MCP Apps — the extension specification

MCP Apps lets an MCP server ship an interactive UI alongside its tools. The UI renders in a sandboxed iframe inside the host (Claude, ChatGPT, M365 Copilot) and talks to the host over JSON-RPC 2.0 carried on postMessage.

Three entities, always:

• Server — registers the tool and the UI resource.
• Host — the chat client. Owns the iframe, mediates every call.
• View (also called the App) — your HTML/JS running inside the iframe.

The View never talks to the server directly. Every server call is proxied by the host, which is what makes the security model auditable.

1. The two registrations

A working MCP App is always exactly two registrations tied by one URI.