Build a Mermaid sequence diagram together

A sequence diagram is the picture you reach for when the question is who calls whom, and in what order. This page is about that specific job: two or more participants, the messages that pass between them, and the difference between a call that waits for an answer and one that does not. You write it as Mermaid sequenceDiagram source in a shared editor, and the preview redraws as you type.

Open Shared Mermaid in a Room

Quick facts

Use: Request and response flows between services
Template: Sequence starter included
Syntax: Mermaid sequenceDiagram
Account: No signup required
Rendering: Local to each browser
Export: Copy source; no file download

How it works

Open Shared Mermaid in the Room, then choose the Sequence starter template to get a working sequenceDiagram to edit.
Rename the participants to the actors, services or teams involved, and rewrite the messages between them.
Talk the flow through in Room voice or chat while everybody watches the same diagram redraw.
Copy the sequenceDiagram source into your design doc, pull request or repository before the Room ends.

What you can do

Write Mermaid sequenceDiagram source with a preview that redraws as you type.
Model sync calls, replies and async messages between named participants.
Start from a Sequence template with a request, a response and a follow-up message already written.
Keep the diagram and the conversation in the same temporary Room.
Copy the source into a design review, an ADR, a wiki page or a pull request.

Why use it in a Free4Chat Room

The reason to do it in a Room is that sequence diagrams are usually wrong in a way only the other engineer can see. Instead of describing the flow in a call and then sending a diagram to be corrected afterwards, you keep the source open while you talk: somebody edits, everybody sees the same picture, and the disagreement resolves into the diagram rather than into a comment thread. Start from the Sequence template if you want a worked example of the syntax to edit.

Limits & privacy

It renders Mermaid, not a canvas. There is no drag-and-drop actor placement, no automatic code tracing, no runtime introspection and no import from an API specification—the diagram is exactly the text you write. Only that text is shared between participants and each browser draws it locally, so nothing is uploaded to a rendering service. The source is capped at 12 KiB, there is no document history and no file download, so copy the source before the Room ends.

Frequently asked questions

What is a sequence diagram for?

Showing the order in which messages pass between two or more participants over time. It is the right diagram when the question is not what the system contains but who calls whom, in what order, and which calls wait for a reply. Typical uses are an API request lifecycle, a checkout or login flow, and a review of how two services are expected to talk to each other.

Does it generate the diagram from my code or an OpenAPI spec?

No. You write the Mermaid sequenceDiagram text yourself. There is no code tracing, no runtime introspection, no import from an API specification and no AI generation. That keeps the diagram an explicit, reviewable artifact rather than a snapshot that silently drifts from the implementation.

Can I drag actors around on the diagram?

No. This is a text editor with a preview, not a visual diagram canvas. Participants appear in the order they are declared in the source, and rearranging them means editing that order rather than dragging a box.

Can we edit the same sequence diagram at once?

Yes. Everybody in the Room edits the same source and sees the same rendered diagram. Your preview updates on each keystroke, and the Room is told about your edit once your typing settles. If two people genuinely edit the same diagram at the same moment, one version becomes the Room's diagram and the other person is told whose version won instead of losing their text silently.

How do I get the diagram into our repository?

Copy source, then paste the Mermaid text into wherever your docs are rendered—a Markdown file, a README, a wiki, an issue or a pull request. Many code hosts and documentation tools render Mermaid natively. There is no image download in the Room sandbox.

Is this the same App as the general Mermaid editor?

It is the same runtime and the same shared editor, approached from the sequence-diagram task. Both launch the identical tool; this page covers the request-flow workflow and its template, and the main Mermaid editor page covers diagramming more generally.