design-system

When this skill is invoked:

1. Parse Arguments & Validate

Resolve the review mode (once, store for all gate spawns this run):

1. If --review [full|lean|solo] was passed → use that
2. Else read production/review-mode.txt → use that value
3. Else → default to lean

See .claude/docs/director-gates.md for the full check pattern.

A system name or retrofit path is required. If missing:

1. Check if design/gdd/systems-index.md exists.
2. If it exists: read it, find the highest-priority system with status "Not Started" or equivalent, and use AskUserQuestion:
   • Prompt: "The next system in your design order is [system-name] ([priority] | [layer]). Start designing it?"
   • Options: [A] Yes — design [system-name] / [B] Pick a different system / [C] Stop here
   • If [A]: proceed with that system name. If [B]: ask which system to design (plain text). If [C]: exit.
3. If no systems index exists, fail with:

   "Usage: /design-system <system-name> — e.g., /design-system movement Or to fill gaps in an existing GDD: /design-system retrofit design/gdd/[system-name].md No systems index found. Run /map-systems first to map your systems and get the design order."

Detect retrofit mode: If the argument starts with retrofit or the argument is a file path to an existing .md file in design/gdd/, enter retrofit mode:

1. Read the existing GDD file.
2. Identify which of the 8 required sections are present (scan for section headings). Required sections: Overview, Player Fantasy, Detailed Design/Rules, Formulas, Edge Cases, Dependencies, Tuning Knobs, Acceptance Criteria.
3. Identify which sections contain only placeholder text ([To be designed] or equivalent — blank, a single line, or obviously incomplete).
4. Present to the user before doing anything:

   ## Retrofit: [System Name]
   File: design/gdd/[filename].md
   
   Sections already written (will not be touched):
   ✓ [section name]
   ✓ [section name]
   
   Missing or incomplete sections (will be authored):
   ✗ [section name] — missing
   ✗ [section name] — placeholder only
   

5. Ask: "Shall I fill the [N] missing sections? I will not modify any existing content."
6. If yes: proceed to Phase 2 (Gather Context) as normal, but in Phase 3 skip creating the skeleton (file already exists) and in Phase 4 skip sections that are already complete. Only run the section cycle for missing/ incomplete sections.
7. Never overwrite existing section content. Use Edit tool to replace only [To be designed] placeholders or empty section bodies.

If NOT in retrofit mode, normalize the system name to kebab-case for the filename (e.g., "combat system" becomes combat-system).

2. Gather Context (Read Phase)

Read all relevant context before asking the user anything. This is the skill's primary advantage over ad-hoc design — it arrives informed.

2a: Required Reads

• Game concept: Read design/gdd/game-concept.md — fail if missing:

  "No game concept found. Run /brainstorm first."

• Systems index: Read design/gdd/systems-index.md — fail if missing:

  "No systems index found. Run /map-systems first to map your systems."

• Target system: Find the system in the index. If not listed, warn:

  "[system-name] is not in the systems index. Would you like to add it, or design it as an off-index system?"

• Entity registry: Read design/registry/entities.yaml if it exists. Extract all entries referenced by or relevant to this system (grep referenced_by.*[system-name] and source.*[system-name]). Hold these in context as known facts — values that other GDDs have already established and this GDD must not contradict.
• Reflexion log: Read docs/consistency-failures.md if it exists. Extract entries whose Domain matches this system's category. These are recurring conflict patterns — present them under "Past failure patterns" in the Phase 2d context summary so the user knows where mistakes have occurred before in this domain.

2b: Dependency Reads

From the systems index, identify:

• Upstream dependencies: Systems this one depends on. Read their GDDs if they exist (these contain decisions this system must respect).
• Downstream dependents: Systems that depend on this one. Read their GDDs if they exist (these contain expectations this system must satisfy).

For each dependency GDD that exists, extract and hold in context:

• Key interfaces (what data flows between the systems)
• Formulas that reference this system's outputs
• Edge cases that assume this system's behavior
• Tuning knobs that feed into this system

2c: Optional Reads

• Game pillars: Read design/gdd/game-pillars.md if it exists
• Existing GDD: Read design/gdd/[system-name].md if it exists (resume, don't restart from scratch)
• Related GDDs: Glob design/gdd/*.md and read any that are thematically related (e.g., if designing a system that overlaps with another in scope, read the related GDD even if it's not a formal dependency)

2d: Present Context Summary

Before starting design work, present a brief summary to the user:

Designing: [System Name]

• Priority: [from index] | Layer: [from index]
• Depends on: [list, noting which have GDDs vs. undesigned]
• Depended on by: [list, noting which have GDDs vs. undesigned]
• Existing decisions to respect: [key constraints from dependency GDDs]
• Pillar alignment: [which pillar(s) this system primarily serves]
• Known cross-system facts (from registry):
  • [entity_name]: [attribute]=[value], [attribute]=[value] (owned by [source GDD])
  • [item_name]: [attribute]=[value], [attribute]=[value] (owned by [source GDD])
  • [formula_name]: variables=[list], output=[min–max] (owned by [source GDD])
  • [constant_name]: [value] [unit] (owned by [source GDD]) (These values are locked — if this GDD needs different values, surface the conflict before writing. Do not silently use different numbers.)

If no registry entries are relevant: omit the "Known cross-system facts" section.

If any upstream dependencies are undesigned, warn:

"[dependency] doesn't have a GDD yet. We'll need to make assumptions about its interface. Consider designing it first, or we can define the expected contract and flag it as provisional."

2e: Technical Feasibility Pre-Check

Before asking the user to begin designing, load engine context and surface any constraints or knowledge gaps that will shape the design.

Step 1 — Determine the engine domain for this system: Map the system's category (from systems-index.md) to an engine domain:

Step 2 — Read engine context (if available):

• Read .claude/docs/technical-preferences.md to identify the engine and version
• If engine is configured, read docs/engine-reference/[engine]/VERSION.md
• Read docs/engine-reference/[engine]/modules/[domain].md if it exists
• Read docs/engine-reference/[engine]/breaking-changes.md for domain-relevant entries
• Glob docs/architecture/adr-*.md and read any ADRs whose domain matches (check the Engine Compatibility table's "Domain" field)

Step 3 — Present the Feasibility Brief:

If engine reference docs exist, present before starting design:

## Technical Feasibility Brief: [System Name]
Engine: [name + version]
Domain: [domain]

### Known Engine Capabilities (verified for [version])
- [capability relevant to this system]
- [capability 2]

### Engine Constraints That Will Shape This Design
- [constraint from engine-reference or existing ADR]

### Knowledge Gaps (verify before committing to these)
- [post-cutoff feature this design might rely on — mark HIGH/MEDIUM risk]

### Existing ADRs That Constrain This System
- ADR-XXXX: [decision summary] — means [implication for this GDD]
  (or "None yet")

If no engine reference docs exist (engine not yet configured), show a short note:

"No engine configured yet — skipping technical feasibility check. Run /setup-engine before moving to architecture if you haven't already."

Step 4 — Ask before proceeding:

Use AskUserQuestion:

• "Any constraints to add before we begin, or shall we proceed with these noted?"
  • Options: "Proceed with these noted", "Add a constraint first", "I need to check the engine docs — pause here"

Use AskUserQuestion:

• "Ready to start designing [system-name]?"
  • Options: "Yes, let's go", "Show me more context first", "Design a dependency first"

3. Create File Skeleton

Once the user confirms, immediately create the GDD file with empty section headers. This ensures incremental writes have a target.

Use the template structure from .claude/docs/templates/game-design-document.md:

# [System Name]

> **Status**: In Design
> **Author**: [user + agents]
> **Last Updated**: [today's date]
> **Implements Pillar**: [from context]

## Overview

[To be designed]

## Player Fantasy

[To be designed]

## Detailed Design

### Core Rules

[To be designed]

### States and Transitions

[To be designed]

### Interactions with Other Systems

[To be designed]

## Formulas

[To be designed]

## Edge Cases

[To be designed]

## Dependencies

[To be designed]

## Tuning Knobs

[To be designed]

## Visual/Audio Requirements

[To be designed]

## UI Requirements

[To be designed]

## Acceptance Criteria

[To be designed]

## Open Questions

[To be designed]

Ask: "May I create the skeleton file at design/gdd/[system-name].md?"

After writing, update production/session-state/active.md:

• Use Glob to check if the file exists.
• If it does not exist: use the Write tool to create it. Never attempt Edit on a file that may not exist.
• If it already exists: use the Edit tool to update the relevant fields.

File content:

• Task: Designing [system-name] GDD
• Current section: Starting (skeleton created)
• File: design/gdd/[system-name].md

4. Section-by-Section Design

Walk through each section in order. For each section, follow this cycle:

The Section Cycle

Context  ->  Questions  ->  Options  ->  Decision  ->  Draft  ->  Approval  ->  Write

1. Context: State what this section needs to contain, and surface any relevant decisions from dependency GDDs that constrain it.

2. Questions: Ask clarifying questions specific to this section. Use AskUserQuestion for constrained questions, conversational text for open-ended exploration.

3. Options: Where the section involves design choices (not just documentation), present 2-4 approaches with pros/cons. Explain reasoning in conversation text, then use AskUserQuestion to capture the decision.

4. Decision: User picks an approach or provides custom direction.

5. Draft: Write the section content in conversation text for review. Flag any provisional assumptions about undesigned dependencies.

6. Approval: Immediately after the draft — in the SAME response — use AskUserQuestion. NEVER use plain text. NEVER skip this step.

   • Prompt: "Approve the [Section Name] section?"
   • Options: [A] Approve — write it to file / [B] Make changes — describe what to fix / [C] Start over

   The draft and the approval widget MUST appear together in one response. If the draft appears without the widget, the user is left at a blank prompt with no path forward — this is a protocol violation.

7. Write: Use the Edit tool to replace the placeholder with the approved content. CRITICAL: Always include the section heading in the old_string to ensure uniqueness — never match [To be designed] alone, as multiple sections use the same placeholder and the Edit tool requires a unique match. Use this pattern:

   old_string: "## [Section Name]\n\n[To be designed]"
   new_string: "## [Section Name]\n\n[approved content]"
   

   Confirm the write.

8. Registry conflict check (Sections C and D only — Detailed Design and Formulas): After writing, scan the section content for entity names, item names, formula names, and numeric constants that appear in the registry. For each match:

   • Compare the value just written against the registry entry.
   • If they differ: surface the conflict immediately before starting the next section. Do not continue silently.

     "Registry conflict: [name] is registered in [source GDD] as [registry_value]. This section just wrote [new_value]. Which is correct?"

   • If new (not in registry): flag it as a candidate for registry registration (will be handled in Phase 5).

After writing each section, update production/session-state/active.md with the completed section name. Use Glob to check if the file exists — use Write to create it if absent, Edit to update it if present.

Section-Specific Guidance

Each section has unique design considerations and may benefit from specialist agents:

Section A: Overview

Goal: One paragraph a stranger could read and understand.

Derive recommended options before building the widget: Read the system's category and layer from the systems index (already in context from Phase 2), then determine the recommended option for each tab:

• Framing tab: Foundation/Infrastructure layer → [A] recommended. Player-facing categories (Combat, UI, Dialogue, Character, Animation, Visual Effects, Audio) → [C] Both recommended.
• ADR ref tab: Glob docs/architecture/adr-*.md and grep for the system name in the GDD Requirements section of any ADR. If a matching ADR is found → [A] Yes — cite the ADR recommended. If none found → [B] No recommended.
• Fantasy tab: Foundation/Infrastructure layer → [B] No recommended. All other categories → [A] Yes recommended.

Append (Recommended) to the appropriate option text in each tab.

Framing questions (ask BEFORE drafting): Use AskUserQuestion with a multi-tab widget:

• Tab "Framing" — "How should the overview frame this system?" Options: [A] As a data/infrastructure layer (technical framing) / [B] Through its player-facing effect (design framing) / [C] Both — describe the data layer and its player impact
• Tab "ADR ref" — "Should the overview reference the existing ADR for this system?" Options: [A] Yes — cite the ADR for implementation details / [B] No — keep the GDD at pure design level
• Tab "Fantasy" — "Does this system have a player fantasy worth stating?" Options: [A] Yes — players feel it directly / [B] No — pure infrastructure, players feel what it enables

Use the user's answers to shape the draft. Do NOT answer these questions yourself and auto-draft.

Questions to ask:

• What is this system in one sentence?
• How does a player interact with it? (active/passive/automatic)
• Why does this system exist — what would the game lose without it?

Cross-reference: Check that the description aligns with how the systems index describes it. Flag discrepancies.

Design vs. implementation boundary: Overview questions must stay at the behavior level — what the system does, not how it is built. If implementation questions arise during the Overview (e.g., "Should this use an Autoload singleton or a signal bus?"), note them as "→ becomes an ADR" and move on. Implementation patterns belong in /architecture-decision, not the GDD. The GDD describes behavior; the ADR describes the technical approach used to achieve it.

Section B: Player Fantasy

Goal: The emotional target — what the player should feel.

Derive recommended option before building the widget: Read the system's category and layer from Phase 2 context:

• Player-facing categories (Combat, UI, Dialogue, Character, Animation, Audio, Level/World) → [A] Direct recommended
• Foundation/Infrastructure layer → [B] Indirect recommended
• Mixed categories (Camera/input, Economy, AI with visible player effects) → [C] Both recommended

Append (Recommended) to the appropriate option text.

Framing question (ask BEFORE drafting): Use AskUserQuestion:

• Prompt: "Is this system something the player engages with directly, or infrastructure they experience indirectly?"
• Options: [A] Direct — player actively uses or feels this system / [B] Indirect — player experiences the effects, not the system / [C] Both — has a direct interaction layer and infrastructure beneath it

Use the answer to frame the Player Fantasy section appropriately. Do NOT assume the answer.

Questions to ask:

• What emotion or power fantasy does this serve?
• What reference games nail this feeling? What specifically creates it?
• Is this a "system you love engaging with" or "infrastructure you don't notice"?

Cross-reference: Must align with the game pillars. If the system serves a pillar, quote the relevant pillar text.

Agent delegation (MANDATORY): After the framing answer is given but before drafting, spawn creative-director via Task:

• Provide: system name, framing answer (direct/indirect/both), game pillars, any reference games the user mentioned, the game concept summary
• Ask: "Shape the Player Fantasy for this system. What emotion or power fantasy should it serve? What player moment should we anchor to? What tone and language fits the game's established feeling? Be specific — give me 2-3 candidate framings."
• Collect the creative-director's framings and present them to the user alongside the draft.

Do NOT draft Section B without first consulting creative-director. The framing answer tells us what kind of fantasy it is; the creative-director shapes how it's described — tone, language, the specific player moment to anchor to.

Section C: Detailed Design (Core Rules, States, Interactions)

Goal: Unambiguous specification a programmer could implement without questions.

This is usually the largest section. Break it into sub-sections:

1. Core Rules: The fundamental mechanics. Use numbered rules for sequential processes, bullets for properties.
2. States and Transitions: If the system has states, map every state and every valid transition. Use a table.
3. Interactions with Other Systems: For each dependency (upstream and downstream), specify what data flows in, what flows out, and who owns the interface.

Questions to ask:

• Walk me through a typical use of this system, step by step
• What are the decision points the player faces?
• What can the player NOT do? (Constraints are as important as capabilities)

Agent delegation (MANDATORY): Before drafting Section C, spawn specialist agents via Task in parallel:

• Look up the system category in the routing table (Section 6 of this skill)
• Spawn the Primary Agent AND Supporting Agent(s) listed for this category
• Provide each agent: system name, game concept summary, pillar set, dependency GDD excerpts, the specific section being worked on
• Collect their findings before drafting
• Surface any disagreements between agents to the user via AskUserQuestion
• Draft only after receiving specialist input

Do NOT draft Section C without first consulting the appropriate specialists. A systems-designer reviewing rules and mechanics will catch design gaps the main session cannot.

Cross-reference: For each interaction listed, verify it matches what the dependency GDD specifies. If a dependency defines a value or formula and this system expects something different, flag the conflict.

Section D: Formulas

Goal: Every mathematical formula, with variables defined, ranges specified, and edge cases noted.

Completion Steering — always begin each formula with this exact structure:

The [formula_name] formula is defined as:

`[formula_name] = [expression]`

**Variables:**
| Variable | Symbol | Type | Range | Description |
|----------|--------|------|-------|-------------|
| [name] | [sym] | float/int | [min–max] | [what it represents] |

**Output Range:** [min] to [max] under normal play; [behaviour at extremes]
**Example:** [worked example with real numbers]

Do NOT write [Formula TBD] or describe a formula in prose without the variable table. A formula without defined variables cannot be implemented without guesswork.

Questions to ask:

• What are the core calculations this system performs?
• Should scaling be linear, logarithmic, or stepped?
• What should the output ranges be at early/mid/late game?

Agent delegation (MANDATORY): Before proposing any formulas or balance values, spawn specialist agents via Task in parallel:

• Always spawn systems-designer: provide Core Rules from Section C, tuning goals from user, balance context from dependency GDDs. Ask them to propose formulas with variable tables and output ranges.
• For economy/cost systems, also spawn economy-designer: provide placement costs, upgrade cost intent, and progression goals. Ask them to validate cost curves and ratios.
• Present the specialists' proposals to the user for review via AskUserQuestion
• The user decides; the main session writes to file
• Do NOT invent formula values or balance numbers without specialist input. A user without balance design expertise cannot evaluate raw numbers — they need the specialists' reasoning.

Cross-reference: If a dependency GDD defines a formula whose output feeds into this system, reference it explicitly. Don't reinvent — connect.

Section E: Edge Cases

Goal: Explicitly handle unusual situations so they don't become bugs.

Completion Steering — format each edge case as:

• If [condition]: [exact outcome]. [rationale if non-obvious]

Example (adapt terminology to the game's domain):

• If [resource] reaches 0 while [protective condition] is active: hold at minimum until condition ends, then apply consequence.
• If two [triggers/events] fire simultaneously: resolve in [defined priority order]; ties use [defined tiebreak rule].

Do NOT write vague entries like "handle appropriately" — each must name the exact condition and the exact resolution. An edge case without a resolution is an open design question, not a specification.

Questions to ask:

• What happens at zero? At maximum? At out-of-range values?
• What happens when two rules apply at the same time?
• What happens if a player finds an unintended interaction? (Identify degenerate strategies)

Agent delegation (MANDATORY): Spawn systems-designer via Task before finalising edge cases. Provide: the completed Sections C and D, and ask them to identify edge cases from the formula and rule space that the main session may have missed. For narrative systems, also spawn narrative-director. Present their findings and ask the user which to include.

Cross-reference: Check edge cases against dependency GDDs. If a dependency defines a floor, cap, or resolution rule that this system could violate, flag it.

Section F: Dependencies

Goal: Map every system connection with direction and nature.

This section is partially pre-filled from the con