/mermaid-skill

Use Case Diagram Syntax

Use case diagrams (UML) show how actors interact with a system and its use cases. Available as usecase-beta (v11.17+).

Basic Structure

usecase-beta
direction LR
actor Customer("Customer")
systemBoundary "Order system"
  Checkout("Place order")
end
Customer --> Checkout

• usecase-beta — required first line
• direction — TD, TB, BT, LR, or RL
• Actors: actor Name("Display label") — stick figure by default
• Use cases: Name("label") = ellipse, Name[label] = rectangle
• systemBoundary id["Title"] ... end — groups actors/use cases (default rect, or @{ type: package })

Actor Variants

usecase-beta
actor Normal("Normal actor")
actor Hollow("Hollow actor")@{ type: hollow }
actor Awesome("Awesome actor")@{ type: awesome }
actor Icon("Icon actor")@{ icon: "fa:user" }
actor B2B("Sales agent")@{ business: true } <<Employee>>
Normal --> Manage

• type: hollow | awesome (icon actors via icon: "fa:name")
• business: true adds the conventional business slash
• <<stereotype>> adds a visible stereotype above the label

Relationships

usecase-beta
actor Admin
actor Person
Checkout
Payment
ApplyCoupon
Admin --|> Person                %% generalization
Checkout ..> : include Payment   %% include
ApplyCoupon ..> : extend Checkout %% extend
User --> Start                   %% plain association
User --o Start                   %% circle (aggregation-like)
User --x Finish                  %% cross

Notes and JSON Tables

usecase-beta
note for Login "`Requires an **active session**`"
actor User
Login("Sign in")
User --> Login

json Payload@{
  "status": "pending",
  "items": [{ "name": "Book" }]
}:::data
Inspect --> Payload

• note for <target> "text" — attaches to an actor or use case
• json <id>@{ ... } — renders a JSON object as a table node
• Nested values use paths: address.city, items[0].name

Styling

Classes and direct styles work like other Mermaid types: classDef, class, style, or ::: suffix on declarations. Use %% for comments; // and # are NOT comments in usecase diagrams.