Mermaid Sequence Diagram Cheat Sheet

Participants

Declare them to control the order. Use actor for the stick figure.

sequenceDiagram
  participant Alice
  actor Bob
  Bob->>Alice: Hi Alice
  Alice->>Bob: Hi Bob

A filled arrowhead is a call, an open one is a reply, a line with no head is just a note.

sequenceDiagram
  Alice->>John: Hello John
  John-->>Alice: Hi Alice
  John-)Alice: A one-way note

A plus opens a bar on the lifeline, a minus closes it. The bar shows how long the call took.

sequenceDiagram
  Alice->>+John: Hello
  John-->>-Alice: Hi

Notes attach to a participant, or float over a range of them.

sequenceDiagram
  Alice->>John: Hello
  Note right of John: He answers later
  Note over Alice,John: A shared note

A labelled block that repeats. Anything inside runs once per iteration.

sequenceDiagram
  Alice->>John: Hello
  loop Every minute
    John->>Alice: Still there?
  end

alt and else are the if/else of a sequence diagram.

sequenceDiagram
  Alice->>John: Hello
  alt John is a robot
    John-->>Alice: Beep
  else John is a human
    John-->>Alice: Hello there
  end

autonumber puts a number on every message, in order.

sequenceDiagram
  autonumber
  Alice->>John: Hello
  John-->>Alice: Hi
  Alice->>John: Still there?

opt marks a block that may or may not happen.

sequenceDiagram
  Alice->>John: Hello
  opt If he feels like it
    John-->>Alice: Hi
  end