Mermaid Diagrams Skill

Generate diagrams from text using Mermaid.

Requirements

• @mermaid-js/mermaid-cli (npm install -g @mermaid-js/mermaid-cli)

Usage

1. Write Mermaid syntax to a .mmd file
2. Run mmdc to render PNG/SVG
3. Return the image to the user

Basic Command

mmdc -i input.mmd -o output.png -t dark -b transparent

Options

Flag	Description
-i	Input .mmd file
-o	Output file (.png, .svg, .pdf)
-t	Theme: default, dark, forest, neutral
-b	Background color (transparent, hex)
-w	Width in pixels (default: 800)
-H	Height in pixels
-s	Scale factor (default: 1, use 2-3 for high-res)
-c	Config JSON file for advanced theming

Supported Diagram Types

• Flowchart: graph TD / graph LR
• Sequence: sequenceDiagram
• Class: classDiagram
• State: stateDiagram-v2
• ER: erDiagram
• Gantt: gantt
• Pie: pie
• Mindmap: mindmap
• Timeline: timeline
• Git graph: gitGraph
• Quadrant: quadrantChart
• Block: block-beta

Workflow

1. Determine the best diagram type for what the user wants
2. Write the .mmd file to /tmp/mermaid-<name>.mmd
3. Render: mmdc -i /tmp/mermaid-<name>.mmd -o /tmp/mermaid-<name>.png -t dark -b transparent -s 2
4. Show the image to the user via Read tool
5. Clean up temp files if not needed

Example: Architecture Diagram

graph TD
    A[Client] -->|HTTPS| B[API Gateway]
    B --> C[Auth Service]
    B --> D[App Server]
    D --> E[(PostgreSQL)]
    D --> F[(Redis Cache)]

Example: Sequence Diagram

sequenceDiagram
    participant U as User
    participant A as API
    participant DB as Database
    U->>A: POST /login
    A->>DB: Validate credentials
    DB-->>A: User record
    A-->>U: JWT token

Tips

• Use graph LR for left-to-right flow, graph TD for top-down
• Keep node labels short — detail goes in tooltips or notes
• Use subgraphs to group related components
• For dark backgrounds, use -t dark -b transparent
• Scale -s 2 or -s 3 for sharp images on retina displays
• Always use pty: false when calling mmdc

Theming

For custom colors, create a config JSON:

{
  "theme": "base",
  "themeVariables": {
    "primaryColor": "#f59e0b",
    "primaryTextColor": "#1a1a1a",
    "primaryBorderColor": "#d97706",
    "lineColor": "#6b7280",
    "secondaryColor": "#10b981",
    "tertiaryColor": "#3b82f6"
  }
}

Then: mmdc -i input.mmd -o output.png -c config.json