Compiling first is what makes this usable in a loop: the model gets told exactly where the syntax broke, fixes it, and only then renders — and in ASCII mode it can read its own output and iterate without a human looking at the picture.
A Go server for D2, the declarative diagramming language. It compiles D2 source to catch syntax errors before anything renders, produces the diagram in the format you asked for, and carries a built-in cheat sheet so the model has the syntax to hand rather than guessing at it.
- `compile-d2` — validates D2 source and surfaces syntax errors, taking either `code` or a `file_path`
- `render-d2` — renders to `png`, `svg` or `ascii`, with `ascii_mode` choosing `extended` Unicode box drawing or `standard` basic characters; ASCII is the one an LLM can read back
- `fetch_d2_cheat_sheet` — a Markdown reference covering shapes, styling and layout, useful as support material in a prompt
- Optional file output with `--write-files`, so rendered diagrams land in a mounted directory rather than only in the response
Install with `go install github.com/h0rv/d2-mcp@latest`, build from source, or run the container image `ghcr.io/h0rv/d2-mcp:main`. PNG output needs `rsvg-convert` from librsvg present — install it with brew, apt or apk — and without it the server removes `png` from the format enum and falls back to SVG rather than failing; the Docker image ships librsvg and fallback fonts, so PNG works there out of the box. Defaults to stdio; `--transport http` serves the streamable endpoint at `/mcp` and `--transport sse` the legacy one, with `MCP_TRANSPORT` and `PORT` as environment overrides. Writing files requires a bind mount, since the container has no view of your working tree otherwise.
One command — go install github.com/h0rv/d2-mcp@latest