Build an MCP App (Interactive UI Widgets) An MCP app is a standard MCP server that also serves UI resources — interactive components rendered inline in the chat surface. Build once, runs in Claude and ChatGPT and any other host that implements the apps surface. The UI layer is additive . Under the hood it's still tools, resources, and the same wire protocol. If you haven't built a plain MCP server before, the skill covers the base layer. This skill adds widgets on top. Testing in Claude: Add the server as a custom connector in claude.ai (via a Cloudflare tunnel for local dev) — this exercises…