Technical Design Document Skill

When to Use

• Designing a new feature or system
• Documenting architecture decisions (ADR/RFC)
• Planning refactoring or optimization work

Execution Flow

1. Assess Complexity

Level	Scope	Sections Required
Small	Single component, <100 LOC	TL;DR, Design, Implementation
Medium	Cross-component, API changes	+ Background, Solution Analysis
Large	System-level, new service	Full template

2. Gather Context

Before writing, explore the codebase:

• Identify affected components (grep/glob for related code)
• Read existing implementations and patterns
• Note dependencies and potential side effects
• Check for similar solutions already in codebase

3. Write Document

Follow the template structure below, scaled to complexity level.

4. Verify Before Handoff

• [ ] Problem clearly defined (what breaks if we do nothing?)
• [ ] Options compared with trade-offs (not just one solution)
• [ ] Decision rationale documented
• [ ] Diagrams illustrate key flows
• [ ] Implementation steps are concrete and actionable
• [ ] Risks identified with mitigations

Document Template

# [Feature/System Name] Technical Design

## TL;DR
- 3-5 bullets: problem, solution, key decisions, expected outcome

## Background (Medium/Large)

### Current State
- Existing behavior and limitations

### Problem Statement
- What breaks if we do nothing?
- Who is affected and how?

### Goals / Non-Goals
- Goals: what this design achieves
- Non-Goals: explicitly out of scope

## Solution Analysis (Medium/Large)

### Option 1: [Name]
Pros: ...
Cons: ...

### Option 2: [Name]
Pros: ...
Cons: ...

### Comparison
| Criteria | Option 1 | Option 2 |
|----------|----------|----------|
| Performance | ... | ... |
| Complexity | ... | ... |

### Recommendation
Selected: Option X
Rationale: [why]

## Detailed Design

### Architecture
[Mermaid diagram - see examples below]

### Component Design
- Responsibilities
- Interfaces
- Dependencies

### Data Model (if applicable)
[Schema or structure]

### API Design (if applicable)
[Endpoints, request/response]

## Implementation Plan

### Phase 1: [Name]
- [ ] Task 1
- [ ] Task 2

### Migration Strategy (if applicable)

## Risk Assessment

| Risk | Probability | Impact | Mitigation |
|------|-------------|--------|------------|
| ... | High/Med/Low | High/Med/Low | ... |

## References
- Related docs, external resources

Mermaid Diagram Examples

Architecture (flowchart):

flowchart TD
    A[Client] --> B[API Gateway]
    B --> C[Service]
    C --> D[(Database)]

Sequence:

sequenceDiagram
    Client->>Server: Request
    Server->>DB: Query
    DB-->>Server: Result
    Server-->>Client: Response

State:

stateDiagram-v2
    [*] --> Pending
    Pending --> Processing: start
    Processing --> Done: complete
    Processing --> Failed: error

Handling Feedback

When user requests changes:

1. Understand which section needs revision
2. Update only affected sections
3. Ensure changes don't contradict other sections
4. Re-verify the checklist items related to changes

Output Location

• Check if project has docs/, ai_docs/, or design/ directory
• Ask user if location is unclear
• Use descriptive filename: design-[feature-name].md