Definition
MCP Apps is an official model-context-protocol extension (SEP-1865) that lets servers ship interactive HTML UIs rendered by hosts in sandboxed iframes, with actions audited via the same JSON-RPC path as tool calls.
Key Points
- Tools declare UI templates ahead of time for prefetch, cache, and security review
- Bidirectional host↔UI communication uses JSON-RPC / postMessage
- Graduates from experimental to official with the 2026-07-28 extensions framework