Design Document Creation Guide

Complete guide for creating design documents that define functional specifications based on PRD phases.

Quick Start

Create design docs that translate PRD phases into detailed functional specifications. Design docs bridge the gap between product requirements and implementation.

Design Doc Structure

A design doc should include:

markdown

# Design Document: {{Feature/Component Name}}

## Metadata
- **Related PRD**: {{PRD name and phase}}
- **Phase**: {{Phase number and name}}
- **Status**: {{draft|review|approved|implemented}}
- **Last Updated**: {{date}}
- **Author**: {{designer/engineer}}
- **Reviewers**: {{reviewers}}

## Overview
{{Brief description of what this design doc covers}}

## Context
- **Problem**: {{What problem this solves (from PRD)}}
- **Goals**: {{What this design achieves (from PRD phase)}}
- **Scope**: {{What's in scope and out of scope}}

## Functional Specifications

### Feature: {{Feature Name}}
**Description**: {{What this feature does}}
**User Story**: {{From PRD}}
**Acceptance Criteria**: {{From PRD}}

**Functional Requirements**:
- FR-1: {{Requirement}}
- FR-2: {{Requirement}}

**User Flows**:
1. {{Step 1}}
2. {{Step 2}}
3. {{Step 3}}

**Edge Cases**:
- {{Edge case 1}}: {{How to handle}}
- {{Edge case 2}}: {{How to handle}}

### Feature: {{Feature Name}}
[Same structure for each feature]

## Technical Design

### Architecture
{{High-level architecture diagram or description}}

### Components
- **Component 1**: {{Purpose and responsibility}}
- **Component 2**: {{Purpose and responsibility}}

### Data Models
```typescript
// Example data structures
interface {{ModelName}} {
  // Fields
}

APIs/Interfaces

•
API Endpoint: {{Method}} {{Path}}
- •Request: {{Structure}}
- •Response: {{Structure}}
- •Errors: {{Error cases}}

Algorithms/Logic

Implementation Plan

Tasks

• {{Task 1}}
• {{Task 2}}
• {{Task 3}}

Dependencies

•Requires: {{What must be done first}}
•Blocks: {{What this unblocks}}

Testing Strategy

•Unit tests: {{What to test}}
•Integration tests: {{What to test}}
•E2E tests: {{What to test}}

Non-Functional Requirements

Performance

•Response time: {{target}}
•Throughput: {{target}}
•Scalability: {{requirements}}

Security

•Authentication: {{requirements}}
•Authorization: {{requirements}}
•Data protection: {{requirements}}

Reliability

•Uptime: {{target}}
•Error handling: {{approach}}
•Monitoring: {{what to monitor}}

Design Decisions

Decision 1: {{Decision}}

Context: {{Why this decision was needed}} Options Considered: {{Alternatives}} Chosen Solution: {{What was chosen}} Rationale: {{Why this was chosen}} Trade-offs: {{What was given up}}

Decision 2: {{Decision}}

[Same structure]

Open Questions

• {{Question 1}}
• {{Question 2}}

Risks & Mitigations

•
Risk 1: {{Description}}
- •Mitigation: {{How to address}}
•
Risk 2: {{Description}}
- •Mitigation: {{How to address}}

Success Metrics

•{{Metric 1}}: {{How to measure}}
•{{Metric 2}}: {{How to measure}}

References

•PRD: {{Link to PRD and phase}}
•Related Design Docs: {{Links to related docs}}
•Technical Docs: {{Links to relevant technical docs}}
•AGENTS.md: {{Relevant sections}}

code


## From PRD Phase to Design Doc

### Mapping PRD to Design

1. **Identify Phase Scope**
   - Review PRD phase goals and deliverables
   - Break down into features/components
   - Each major feature may need its own design doc

2. **Extract Requirements**
   - User stories from PRD
   - Acceptance criteria
   - Success metrics
   - Constraints and assumptions

3. **Define Functional Specs**
   - Translate user stories into functional requirements
   - Define user flows
   - Identify edge cases
   - Specify data needs

4. **Design Technical Solution**
   - Architecture and components
   - Data models
   - APIs and interfaces
   - Algorithms and logic

5. **Plan Implementation**
   - Break into tasks
   - Identify dependencies
   - Define testing approach

## Functional Specification Guidelines

### Writing Good Functional Requirements

**Format**: "The system SHALL [action] [condition]"

Examples:
- ✅ "The system SHALL validate email format before submission"
- ✅ "The system SHALL display error message when validation fails"
- ❌ "The system should be user-friendly"
- ❌ "The system needs to work well"

### User Flows

Document complete user journeys:
1. Start with user goal
2. Include all steps (happy path)
3. Document alternative paths
4. Include error handling
5. Show end states

### Edge Cases

Consider:
- Empty/null inputs
- Invalid inputs
- Boundary conditions
- Concurrent operations
- Failure scenarios
- Data inconsistencies

## Technical Design Guidelines

### Architecture Decisions

Document:
- Why this architecture was chosen
- What alternatives were considered
- What trade-offs were made
- How it fits with existing system (reference AGENTS.md)

### Component Design

For each component:
- Single responsibility
- Clear interfaces
- Dependencies documented
- Integration points identified

### Data Models

Define:
- Required fields
- Optional fields
- Validation rules
- Relationships
- Indexes (if applicable)

## Design Doc Best Practices

### Clarity
- Use diagrams for complex flows
- Include examples
- Define terminology
- Reference existing patterns from AGENTS.md

### Completeness
- Cover all features from PRD phase
- Address all acceptance criteria
- Include error handling
- Document edge cases

### Actionability
- Tasks are specific and clear
- Dependencies are explicit
- Testing strategy is defined
- Success metrics are measurable

## Design Doc to Implementation Handoff

The design doc should provide:
1. **Clear functional specs** - What to build
2. **Technical design** - How to build it
3. **Implementation plan** - Tasks and order
4. **Testing strategy** - How to validate
5. **Success criteria** - How to know it's done

## Quality Checklist

Before finalizing a design doc, ensure:
- ✅ All PRD phase requirements are addressed
- ✅ Functional specs are clear and testable
- ✅ Technical design follows project patterns (AGENTS.md)
- ✅ Implementation plan is actionable
- ✅ Testing strategy is defined
- ✅ Dependencies are documented
- ✅ Open questions are identified
- ✅ Success metrics align with PRD

## Integration with Project

Reference AGENTS.md for:
- Architectural patterns
- Development conventions
- Code style guidelines
- Testing strategies
- Deployment processes

Link to:
- PRD phase this implements
- Related design docs
- Implementation tasks/issues