Mermaid Syntax Reference

Quick reference for generating valid Mermaid diagram code.

Flowchart

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

Direction

• TD / TB - Top to bottom
• BT - Bottom to top
• LR - Left to right
• RL - Right to left

Node Shapes

• A[Text] - Rectangle
• A(Text) - Rounded rectangle
• A([Text]) - Stadium/pill
• A[[Text]] - Subroutine
• A[(Text)] - Cylinder (database)
• A((Text)) - Circle
• A>Text] - Asymmetric
• A{Text} - Diamond (decision)
• A{{Text}} - Hexagon
• A[/Text/] - Parallelogram
• A[\Text\] - Parallelogram alt
• A[/Text\] - Trapezoid
• A[\Text/] - Trapezoid alt

Edge Styles

• A --> B - Arrow
• A --- B - Line
• A -.-> B - Dotted arrow
• A ==> B - Thick arrow
• A -->|text| B - Arrow with label (preferred)
• A ---|text| B - Line with label (preferred)

Important: Always use pipe syntax -->|label| for edge labels. The space-dash syntax -- label --> can cause incomplete renders.

Subgraphs

graph TD
    subgraph Group1 [Label]
        A --> B
    end
    subgraph Group2
        C --> D
    end
    B --> C

Sequence Diagram

sequenceDiagram
    participant A as Alice
    participant B as Bob
    A->>B: Hello
    B-->>A: Hi there
    A->>+B: Start process
    B-->>-A: Done

Arrow Types

• ->> - Solid arrow
• -->> - Dashed arrow
• -x - Solid with x
• --x - Dashed with x
• -) - Solid open arrow
• --) - Dashed open arrow

Activations

• + after arrow activates participant
• - after arrow deactivates participant

Notes and Boxes

sequenceDiagram
    Note over A,B: Shared note
    Note right of A: Side note
    rect rgb(200, 220, 255)
        A->>B: In a box
    end

Loops and Conditionals

sequenceDiagram
    loop Every minute
        A->>B: Ping
    end
    alt Success
        B-->>A: Pong
    else Failure
        B-->>A: Error
    end
    opt Optional
        A->>B: Extra step
    end

State Diagram

stateDiagram-v2
    [*] --> Idle
    Idle --> Processing : start
    Processing --> Done : complete
    Processing --> Error : fail
    Error --> Idle : reset
    Done --> [*]

Composite States

stateDiagram-v2
    state Active {
        [*] --> Running
        Running --> Paused : pause
        Paused --> Running : resume
    }
    Idle --> Active : activate
    Active --> Idle : deactivate

Notes

stateDiagram-v2
    State1 : Description here
    note right of State1
        Additional info
    end note

Class Diagram

classDiagram
    class Animal {
        +String name
        +int age
        +makeSound() void
    }
    class Dog {
        +bark() void
    }
    Animal <|-- Dog : extends

Relationships

• <|-- - Inheritance
• *-- - Composition
• o-- - Aggregation
• --> - Association
• -- - Link (solid)
• ..> - Dependency
• ..|> - Realisation
• .. - Link (dashed)

Cardinality

classDiagram
    Customer "1" --> "*" Order
    Order "1" --> "1..*" LineItem

Visibility

• + Public
• - Private
• # Protected
• ~ Package/Internal

Entity-Relationship Diagram

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    PRODUCT }|..|{ LINE-ITEM : "ordered in"

Relationship Types

• || - Exactly one
• |{ - One or more
• o{ - Zero or more
• o| - Zero or one

Identifying vs Non-identifying

• -- - Identifying (solid)
• .. - Non-identifying (dashed)

Attributes

erDiagram
    CUSTOMER {
        string id PK
        string name
        string email UK
    }
    ORDER {
        int id PK
        string customer_id FK
        date created_at
    }

Styling

CSS Classes

graph TD
    A:::highlight --> B
    classDef highlight fill:#f96,stroke:#333

Inline Styles

graph TD
    A --> B
    style A fill:#bbf,stroke:#333

Tips

1. Escape special characters: Use quotes for labels with special chars: A["Label with (parens)"]
2. Multi-line labels: Use <br/> for line breaks
3. Comments: Use %% for comments that won't render
4. IDs vs Labels: Node IDs should be simple, labels can be complex: node1["Complex Label Here"]