I hacked the new MCP UI extension 😎
By which I mean I read the spec and used it exactly as documented, for a dumb little game. 😅
MCP Apps is the official MCP UI extension. Short version: your server declares an HTML resource under ui://, links it to a tool via _meta.ui.resourceUri, and the host renders that HTML in a sandboxed iframe right in the conversation. The iframe talks back over JSON-RPC, so it can call tools. Not a rendered screenshot, a live UI.
Everyone is shipping dashboards and forms with it. I put a game loop in there.
The tool starts a run, the iframe is a real one button arcade game. When you crash, the widget sends the seed and the ticks you jumped at back to the server, the server replays them through the engine and computes the score. The model never touches the number.