PlantUML Viewer — Local UML Diagram Tool

📐 Draws .puml files and plantuml code blocks as UML diagrams in the page. No server, works offline. Free UML diagram tool.

📐 PlantUML Viewer draws your diagrams inside the browser. The whole PlantUML engine ships in the extension, so a .puml file or a plantuml code block turns into a picture with no server, no account and no network. It works offline, with the network unplugged. ⚡ WHERE IT WORKS 🔧 GitLab, Bitbucket, Gitea, Forgejo, Codeberg, Confluence and Azure DevOps. Open a .puml file, or any page holding a plantuml code block, and the diagram appears where the markup was. One click puts the source back. 🏠 Self-hosted too. Add your own GitLab or Confluence address in settings and it renders there as well, on an internal network that never reaches the public internet. 📄 Raw files and local files. A .puml on your own disk opens as a diagram, as does any plain text file beginning with an @start directive. ⚡ WHAT IT DRAWS 🧠 Every PlantUML syntax the engine understands: UML sequence diagram, UML class diagram, UML activity diagram, UML use case diagram, state diagram, component diagram, mindmap, Gantt chart, WBS, JSON, Ditaa and Graphviz Dot. ✍️ It is not a diagram maker. You write the PlantUML syntax, the extension draws it — so a UML chart stays a text file your team can review, diff and merge like any other. 🎨 C4 travels with it. The C4 library is bundled, so !include <C4/C4_Container> resolves from inside the extension rather than reaching out to GitHub the way C4 normally does. 🖼️ SVG output. Labels stay selectable, the picture stays sharp at any zoom, and Chinese, Japanese and Cyrillic text stays readable, because your own system fonts draw it. 🔒 NOTHING LEAVES YOUR MACHINE This is a property of how it is built, not a setting you have to trust. There is no render server to configure and no address to check. It replaces the plantuml online round-trip that every other viewer depends on. The extension ships asking for one permission, storage, and holds no access to any website beyond the ones it draws diagrams on. ⚙️ No account, no signup, no analytics, no telemetry. An include pointing at a URL is skipped rather than fetched, and the extension says so, because honouring it would send your diagram somewhere. 📦 MANIFEST V3 Written for Manifest V3, so it will not be switched off the way the old MV2 PlantUML extensions were. Runs in Chrome, Edge, Brave, Opera and any Chromium browser. There is no UML software to install beyond it, and no Java. ✨ WHY NOT AN ONLINE UML EDITOR Because the file is already in front of you. Pasting markup into an online UML editor, draw.io or Lucidchart means lifting source out of your repository and into somebody else's tab, which is exactly what you cannot do with an architecture diagram of a system you are paid to keep quiet about. A free UML diagram tool that reads files where they already live is quicker, and one that never transmits them is safer. 🚀 GET STARTED Install it and open a .puml file. Made for developers, architects, technical writers, and anyone reviewing a plantuml diagram in a merge request. Note: github.com is deliberately left alone, because PlantUML publish their own extension for it.