How to Create Sequence Diagrams Online Free — API & System Flows

What Is a Sequence Diagram?

A sequence diagram shows how systems, services, or people interact over time. Each participant gets a vertical lifeline, and arrows between lifelines show messages, API calls, or events in chronological order. They are essential for documenting microservice architectures, API integrations, and authentication flows.

How to Create a Sequence Diagram

1. Open the Sequence Diagram tool
2. Define participants (e.g., "Client", "API", "Database")
3. Type interactions using arrow syntax (e.g., "Client->>API: GET /users")
4. The diagram renders in real-time
5. Export as PNG or SVG

Everything happens in your browser. No account needed, no watermarks on exports.

Common Use Cases

• API documentation — show request/response flows between client, server, and database
• Authentication flows — document OAuth, JWT, or SSO login sequences
• Microservice communication — visualize how services talk to each other
• Webhook flows — show event-driven architectures with callbacks
• Technical specs — include in design documents and architecture reviews

Syntax Tips

• Solid arrows: A->>B: Request for synchronous calls
• Dashed arrows: B-->>A: Response for return messages
• Notes: Add context notes alongside interactions
• Loops & conditions: Wrap interactions in loop or alt blocks for conditional logic