Mermaid Sequence Diagrams

Generate, review, and fix Mermaid sequence diagrams that are syntactically correct, visually clear, and follow established best practices.

This skill is intentionally Mermaid-specific: keep the output in Mermaid sequenceDiagram syntax.

Generating a New Diagram

1. Identify participants. List every system, service, or actor involved. Assign short IDs with descriptive aliases: participant OMS as Order Management Service.
2. Use actor for human users (renders stick figure), participant for services, database for data stores, queue for message brokers.
3. Determine the flow type:
   • Synchronous request/response → ->> / -->> arrows.
   • Async fire-and-forget → -) / --) arrows.
   • Failed/rejected → -x / --x arrows.
   • Bidirectional (v11+) → <<->> / <<-->> arrows.
4. Declare all participants explicitly at the top to control left-to-right ordering. Place the initiator on the far left.
5. Write messages one per line. Keep message labels concise (e.g., POST /orders, 201 Created). Move payload details to Note blocks.
6. Add autonumber when the diagram has 5+ messages.
7. Use +/- shorthand for activations on arrows (e.g., ->>+Server to activate, -->>-Client to deactivate).
8. Model error paths with alt/else. Never produce a happy-path-only diagram — always include at least one failure branch.
9. Use control flow blocks as needed — read references/02-control-flow-and-notes.md for alt, opt, loop, par, critical, break, and rect.
10. Group related participants with box when there are 4+ participants across distinct layers.
11. Add Note over blocks sparingly for protocol details, SLA info, or phase labels.
12. Verify the output against the syntax rules below before presenting.
13. For styling, themes, or configuration directives, read references/03-styling-and-best-practices.md.
14. For real-world pattern inspiration (OAuth, webhooks, sagas, circuit breakers, retries), read references/04-real-world-examples.md.

Reviewing / Fixing an Existing Diagram

1. Check for syntax errors using the table below. Fix any found.
2. Verify every activate has a matching deactivate (or use +/- shorthand consistently).
3. Verify every alt, opt, loop, par, critical, break, and rect block has a matching end.
4. Check for design anti-patterns — read references/03-styling-and-best-practices.md for the full list.
5. If the diagram exceeds 20 messages or 7 participants, recommend splitting into focused sub-diagrams.
6. Ensure arrow style usage is consistent (don't mix sync and async arrows without clear intent).
7. Confirm participants are declared explicitly with aliases if any name exceeds ~15 characters.

Refactoring Large Diagrams

1. Identify logical phases (e.g., authentication, data processing, notification).
2. Split into one diagram per phase, with a brief prose description connecting them.
3. Keep shared participant declarations consistent across sub-diagrams.
4. Each sub-diagram should have ≤ 15-20 messages and ≤ 6-7 participants.

Syntax Rules — Common Errors

Mistake	Fix
Smart quotes " "	Use straight quotes "
Unicode arrows → ⇒	Use ASCII arrows ->> -->
Em/en dashes — –	Use plain hyphens -
The word end in labels	Wrap in quotes: "end", brackets: [end], or parens: (end)
Multiple statements on one line	One statement per line
Spaces in participant IDs	Use CamelCase or snake_case IDs with alias for display name
Semicolons in text	Escape as #59;
Unbalanced brackets	Ensure every [ has ], every ( has )

Output Template

Use this skeleton as the starting point for every diagram:

sequenceDiagram
    autonumber
    participant A as Service A
    participant B as Service B

    A->>+B: Request
    B-->>-A: Response

References

• references/01-syntax-fundamentals.md — Participants, actors, specialized types, arrow reference, activation syntax.
• references/02-control-flow-and-notes.md — alt/opt/loop/par/critical/break/rect blocks, notes, sequence numbers, boxes, create/destroy.
• references/03-styling-and-best-practices.md — Themes, configuration, design principles, message labeling, anti-patterns, version compatibility.
• references/04-real-world-examples.md — JWT auth, OAuth 2.0, webhooks, saga pattern, event-driven microservices, retry backoff, circuit breaker.
• references/05-cheat-sheet.md — Arrow quick reference, control flow quick reference, participant types, escape sequences, CLI validation commands.