Mermaid Diagrams

Create professional software diagrams using Mermaid's text-based syntax. Mermaid renders diagrams from simple text definitions, making diagrams version-controllable, easy to update, and maintainable alongside code.

Installation

OpenClaw / Moltbot / Clawbot

npx clawhub@latest install mermaid-diagrams

Core Syntax

All Mermaid diagrams follow this pattern:

diagramType
  definition content

Key principles:

• First line declares diagram type (e.g., classDiagram, sequenceDiagram, flowchart)
• Use %% for comments
• Line breaks and indentation improve readability but aren't required
• Unknown words break diagrams; invalid parameters fail silently

Diagram Type Selection

Type	Use For	Reference
Class Diagrams	Domain modeling, OOP design, entity relationships	references/class-diagrams.md
Sequence Diagrams	API flows, auth flows, component interactions	references/sequence-diagrams.md
Flowcharts	Processes, algorithms, decision trees, user journeys	references/flowcharts.md
ERD	Database schemas, table relationships, data modeling	references/erd-diagrams.md
C4 Diagrams	System context, containers, components, architecture	references/c4-diagrams.md
State Diagrams	State machines, lifecycle states	—
Git Graphs	Branching strategies	—
Gantt Charts	Project timelines, scheduling	—

For styling, themes, and layout options: See references/advanced-features.md

Quick Start Examples

Class Diagram (Domain Model)

classDiagram
    Title -- Genre
    Title *-- Season
    Title *-- Review
    User --> Review : creates

    class Title {
        +string name
        +int releaseYear
        +play()
    }

    class Genre {
        +string name
        +getTopTitles()
    }

Sequence Diagram (API Flow)

sequenceDiagram
    participant User
    participant API
    participant Database

    User->>API: POST /login
    API->>Database: Query credentials
    Database-->>API: Return user data
    alt Valid credentials
        API-->>User: 200 OK + JWT token
    else Invalid credentials
        API-->>User: 401 Unauthorized
    end

Flowchart (User Journey)

flowchart TD
    Start([User visits site]) --> Auth{Authenticated?}
    Auth -->|No| Login[Show login page]
    Auth -->|Yes| Dashboard[Show dashboard]
    Login --> Creds[Enter credentials]
    Creds --> Validate{Valid?}
    Validate -->|Yes| Dashboard
    Validate -->|No| Error[Show error]
    Error --> Login

ERD (Database Schema)

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : includes

    USER {
        int id PK
        string email UK
        string name
        datetime created_at
    }

    ORDER {
        int id PK
        int user_id FK
        decimal total
        datetime created_at
    }

Best Practices

1. Start simple — begin with core entities/components, add details incrementally
2. Use meaningful names — clear labels make diagrams self-documenting
3. Comment extensively — use %% comments to explain complex relationships
4. Keep focused — one diagram per concept; split large diagrams into multiple views
5. Version control — store .mmd files alongside code for easy updates
6. Add context — include titles and notes to explain diagram purpose
7. Iterate — refine diagrams as understanding evolves

Configuration and Theming

Configure diagrams using frontmatter:

---
config:
  theme: base
  themeVariables:
    primaryColor: "#ff6b6b"
---
flowchart LR
    A --> B

Available themes: default, forest, dark, neutral, base

Layout options:

• layout: dagre (default) — classic balanced layout
• layout: elk — advanced layout for complex diagrams

Look options:

• look: classic — traditional Mermaid style
• look: handDrawn — sketch-like appearance

Rendering and Export

Native support in:

• GitHub/GitLab — automatically renders in Markdown
• VS Code — with Markdown Mermaid extension
• Notion, Obsidian, Confluence — built-in support

Export options:

• Mermaid Live Editor — online editor with PNG/SVG export
• Mermaid CLI — npm install -g @mermaid-js/mermaid-cli then mmdc -i input.mmd -o output.png

When to Create Diagrams

Always diagram when:

• Starting new projects or features
• Documenting complex systems
• Explaining architecture decisions
• Designing database schemas
• Planning refactoring efforts
• Onboarding new team members

Use diagrams to:

• Align stakeholders on technical decisions
• Document domain models collaboratively
• Visualize data flows and system interactions
• Plan before coding
• Create living documentation that evolves with code

Common Pitfalls

• Breaking characters — avoid {} in comments; escape special characters
• Syntax errors — misspellings break diagrams; validate in Mermaid Live
• Overcomplexity — split complex diagrams into multiple focused views
• Missing relationships — document all important connections between entities
• Stale diagrams — a wrong diagram is worse than no diagram; update when systems change

NEVER Do

1. NEVER create diagrams with more than 15 nodes — they become unreadable; split into multiple focused diagrams
2. NEVER leave arrows unlabeled — every connection should explain the relationship or data flow
3. NEVER create diagrams without a title or caption — context-free diagrams are useless outside the author's head
4. NEVER use diagrams as the sole documentation — pair diagrams with prose that explains the "why"
5. NEVER let diagrams go stale — update diagrams when architecture changes; stale diagrams mislead
6. NEVER use Mermaid for data visualization — Mermaid is for architecture and flow diagrams, not charts of business data