Mermaid Diagrams Creator

Mermaid Diagrams Skill

Create Mermaid diagrams that render correctly and communicate clearly.

Key Feature: This skill creates both .mermaid source files AND image files (PNG/SVG/PDF).

Two-Step Process

Every diagram creation involves:

1. Create .mermaid file - Editable source that renders as visual artifact in Claude
2. Generate image file - Portable PNG/SVG/PDF using mermaid CLI (MANDATORY)

Never skip the image generation step! Users need the image file for sharing, documentation, and presentations.

Workflow

IMPORTANT: Always complete ALL steps below. Do not skip image generation.

1. Identify the best diagram type for the request
2. Generate clean Mermaid syntax following the patterns below
3. Save as .mermaid file to /mnt/user-data/outputs/
4. Generate image (PNG by default) using mermaid CLI - THIS STEP IS MANDATORY
5. Verify both .mermaid and image file were created successfully

Step-by-Step Execution

After creating the .mermaid file, you MUST immediately run:

mmdc -i /mnt/user-data/outputs/diagram-name.mermaid -o /mnt/user-data/outputs/diagram-name.png -b white

If mmdc is not available, check installation and guide the user to install it.

Mermaid CLI Setup

Installation

The mermaid CLI (@mermaid-js/mermaid-cli) is required for image generation. Check if it's installed:

mmdc --version

If not installed, install it globally using npm:

npm install -g @mermaid-js/mermaid-cli

Or use npx to run without global installation:

npx -p @mermaid-js/mermaid-cli mmdc --version

Image Generation

After creating a .mermaid file, automatically generate an image:

PNG (default, good for documentation):

mmdc -i /mnt/user-data/outputs/diagram-name.mermaid -o /mnt/user-data/outputs/diagram-name.png

SVG (scalable, best for web):

mmdc -i /mnt/user-data/outputs/diagram-name.mermaid -o /mnt/user-data/outputs/diagram-name.svg

PDF (good for print):

mmdc -i /mnt/user-data/outputs/diagram-name.mermaid -o /mnt/user-data/outputs/diagram-name.pdf

Configuration Options

Background color:

mmdc -i input.mermaid -o output.png -b white
# or transparent
mmdc -i input.mermaid -o output.png -b transparent

Custom theme:

mmdc -i input.mermaid -o output.png -t dark
# themes: default, dark, forest, neutral

Custom size/scale:

mmdc -i input.mermaid -o output.png -w 1920 -H 1080
# or use scale factor
mmdc -i input.mermaid -o output.png -s 2

Custom config file:

mmdc -i input.mermaid -o output.png -c config.json

Example config.json:

{
  "theme": "default",
  "themeVariables": {
    "primaryColor": "#ff6b6b",
    "primaryTextColor": "#fff",
    "primaryBorderColor": "#000",
    "lineColor": "#333",
    "secondaryColor": "#4ecdc4",
    "tertiaryColor": "#ffe66d"
  },
  "flowchart": {
    "curve": "basis",
    "padding": 15
  }
}

Diagram Type Selection

Quick Syntax Reference

Flowchart

flowchart TB
    A[Start] --> B{Decision?}
    B -->|Yes| C[Action 1]
    B -->|No| D[Action 2]
    C --> E[End]
    D --> E

Node shapes: [rect] (rounded) {diamond} ([stadium]) [[subroutine]] [(database)] ((circle)) >flag] {{hexagon}}

Sequence Diagram

sequenceDiagram
    participant U as User
    participant S as Server
    participant D as Database
    
    U->>S: POST /api/data
    activate S
    S->>D: INSERT query
    D-->>S: Success
    S-->>U: 201 Created
    deactivate S

Arrows: ->> (solid async), -->> (dotted response), --) (async no arrow), -x (lost message)

Class Diagram

classDiagram
    class User {
        +String name
        +String email
        +login() bool
        +logout() void
    }
    class Admin {
        +List~Permission~ permissions
        +grantAccess(User) void
    }
    User <|-- Admin : extends
    User "1" --> "*" Order : places

Relations: <|-- (inheritance), *-- (composition), o-- (aggregation), --> (association), ..> (dependency)

ER Diagram

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : "ordered in"
    
    USER {
        int id PK
        string email UK
        string name
    }
    ORDER {
        int id PK
        int user_id FK
        date created_at
    }

Cardinality: || (one), o| (zero or one), }| (one or more), }o (zero or more)

State Diagram

stateDiagram-v2
    [*] --> Idle
    Idle --> Processing : submit
    Processing --> Success : complete
    Processing --> Failed : error
    Failed --> Idle : retry
    Success --> [*]

Gantt Chart

gantt
    title Project Timeline
    dateFormat YYYY-MM-DD
    section Phase 1
        Task A :a1, 2024-01-01, 30d
        Task B :after a1, 20d
    section Phase 2
        Task C :2024-02-15, 25d

Best Practices

Node IDs: Use meaningful names (userService, validateInput) not single letters (A, B).

Labels: Keep under 40 characters. Use <br> for line breaks in longer text.

Direction: Use TB (top-bottom) for hierarchies, LR (left-right) for sequences/timelines.

Subgraphs: Group related nodes to improve readability:

flowchart TB
    subgraph Frontend
        UI[Web UI]
        Mobile[Mobile App]
    end
    subgraph Backend
        API[API Gateway]
        Auth[Auth Service]
    end
    UI --> API
    Mobile --> API
    API --> Auth

Styling: Apply sparingly for emphasis:

flowchart LR
    A --> B --> C
    style B fill:#f96,stroke:#333
    linkStyle 1 stroke:#f00,stroke-width:2px

Common Pitfalls

Output

CRITICAL: Always create BOTH files - never just the .mermaid file alone!

Save diagrams to /mnt/user-data/outputs/ with both source and rendered formats:

1. Source file: diagram-name.mermaid (editable source, renders as visual artifact in Claude)
2. Image file: diagram-name.png (portable image for sharing/documentation) - REQUIRED

Mandatory Image Generation

You MUST always generate an image file after creating the .mermaid file.

Default command (PNG with white background):

mmdc -i /mnt/user-data/outputs/diagram-name.mermaid -o /mnt/user-data/outputs/diagram-name.png -b white

Execution checklist:

• ✅ Write .mermaid file
• ✅ Run mmdc command to generate PNG
• ✅ Verify both files exist
• ✅ Show user both file paths

If mmdc command fails:

1. Check if mmdc is installed: which mmdc or mmdc --version
2. If not installed, guide user to install: npm install -g @mermaid-js/mermaid-cli
3. Retry image generation after installation
4. If still fails, check troubleshooting section below

This provides both the editable source and a portable image for sharing, documentation, or presentation.

Image Format Selection

Choose the format based on use case:

Troubleshooting Image Generation

Puppeteer/Chromium Issues

If image generation fails with Puppeteer errors:

# Install Chromium dependencies (Linux)
sudo apt-get install -y libgbm-dev libxkbcommon-x11-0 libgtk-3-0 libx11-xcb1 libnss3 libxss1 libasound2

# Or use system Chromium (set environment variable)
export PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true
export PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium

Permission Errors

If output directory is not writable:

# Check directory permissions
ls -la /mnt/user-data/outputs/

# Create directory if missing
mkdir -p /mnt/user-data/outputs/

Syntax Errors

If image generation fails, first verify the .mermaid file syntax:

1. Check the source .mermaid file for syntax errors
2. Test in the Mermaid Live Editor
3. Fix syntax issues and regenerate image

Large Diagrams

For very large or complex diagrams:

# Increase viewport size
mmdc -i input.mermaid -o output.png -w 3840 -H 2160

# Increase scale for better quality
mmdc -i input.mermaid -o output.png -s 3

# Use SVG for infinite scaling
mmdc -i input.mermaid -o output.svg

Complete Example Workflow

Here's exactly what you should do when a user asks for a diagram:

User request: "Create a flowchart showing the authentication process"

Your actions:

1. Create the .mermaid file:

# Write the mermaid content to file
cat > /mnt/user-data/outputs/auth-flow.mermaid << 'EOF'
flowchart TB
    Start[User Login] --> ValidateInput{Valid Credentials?}
    ValidateInput -->|Yes| CheckDB[Query Database]
    ValidateInput -->|No| Error[Show Error]
    CheckDB --> UserExists{User Exists?}
    UserExists -->|Yes| GenerateToken[Generate JWT Token]
    UserExists -->|No| Error
    GenerateToken --> Success[Login Success]
    Error --> End[End]
    Success --> End
EOF

2. Generate the PNG image:

# MANDATORY: Generate image from .mermaid file
mmdc -i /mnt/user-data/outputs/auth-flow.mermaid -o /mnt/user-data/outputs/auth-flow.png -b white

3. Verify and inform user:

# Check both files exist
ls -lh /mnt/user-data/outputs/auth-flow.*

# Output to user:
# ✅ Created auth-flow.mermaid (source file)
# ✅ Created auth-flow.png (image file - 45 KB)

What NOT to do:

• ❌ Create only the .mermaid file and stop
• ❌ Tell the user to run mmdc themselves
• ❌ Skip image generation because you think it's optional
• ❌ Forget to verify both files were created

If mmdc is not installed:

1. Check: which mmdc
2. If not found, inform user: "mmdc not found. Installing mermaid CLI..."
3. Guide installation: npm install -g @mermaid-js/mermaid-cli
4. Retry image generation after installation

Quick Pre-Flight Check

Before creating any diagram, verify mermaid CLI is available:

# Check if mmdc is installed
mmdc --version

If this succeeds, proceed with diagram creation. If it fails, install mermaid CLI first.

For detailed syntax reference and advanced patterns, see references/syntax-guide.md.