MCP Apps: declare your UI templates ahead of time so hosts can prefetch and security-review them

Model Context Protocol Blog·medium signal

MCP Apps standardizes the ad-hoc embedded-UI experiments into server-rendered HTML displayed in a sandboxed iframe. The design constraint worth copying: tools declare their UI templates in advance rather than streaming markup at call time, so the host can prefetch, cache, and security-review the template before anything executes. Every action taken inside the rendered UI routes back through the same JSON-RPC base protocol, inheriting the identical consent and audit path as a direct tool call.