What it solves
MCP Apps links an MCP tool to a ui:// resource. A compatible host fetches the HTML view, renders it in a sandboxed iframe and brokers JSON-RPC communication between the view, model context and server tools.
Where it sits
This is a host-embedded application mechanism. It is not a portable declarative component tree like A2UI and not a general agent-to-owned-frontend event protocol like AG-UI.
Host dependency
Support varies by client. Confirm capability negotiation and the client matrix before choosing MCP Apps as a distribution target.
Security boundary
Use the mandatory sandbox, declare the narrowest content security policy and permissions, treat tool results as untrusted, and keep server-side authorization independent from anything displayed by the view.
Minimal example
MCP tool result → references ui://atlas/comparison
compatible host → fetches resource → sandboxed iframe
iframe action → host bridge → authorized MCP tool
Capability detection and a text fallback are required because not every MCP client renders Apps.