MCP Apps (UI extension) — staged 2026-08-11

Provenance

File	Source	Fetched	Completeness
ext-apps-spec.md	github.com/modelcontextprotocol/ext-apps specification/2026-01-26/apps.mdx (via 2026-08-10 baseline, pinned commit 92f46a5)	2026-08-10, re-checked 2026-08-11	COMPLETE
spec.types.ts	same repo, TypeScript type source of truth	2026-08-10	COMPLETE
patterns.md, csp-cors.md	same repo	2026-08-10	COMPLETE

Re-checked today against https://modelcontextprotocol.io/extensions/apps/overview.md (fetched 2026-08-11, COMPLETE). No drift: the spec directory upstream is still specification/2026-01-26/, the wire model is unchanged.

Version

Extension spec revision 2026-01-26. This is an extension, versioned independently of the core MCP protocol revision (2026-07-28).

Model (unchanged since baseline)

1. A tool declares _meta.ui.resourceUri → a ui:// resource. Hosts may preload it before the tool is called.
2. Host fetches the UI resource (an HTML page, usually with JS/CSS inlined).
3. Host renders it in a sandboxed iframe. _meta.ui.csp controls which external origins may be loaded; _meta.ui.permissions requests extra capabilities (mic, camera, …).
4. App ↔ host speak a JSON-RPC dialect over postMessage: some methods shared with core MCP (tools/call), some new with a ui/ prefix (ui/initialize, …).

Host support as of 2026-08-11: Claude, Claude Desktop, VS Code GitHub Copilot, Microsoft 365 Copilot, Goose, Postman, MCPJam, Archestra.AI. Client-side libs: @modelcontextprotocol/ext-apps (App + AppBridge), @mcp-ui/client.

API docs / generated registry: https://apps.extensions.modelcontextprotocol.io/api/