MCP-UI is a monorepo SDK implementing the MCP Apps standard for rendering interactive UIs over the Model Context Protocol. It provides server-side tools (in TypeScript, Python, and Ruby) to create UI resources, and a client-side React SDK to render them in sandboxed iframes. The architecture centers on a secure double-iframe proxy pattern that isolates third-party HTML and external URLs, with JSON-RPC communication via AppBridge. The codebase is well-structured with comprehensive tests, semantic release automation, and cloudflare workers example, demonstrating strong engineering practices across multiple languages.