Diagram Fences

What it is

BB draws Mermaid in chat and in Markdown previews, and nothing else. Diagram Fences adds the rest of the toolchain developers actually use: PlantUML, Graphviz, D2 and the wider Kroki family, as fenced code blocks that render where you read them.

Where it renders

• Assistant replies: a plantuml, dot or d2 block becomes a diagram with a source toggle, copy, and open-in-tab controls.
• Markdown file previews: the same blocks draw in the right panel, where BB's code-fence slot does not reach. A content script swaps the code block for the diagram and leaves the file untouched. The panel must be BB's built-in preview; when another plugin opens .md files, pick Built-in preview under Settings → Files → File openers.

Local engines

Settings lists each engine with the binary it resolved, its origin, and its own buttons:

• Graphviz uses dot when it is installed and a bundled WASM build otherwise, so boxes-and-arrows always work offline.
• PlantUML is the official native binary, downloaded from the settings screen or on first use — no Java, no server. It draws in about 0.2 s.
• D2 is the official binary, installed the same way; Kroki covers it until then.
• A custom path can point any engine at a binary you already have.

Everything else goes to Kroki, public or your own instance.

Cost

Each engine runs per diagram and exits; at most two run at a time. Diagrams are cached by content hash, and a preview draws only the fences near the viewport, so scrolling a long file or switching threads costs nothing.

Modes

Auto (default) picks a local engine when one exists and kroki.io otherwise. Local only refuses to leave the machine. kroki.io and Own Kroki URL cover the hosted cases, including a self-hosted Kroki for private diagrams.

Safety

Rendered SVG is shown through an <img> blob, so a diagram cannot run script in the bb page. Files are written only inside the plugin's own data directory, and diagram source reaches kroki.io only in the modes that name it.