Design Doc — Engineering Design Document Generator

Generate complete, publication-ready design documents with embedded Mermaid diagrams from a single prompt.

Philosophy

One-shot output — Full document from single prompt
Diagrams first-class — Every doc includes 2-5 Mermaid diagrams
Opinionated defaults — Make reasonable choices, document in Decision Log

Workflow

Parse request — Extract what, why, who, constraints, scope
Clarify only if critical — ONE round of ≤4 questions if truly vague
Select diagrams — Use decision matrix below
Generate document — Use template, all sections substantive
Save — .output/design-doc-{topic}-{timestamp}.md

Diagram Selection Matrix

Scenario	Diagrams
New service/microservice	C4 Context, C4 Container, Sequence, ERD
Feature in existing system	Component, Sequence, State (if stateful)
API design	C4 Context, Sequence, Class, ERD
Data pipeline/ETL	Flowchart, C4 Container, ERD
Auth/authorization	Sequence, C4 Context, State
Event-driven	C4 Container, Sequence, Flowchart
Migration/refactor	C4 Container (before+after), Flowchart

Always include: 1 context-level diagram + 1 flow diagram

Document Template

# Design Document: {Title}

**Author:** {User}
**Date:** {YYYY-MM-DD}
**Status:** Draft

## 1. Overview
### Problem Statement
### Goals
### Non-Goals

## 2. Architecture
### System Context
```mermaid
{C4 Context}

Container View

{C4 Container}

3. Component Design

{Component/Class}

4. Data Model

{ERD}

5. Key Flows

Primary Flow

{Sequence}

6. State Management (if applicable)

{State}

7. API Contracts (if applicable)

8. Non-Functional Requirements

Requirement	Target
Availability	99.9%
Latency (p99)	<200ms

9. Security Considerations

10. Decision Log

#	Decision	Rationale

11. Risks & Mitigations

12. Open Questions


---

## Diagram Styling

```mermaid
%%{init: {'theme': 'default'}}%%
flowchart TD
    A[Service]:::primary --> B[(Database)]:::secondary
    A --> C[External]:::accent

    classDef primary fill:#2374ab,color:#fff
    classDef secondary fill:#57a773,color:#fff
    classDef accent fill:#ff8c42,color:#fff

Quick Commands

User Says	Action
"Design a {system}"	Full design document
"Create design doc for {feature}"	Scoped to feature
"Architect {system}"	Architecture-focused doc
"How should I design {thing}?"	Doc with Decision Log

Reference

references/mermaid-syntax.md — Full syntax for all diagram types

design-doc