SequenceDiagram.org
sequencediagram.org
Purpose-built editor for sequence diagrams, typed rather than dragged
No sign-upOn device
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plantuml.com
Renders UML from a text description, with the source encoded in the URL.
The public renderer for PlantUML, the older and more complete text-to-diagram language: class, sequence, activity, component, state, deployment and timing diagrams from a compact syntax. The encoded source travels in the image URL, so a diagram can be embedded in a wiki or README without storing a file anywhere. Your diagram text is sent to their server to be rendered, which matters if the class names are confidential — self-host in that case.
Questions
Works the moment the page loads. No account, no email, no trial. Diagram source is sent to a public server to render, and the default styling looks its age.
Yes. PlantUML Online Server sends your input to its own servers to process it. That is fine for public material; for anything containing personal data, use one of the on-device tools instead.
Diagram source is sent to a public server to render, and the default styling looks its age.
Alternatives
Same category or same task, ordered by how closely they overlap.
sequencediagram.org
Purpose-built editor for sequence diagrams, typed rather than dragged
diagrams.net
Full diagram editor that works without an account or a saved file
mermaid.live
Write a diagram as text and watch it render beside you
asciiflow.com
Draws boxes and arrows as plain ASCII you can paste into a comment
excalidraw.com
Hand-drawn-looking whiteboard that opens instantly with no sign-up
lucidchart.com
Polished diagramming with real-time collaboration and smart layout