> they will identify and authorize only one : the harness; who sits behind will remain a mystery

This doesn’t sound right to me. To connect to the MCP server the user must authenticate. It’s not like all Claude users connect to Figma using the same account, the user connects using their Figma account. So the app still has a direct customer connection even if they lose the ability to control the UI. Unless you’re imagining some other way this would work that I’m not seeing.