Design System

Purpose: Write or update one game-system GDD from the approved systems map.

Use when:

• a system already exists in design/gdd/systems-index.md
• the project needs one implementable GDD for that specific system

Do not use for:

• creating the overall systems index
• inventing the game concept
• broad stage or project health analysis

Inputs / Required Context:

• required: design/gdd/game-concept.md and design/gdd/systems-index.md
• optional: design/gdd/game-pillars.md, existing design/gdd/[system-name].md, upstream or downstream system docs, relevant prototype reports in prototypes/*/REPORT.md, gamedev/standards/design-docs.md

Outputs / Owned Artifacts:

• owns design/gdd/[system-name].md
• updates the matching row in design/gdd/systems-index.md
• uses gamedev/templates/game-design-document.md
• updates the GDD status block and acceptance criteria so the document matches the strongest confirmed project state

Modes or Arguments:

• ``: normalized to kebab-case for the target GDD path

Execution Rules:

1. Parse the system name and validate that the concept and systems index exist.
2. Stop and route to map-systems if design/gdd/systems-index.md is missing.
3. Stop and refresh the systems index if the requested system is not present there.
4. Summarize the system's layer, priority, dependencies, interfaces, formulas, assumptions, and existing evidence.
5. Create or update design/gdd/[system-name].md using the canonical GDD template and shared design-doc rules.
6. Preserve the project's working or documentation language and player-facing terminology policy when it is already fixed in the concept or technical-preferences docs. Do not mix languages inside the system GDD unless the project language policy explicitly requires it.
7. Keep the system contract platform-agnostic unless a runtime capability, input mode, hardware limit, or delivery constraint is genuinely part of the design.
8. Ensure the Scope Boundaries section clearly separates:

• In MVP
• In Vertical Slice
• Deferred / Later

1. If a relevant prototype report exists, keep an explicit Evidence / Prototype Inputs section that:

• references the report path or paths
• extracts up to 4 key findings
• records adopted baseline values or None adopted yet
• records remaining open questions or evidence gaps

1. If no relevant prototype exists, keep the section and explicitly mark None yet instead of implying certainty.
2. Update the matching row in design/gdd/systems-index.md so status reflects the strongest confirmed state.
3. When syncing status:

• use designed when the GDD exists but prototype findings are not yet folded into it
• use informed-by-prototype when prototype findings and baseline decisions are reflected in the GDD
• never downgrade implemented or integrated

1. Sync the GDD ## Status block to the same strongest confirmed state you are relying on:

• keep Document Status as Draft only while implementation blockers remain
• use Approved when the document is implementation-ready
• use Implemented when production code already exists for the system
• use Integrated when the system is already verified inside the main playable loop

1. If acceptance criteria are already satisfied by the current repository state, check them or rewrite them so the document does not pretend the repo is less complete than it really is.
2. If prototype or playtest evidence changes scope or assumptions, rewrite the affected sections so the document stays internally consistent instead of appending contradictory notes.
3. When the GDD is implementation-ready and the scaffold already exists, end with a handoff to implement-system; otherwise hand off to another design-system or prototype.

Failure / Stop Conditions:

• stop if the game concept is missing
• stop if the requested system is outside the current systems index
• record unknowns as assumptions or open questions instead of inventing detail

Return Format:

• GDD path
• systems-index status update and rationale
• scope boundaries captured
• key rules or decisions captured
• evidence adopted from prototype reports, if any
• open questions that still block implementation
• next recommended skill: design-system, prototype, or implement-system

Example Invocation:

• /design-system combat-loop

Related Skills / Boundary:

• use map-systems to define the system list before writing GDDs
• use prototype when one mechanic is still too risky to spec confidently
• use implement-system once the GDD is stable and a runnable project scaffold exists

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: avelrl
• Source: avelrl/skills
• License: MIT

Install and usage instructions live in the source repository linked above.