Excalidraw offers their own open source first party MCP endpoint[0] and server[1]:

[0]https://mcp.excalidraw.com

[1]https://github.com/excalidraw/excalidraw-mcp

I've had a fairly thorough exploration of how I can give my agent a whiteboard so we can work on architectures together. To my surprise, current solutions (including Excalidraw) were not good enough and didn't deliver what I wanted. I ended up finding Mermaid to be the most agent-friendly medium and coded an Obsidian plugin for it. It works okay; we can work on the same doc while I bring my own AI agent, and we can brainstorm together. https://community.obsidian.md/plugins/mermaid-relay

YMMV, but the value I get from producing a diagram is derived from the thinking. Thinking about what I'm trying to draw leads to understanding about where my/my team's knowledge is poorer, what assumptions we're making, etc.

This is crazy. My claude chess stuff is (edit: was) currently near yours on the front page and noticed your post. I have a project very close than yours that I hesitated to share. From a quick glance, we went for a similar approach. I just open sourced it so that you can compare implementation notes. https://github.com/brumar/whiteboard-agents . It's not thoroughly tested but can be interesting to check.

Posted this somewhere else aswell but I've had good success with https://whiteboard-mcp.com - it seems to work better than other free alternatives for things like architecture diagram creation