| 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:
- Spawn
gat-designer → gat/milestone/m{N}-<name>/<system>/<system>-gdd-m{N}.md
- If the system needs content data, spawn
gat-designer → gat/milestone/m{N}-<name>/<system>/<system>-data-m{N}.md
- Spawn
gat-artist → gat/milestone/m{N}-<name>/<system>/<system>-art-m{N}.md
- 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: 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)
