PlantUML for GitHub

PlantUML for GitHub A Chrome extension that renders plantuml code blocks directly on GitHub and GitHub Enterprise Cloud (*.ghe.com) pages, using the TeaVM-compiled PlantUML engine that runs entirely client-side in your browser. No server. No tokens. No tracking. Minimal permissions. HOW IT WORKS 1. The extension's content script scans every page for plantuml code blocks -- including untagged code fences and AsciiDoc [plantuml] blocks that GitHub renders without a language marker. 2. Each block is replaced with a sandboxed <iframe> packaged inside the extension. 3. The iframe loads the TeaVM-compiled plantuml.js engine and renders the diagram to SVG. 4. The result is displayed inline in the page, inside a small wrapper with a header bar. 5. The header bar shows a toggle button that switches between the rendered diagram and the original PlantUML source. The source view uses GitHub's own syntax highlighting, so it looks exactly as it would without the extension installed. This is the same architecture GitHub already uses for Mermaid -- proving that client-side PlantUML can be integrated natively with zero infrastructure cost. The engine ships with 16 standard-library bundles out of the box (C4, Azure, GCP, Kubernetes, and more), so !include <C4/C4_Context> and similar work without any setup. SECURITY AND PERMISSIONS Minimal permissions: the extension requires no host permissions, no storage, and no tabs API. The only declared permission is clipboardWrite, used solely by the user-triggered "Copy as Bitmap" button to copy the generated diagram to the clipboard. The extension runs under the stock Manifest V3 Content Security Policy for extension pages -- essentially script-src 'self' -- with no relaxation at all. Diagrams that need graph layout (class, component, deployment, state, and use-case diagrams) are laid out by Smetana, PlantUML's own built-in Java port of the Graphviz layout algorithms, compiled into the same plantuml.js file as the rest of the engine. There is no WebAssembly module anywhere in the extension, and no 'wasm-unsafe-eval' directive in the manifest -- nothing to relax. This matters beyond the extension itself: GitHub serves its own Mermaid renderer under a script-src 'self' CSP that blocks WebAssembly, so an engine that needed WASM could never be adopted natively. This one runs as a single JavaScript file under exactly that policy. In short: the engine runs entirely inside a sandboxed iframe with an opaque origin, with no network access and no shared state with the host page. ROADMAP - [x] MVP: detect and render plantuml blocks - [x] Firefox support (Manifest V3 is now supported in Firefox) - [x] "Copy as Bitmap" button - [x] Theme matching (light/dark) -- follows GitHub's color mode - [x] Support puml and wsd language aliases - [x] Standard library includes (C4 and 15 more) - [x] GitHub Enterprise Cloud (*.ghe.com) support - [x] Chrome Web Store publication - [ ] Options page (toggle, performance settings) WHY THIS EXTENSION EXISTS PlantUML support on GitHub has been requested for 4+ years: https://github.com/orgs/community/discussions/10111 The main blocker was performance and infrastructure cost. With the TeaVM-compiled engine, that blocker no longer exists. This extension demonstrates that PlantUML can run natively on GitHub.com with zero server-side changes -- using the exact same sandbox pattern GitHub uses for Mermaid. If you'd like to see this integrated natively, please upvote the discussion: https://github.com/orgs/community/discussions/10111 PRIVACY No tracking. No network requests. No permissions beyond clipboardWrite. Everything runs locally, inside a sandboxed iframe. LINKS Source code: https://github.com/plantuml/plantuml-for-github Discussion: https://github.com/orgs/community/discussions/10111 LICENSE: MIT