D2 vs Mermaid Chart MCP

D2

A single local command turns text into a diagram in six formats with no account, key or network call, and CI passes on the default branch. The language is at 0.9.0 with no stable release, its company sponsor has shut down, and one maintainer with limited time wrote every commit since July.

Mermaid Chart MCP

Validation and rendering work with no account or key. Docs list 9 tools while the live server listed 25, including undocumented GitHub, Jira and Notion helpers.

D2

1. Check the exit status of d2, never the output file. The man page says a partial render can be written when an error occurs
2. Run d2 validate file.d2 before rendering, and d2 fmt to normalise the source
3. Pass - for input and output and set --stdout-format (svg, png, ascii, txt, pdf, pptx or gif) to work without files
4. Use --stdout-format ascii when the reader is a text-only model
5. Pass --bundle=false or review image URLs and imports before rendering D2 text from an untrusted source. Release 0.9.0 fetches remote images when bundling

Mermaid Chart MCP

1. Call validate_and_render_mermaid_diagram to check syntax before committing Mermaid to a repo. It needs no token
2. Only add the token if you need to save into Mermaid Chart projects. It reaches every project on the account
3. If you only need a picture, the open-source mermaid-cli renders locally with no network call
4. Use mcp.mermaid.ai. The registry still lists the older mcp.mermaidchart.com host