The MCP Apps Extension (SEP-1865) aims to standardize interactive user interfaces for the Model Context Protocol, addressing a key request from the MCP community. Current MCP servers primarily exchange text and structured data, which can complicate tasks that require visual representation or complex user input. For instance, a server returning chart data as JSON forces clients to handle the rendering themselves, adding unnecessary complexity. The new extension proposes a structured way to declare UI resources, enabling better interaction between embedded interfaces and host applications, thus reducing the burden on developers. The extension builds on existing work from the MCP-UI project and OpenAI Apps SDK. It introduces a standardized pattern for UI resources using a specific URI scheme, allowing hosts to prefetch templates and improve performance. Communication between UI components and hosts will occur via the established MCP JSON-RPC protocol, ensuring that all interactions are structured and auditable. The initial version supports only HTML content in sandboxed iframes, which provides broad compatibility and a solid foundation for future enhancements. Security is a significant focus, with measures like iframe sandboxing, predeclared templates for review, and auditable communication to protect against malicious servers. The extension is optional, allowing existing implementations to continue functioning without disruption. Developers can gradually integrate UI support, with servers expected to provide text-only fallbacks for tools that have UI capabilities. This approach helps avoid fragmentation within the ecosystem and promotes consistent user experiences. The proposal has been shaped by extensive feedback from the UI Community Working Group, and an early access SDK is available for testing and implementation.