Sequence diagrams from Mermaid source

Sequence is a first-class tab, not a plugin. The source of truth is Mermaid sequenceDiagram — the same text you already keep in .mmd files. There is no separate “sequence DSL” to learn on this tab. Paste, import, or type; the sequence canvas parses that source in the browser.

Why sequence is Mermaid when other tabs are not

Architecture, flow, and Gantt have a native diagram: … language because those boards carry layout pins, C4 levels, BPMN types, or task dates that Mermaid does not model the same way. Sequence messages are already well specified by Mermaid: participants, actors, arrows, and notes. DiagramTool keeps that text as-is. Copying “as Mermaid” and “as source” both return the sequence document.

If you paste a sequenceDiagram while another tab is active, the app tells you to open the Sequence tab (or import the file as a sequence board). That is deliberate — mixing a call timeline into an architecture parse would throw confusing lexer errors.

A payment flow you can paste

sequenceDiagram
title Checkout — Payment Flow
actor User
participant API
participant Ledger
User->>API: POST /pay
API->>Ledger: reserve
Ledger-->>API: ok
API-->>User: 201 Created

Titles come from a title / title: line or the first markdown heading in the file. File → Open accepts .mmd, .mermaid, and plain text that starts with sequenceDiagram. Unsupported Mermaid diagrams (erDiagram, classDiagram, mindmap, state diagrams, pie, gitGraph, and Mermaid C4) are refused rather than half-rendered.

Editing and export

Participant display names can be renamed from the workspace without rewriting every arrow by hand. Export still goes through the shared PNG / SVG / PDF path; the sequence canvas is SVG-backed, so vector export does not depend on the architecture React Flow surface. Boards are stored in the same local workspace as your other modes — see local-first storage.

Sequence does not import draw.io. A diagrams.net file becomes a flow board. If you need both a call trace and a component map, keep two boards in the workspace and switch tabs.