(docs): MCP Apps documentation

[localden]

Summary

Note

This document is heavily inspired by @jonathanhefner's work on the quickstart in the MCP Apps documentation.

Adds documentation for MCP Apps, an extension that enables MCP servers to deliver interactive UI applications directly within conversational clients like Claude.

Preview: https://mcp-staging-localden-mcpapps.mintlify.app/docs/extensions/apps

What's included

Getting started

• Two onboarding paths: AI coding agent workflow (with create-mcp-app skill) and manual setup
• Cross-platform support with macOS/Linux and Windows/PowerShell examples
• Skills directory reference for Claude Code, VS Code/GitHub Copilot, Gemini CLI, Cline, and Goose

Technical content

• How MCP Apps work: The tool + UI resource pattern, with sequence diagram showing the LLM → server → app flow
• Building an MCP App: Complete server and UI implementation using @modelcontextprotocol/ext-apps
• The App API: connect(), callServerTool(), ontoolresult, sendMessage(), sendLog(), sendOpenLink()
• Security model: iframe sandboxing and postMessage-based communication
• Framework support: Emphasizes MCP Apps use standard web primitives (HTML, JS, postMessage); SDK is optional

Testing

• Testing with Claude via cloudflared tunneling and custom connectors
• Testing with the basic-host development tool

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Content written by @localden. PR summary generated with Claude Code.

[idosal]

Great points. One other implication that's deducible but easy to miss -
The MCP app's communication with the host isn't restricted to accessing its own server. The app can effectively delegate actions to whatever tools/MCP servers the user already has connected (subject to host permissions/consent).
For example, the app can drive the model to schedule a meeting, and the model can route that through the user’s existing calendar/email connectors. That way, apps don't need to develop direct integrations with all email providers themselves.