Mermaid Diagrams

Mermaid Diagrams Skill

Use Mermaid syntax to create clear, maintainable diagrams directly in markdown. Mermaid diagrams are rendered by GitHub, GitLab, and many documentation tools.

When to Use Mermaid

• Flowcharts: Process flows, decision trees, algorithms
• Sequence Diagrams: API calls, user interactions, system communication
• Class Diagrams: Object relationships, database schemas
• State Diagrams: State machines, workflow states
• Entity Relationship Diagrams: Database design
• Gantt Charts: Project timelines, task scheduling
• Git Graphs: Branch strategies, release flows

IMPORTANT INSTRUCTIONS

1. NEVER use parentheses inside a label

Using parentheses inside a label causes syntax errors when rendering on GitHub.

NEVER DO THIS:

flowchart TD
  Start[Start agent loop (beginning)]
  Criteria[Define completion criteria checklist 3-5]
  ShowCriteria[Show criteria to user for approval]
  CriteriaOK{User approved (causes errors)}

ALWAYS DO THIS INSTEAD:

flowchart TD
  Start[Start agent loop - beginning]
  Criteria[Define completion criteria checklist 3-5]
  ShowCriteria[Show criteria to user for approval]
  CriteriaOK{User approved}

2. ALWAYS ensure that labels are wrapped in matching brackets

Labels must have matching opening and closing brackets based on their shape.

CORRECT - Matching brackets:

flowchart TD
  Start[Start agent loop]
  Criteria[Define completion criteria checklist 3-5]
  ShowCriteria[Show criteria to user for approval]
  CriteriaOK{User approved}

INCORRECT - Mismatched brackets:

flowchart TD
  Start[Start agent loop]
  Criteria[Define completion criteria checklist 3-5]
  ShowCriteria[Show criteria to user for approval]
  CriteriaOK{User approved]

Notice that the brackets do not match on the label for CriteriaOK ({ opens but ] closes).

Basic Syntax Examples

Flowchart

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

Sequence Diagram

sequenceDiagram
    participant Client
    participant API
    participant Database

    Client->>API: Request data
    API->>Database: Query
    Database-->>API: Results
    API-->>Client: Response

Class, state, and entity relationship diagram examples: see [reference.md](reference.md).

For system architecture diagrams (components, layers, data flows), use the architecture-diagramming skill.

Best Practices

1. Keep it Simple: Start with basic shapes and relationships
2. Use Descriptive Labels: Make node names clear and meaningful
3. Limit Complexity: Break complex diagrams into multiple smaller ones
4. Add Context: Include a brief description above the diagram
5. Test Rendering: Verify diagrams render correctly in your target environment

Common Patterns

Decision Flow

flowchart TD
    Start[Receive Request] --> Auth{Authenticated?}
    Auth -->|No| Reject[Return 401]
    Auth -->|Yes| Valid{Valid Input?}
    Valid -->|No| BadReq[Return 400]
    Valid -->|Yes| Process[Process Request]
    Process --> Success[Return 200]

Syntax Reference

Node Shapes

• [Rectangle] - Basic box
• (Rounded) - Rounded edges
• {Diamond} - Decision point
• ([Stadium]) - Pill shape
• [[Subroutine]] - Double border
• [(Database)] - Cylinder
• ((Circle)) - Circle

Arrow Types

• --> - Solid arrow
• -.-> - Dotted arrow
• ==> - Thick arrow
• --text--> - Labeled arrow
• --- - Line (no arrow)

Styling

flowchart TD
    A[Normal]
    B[Highlighted]

    style B fill:#f96,stroke:#333,stroke-width:4px

Resources

• Mermaid Official Docs
• Live Editor
• GitHub/GitLab automatically render Mermaid in markdown

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: britt
• Source: britt/claude-code-skills
• License: MIT
• Homepage: https://britt.github.io/claude-code-skills/

Install and usage instructions live in the source repository linked above.