Legacy Codebase Reverse Documentation

Reverse Documentation

This skill analyzes existing implementation (code, prototypes, systems) and generates appropriate design or architecture documentation. Use this when:

• You built a feature without writing a design doc first
• You inherited a codebase without documentation
• You prototyped a mechanic and need to formalize it
• You need to document "why" behind existing code

Workflow

Phase 1: Parse Arguments

Format: /reverse-document <type> <path>

Type options:

• design → Generate a game design document (GDD section)
• architecture → Generate an Architecture Decision Record (ADR)
• concept → Generate a concept document from prototype

Path: Directory or file to analyze

• src/gameplay/combat/ → All combat-related code
• src/core/event-system.cpp → Specific file
• prototypes/stealth-mech/ → Prototype directory

Examples:

/reverse-document design src/gameplay/magic-system
/reverse-document architecture src/core/entity-component
/reverse-document concept prototypes/vehicle-combat

Phase 2: Analyze Implementation

Read and understand the code/prototype:

For design docs (GDD):

• Identify mechanics, rules, formulas
• Extract gameplay values (damage, cooldowns, ranges)
• Find state machines, ability systems, progression
• Detect edge cases handled in code
• Map dependencies (what systems interact?)

For architecture docs (ADR):

• Identify patterns (ECS, singleton, observer, etc.)
• Understand technical decisions (threading, serialization, etc.)
• Map dependencies and coupling
• Assess performance characteristics
• Find constraints and trade-offs

For concept docs (prototype analysis):

• Identify core mechanic
• Extract emergent gameplay patterns
• Note what worked vs what didn't
• Find technical feasibility insights
• Document player fantasy / feel

Phase 3: Ask Clarifying Questions

DO NOT just describe the code. ASK about intent:

Design questions:

• "I see a [resource] system that depletes during [activity]. Was this for:
  • Pacing (prevent spam)?
  • Resource management (strategic depth)?
  • Or something else?"
• "The [mechanic] seems central. Is this a core pillar, or supporting feature?"
• "[Value] scales exponentially with [factor]. Intentional design, or needs rebalancing?"

Architecture questions:

• "You're using a service locator pattern. Was this chosen for:
  • Testability (mock dependencies)?
  • Decoupling (reduce hard references)?
  • Or inherited from existing code?"
• "I see manual memory management instead of smart pointers. Performance requirement, or legacy?"

Concept questions:

• "The prototype emphasizes stealth over combat. Is that the intended pillar?"
• "Players seem to exploit the grappling hook for speed. Feature or bug?"

Phase 4: Present Findings

Before drafting, show what you discovered:

I've analyzed [path]/. Here's what I found:

MECHANICS IMPLEMENTED:
- [mechanic-a] with [property] (e.g. timing windows, cooldowns)
- [mechanic-b] (e.g. interaction between two states)
- [resource] system (depletes on [action], regens on [condition])
- [state] system (builds up, triggers [effect])

FORMULAS DISCOVERED:
- [Output] = [formula using discovered variables]
- [Secondary output] = [formula]

UNCLEAR INTENT AREAS:
1. [Resource] system — pacing or resource management?
2. [Mechanic] — core pillar or supporting feature?
3. [Value] scaling — intentional design or needs tuning?

Before I draft the design doc, could you clarify these points?

Wait for user to clarify intent before drafting.

Phase 5: Draft Document Using Template

Based on type, use appropriate template:

Draft structure:

• Capture what exists (mechanics, patterns, implementation)
• Document why it exists (intent clarified with user)
• Identify what's missing (edge cases not handled, gaps in design)
• Flag follow-up work (balance tuning, missing features)

Phase 6: Show Draft and Request Approval

Collaborative protocol:

I've drafted the [system-name] design doc based on your code and clarifications.

[Show key sections: Overview, Mechanics, Formulas, Design Intent]

ADDITIONS I MADE:
- Documented [mechanic] as "[intent]" per your clarification
- Added edge cases not in code (e.g., what if [resource] hits 0 mid-[action]?)
- Flagged balance concern: [scaling type] scaling at [boundary condition]

SECTIONS MARKED AS INCOMPLETE:
- "[System] interaction with [other-system]" (not fully implemented yet)
- "[Variant or feature]" (only [subset] implemented so far)

May I write this to design/gdd/[system-name].md?

Wait for approval. User may request changes before writing.

Phase 7: Write Document with Metadata

When approved, write the file with special markers:

---
status: reverse-documented
source: [path/]
date: [today]
verified-by: [User name]
---

# [System Name] Design

> **Note**: This document was reverse-engineered from the existing implementation.
> It captures current behavior and clarified design intent. Some sections may be
> incomplete where implementation is partial or intent was unclear.

[Rest of document...]

Phase 8: Flag Follow-Up Work

After writing, suggest next steps:

✅ Written to design/gdd/combat-system.md

FOLLOW-UP RECOMMENDED:
1. Run /balance-check on combat formulas (exponential scaling concern)
2. Create ADR for stamina system architecture decision
3. Implement missing edge cases:
   - Stamina depletion mid-combo behavior
   - Stagger state during dodge
4. Extend design doc when weapon variety is implemented

Would you like me to tackle any of these now?

Template Selection Logic

Example Session: Reverse-Document a System

User: /reverse-document design src/gameplay/[system]

Agent: I'll analyze your [system] implementation to create a design doc.
       [Reads code, discovers mechanics, formulas]

Agent: I've analyzed src/gameplay/[system]/. Here's what I found:
       [Shows mechanics, formulas, unclear areas]

       Before drafting, could you clarify:
       1. [Resource] system intent?
       2. Is [mechanic] a core pillar?
       3. [Value] scaling — intentional or needs tuning?

User: 1. [Resource] is for pacing, prevent [unwanted behavior]
      2. [Mechanic] is core — I want [design intent]
      3. Yeah, exponential is too much, should be linear

Agent: Perfect, that clarifies the vision. Let me draft the design doc.
       [Shows draft with clarified intent]

       May I write this to design/gdd/[system-name].md?

User: Yes, write it.

Agent: ✅ Written to design/gdd/[system-name].md
       ✅ Marked as [REVERSE-DOCUMENTED]
       ✅ Flagged [value] scaling for rebalancing

       Next steps:
       - Update [formula] to [corrected scaling]
       - Run /balance-check to validate [curve]
       - Document [mechanic] as core pillar in game-pillars.md

Collaborative Protocol

This skill follows the collaborative design principle:

1. Analyze First: Read code, understand implementation
2. Question Intent: Ask about "why", not just "what"
3. Present Findings: Show discoveries, highlight unclear areas
4. User Clarifies: Separate intent from accidents
5. Draft Document: Create doc based on reality + intent
6. Show Draft: Display key sections, explain additions
7. Get Approval: "May I write to [filepath]?" On approval: Verdict: COMPLETE — document generated. On decline: Verdict: BLOCKED — user declined write.
8. Flag Follow-Up: Suggest related work, don't auto-execute

Never assume intent. Always ask before documenting "why".