MCP Apps just dropped (OpenAI and Anthropic collab) and I think this is huge (blog.modelcontextprotocol.io)

Today the MCP community proposed "MCP Apps," an optional extension to the Model Context Protocol that standardizes how servers deliver interactive user interfaces to hosts. Built by maintainers from OpenAI, Anthropic and the MCP-UI community, it formalizes patterns already proven by MCP-UI and the OpenAI Apps SDK so tools can ship rich, embedded experiences (e.g., chart viewers, multi-field configuration UIs) instead of awkward text exchanges. This reduces fragmentation, enables consistent prefetching and caching of UI templates, and positions MCP as a runtime foundation for agentic apps that coordinate models, UIs, and host applications. Technically, UI templates are declared as predeclared resources using a ui:// URI scheme and referenced in tool metadata (e.g., _meta."ui/resourceUri"). UI ↔ host communication reuses MCP’s JSON‑RPC transport over postMessage and the existing @modelcontextprotocol/sdk, so messages are structured, auditable, and compatible with future MCP features. Initial support is for text/html rendered in sandboxed iframes; other content types are deferred. Security is layered: sandboxing, pre-reviewable templates, loggable JSON‑RPC, and host-enforced user consent. MCP Apps is optional and backward-compatible—servers must provide text-only fallbacks—while SEP-1865, SDKs, and prototypes are available for review and testing.