system listed in the milestone's `m{N}-brief.md` that is not yet designed (progress row Pending), in the brief's order:

1. Spawn `gat-designer` → `gat/milestone/m{N}-<name>/<system>/<system>-gdd-m{N}.md` 2. If the system needs content data, spawn `gat-designer` → `gat/milestone/m{N}-<name>/<system>/<system>-data-m{N}.md` 3. Spawn `gat-artist` → `gat/milestone/m{N}-<name>/<system>/<system>-art-m{N}.md` 4. Update that system's progress row in `m{N}-brief.md` to Designed.

Do NOT design systems belonging to other milestones.

System mode (`/gat-design {milestone} {system | hint}`)

**Step 1** — Spawn `gat-designer` → `gat/milestone/m{N}-<name>/<system>/<system>-gdd-m{N}.md`

• Pass `gat/overview/game.md`, `gat/overview/systems-index.md`, the milestone's `m{N}-brief.md`, system-gdd template
• Pass existing system GDD if present (e.g., an earlier milestone's version of the same system, for reference — a later milestone may redefine it differently)

**Step 2 — Content Fill** (for systems with high content volume, within this milestone's scope)

If the system requires substantial content data, spawn `gat-designer` to write `gat/milestone/m{N}-<name>/<system>/<system>-data-m{N}.md`:

• Pass the system GDD, `gat/overview/game.md`, `gat/overview/systems-index.md`, the milestone brief, content-data template
• Pass existing content doc if present
• The content doc fills specific instances, parameters, sequences, and groups for THIS milestone — the data that instantiates the rules defined in the milestone's system GDD

A system needs a content-data doc when its GDD defines data structures that need many concrete instances (e.g. an `enemy` GDD defines enemy attributes → content doc fills the specific enemies in scope for this milestone).

Systems that are purely mechanical (e.g. `input`, `tbs-scoring`) typically do NOT need content-data docs — their parameters fit within the GDD itself.

**Step 3** — Spawn `gat-artist` → `gat/milestone/m{N}-<name>/<system>/<system>-art-m{N}.md`:

• Pass `gat/overview/game.md`, `gat/overview/art-direction.md`, the milestone's system GDD, the content-data doc (if it exists), the milestone brief, system-art template
• Pass existing system art doc if present

**Step 4 — Update Progress**

After writing a system's docs, update that system's row in the milestone's `m{N}-brief.md` progress tracker (System | Iteration(s) | GDD | Art | Data | Design Status) to mark GDD/Art/Data paths filled and Design Status Designed. If this was the first system designed in the milestone, also set the milestone status from `planned` to `designing` (in both the brief and `gat/milestone/milestone.md`).

Phase 3: Cross-Document Consistency Audit

After ALL documents in the current request are written (all systems in continue mode, or the single system in system mode), run a cross-document consistency audit before the review summary.

**Scope** — audit every newly written or updated document in this request against:

• Every other newly written document in this request (peer conflict check)
• Existing system GDD/docs within the SAME milestone that were NOT touche