Sequence diagram makerfrom plain language, free.

sequenceDiagram
  autonumber
  actor User
  participant App as Mobile App
  participant API as Order Server
  participant Gateway as Payment Gateway
  participant Bank as Card Issuer
  User->>App: Tap "Pay now"
  App->>API: Create order (cart, total)
  API->>Gateway: Create payment intent
  Gateway-->>API: Intent id and client secret
  API-->>App: Client secret
  App->>Gateway: Confirm payment with card
  Gateway->>Bank: Authorise charge
  alt 3-D Secure needed
    Bank-->>Gateway: Challenge required
    Gateway-->>App: Show bank challenge
    User->>App: Approve in banking app
    App->>Gateway: Challenge complete
  end
  Bank-->>Gateway: Approved
  Gateway-->>App: Payment succeeded
  App-->>User: Processing your order
  par Webhook
    Gateway->>API: payment_intent.succeeded
    API->>API: Verify signature, mark order paid
    API-->>Gateway: 200 OK
  and Receipt
    Gateway-)User: Email receipt
  end
  API-)App: Push "Order confirmed"
  App-->>User: Show order status