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 | GDD | Art | Data | Status) to mark GDD/Art/Data paths filled and 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: Review

Summarize what was created or updated for the system(s) in the milestone.

Use AskUserQuestion:

• Design another system in this milestone → Run /gat-design <milestone> <system>
• Continue remaining systems in this milestone → Run /gat-design <milestone>
• Stop here (hand the milestone to engineering when all its systems are Designed)