Agent Skills: Mermaid Diagramming

Mermaid Diagramming

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.

Core Syntax Structure

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; parameters fail silently

Diagram Type Selection Guide

Choose the right diagram type:

1. Class Diagrams - Domain modeling, OOP design, entity relationships

   • Domain-driven design documentation
   • Object-oriented class structures
   • Entity relationships and dependencies

2. Sequence Diagrams - Temporal interactions, message flows

   • API request/response flows
   • User authentication flows
   • System component interactions
   • Method call sequences

3. Flowcharts - Processes, algorithms, decision trees

   • User journeys and workflows
   • Business processes
   • Algorithm logic
   • Deployment pipelines

4. Entity Relationship Diagrams (ERD) - Database schemas

   • Table relationships
   • Data modeling
   • Schema design

5. C4 Diagrams - Software architecture at multiple levels

   • System Context (systems and users)
   • Container (applications, databases, services)
   • Component (internal structure)
   • Code (class/interface level)

6. State Diagrams - State machines, lifecycle states

7. Git Graphs - Version control branching strategies

8. Gantt Charts - Project timelines, scheduling

9. Pie/Bar Charts - Data visualization

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
    }

Detailed References

For in-depth guidance on specific diagram types, see:

• references/class-diagrams.md - Domain modeling, relationships (association, composition, aggregation, inheritance), multiplicity, methods/properties
• references/sequence-diagrams.md - Actors, participants, messages (sync/async), activations, loops, alt/opt/par blocks, notes
• references/flowcharts.md - Node shapes, connections, decision logic, subgraphs, styling
• references/erd-diagrams.md - Entities, relationships, cardinality, keys, attributes
• references/c4-diagrams.md - System context, container, component diagrams, boundaries
• references/advanced-features.md - Themes, styling, configuration, layout options

Important Notes

• Avoid {} in comments — they break diagrams. Use proper escape sequences for special characters.
• Unknown words and misspellings break diagrams silently; parameters fail silently.
• Use %% for comments.
• For theming, styling, and layout configuration, see references/advanced-features.md.
• Export with CLI: npm install -g @mermaid-js/mermaid-cli then mmdc -i input.mmd -o output.png