MCP Apps: Build Interactive UIs in Your MCP Server

MCP servers used to be text-only. You called a tool, got back JSON or a string, and the model formatted it in chat. That changed in early 2026 with SEP-1865 (MCP Apps), an official extension that lets your server ship a real, sandboxed UI right inside Claude, ChatGPT, or any host that supports the spec.

This guide builds a tiny “Get Server Time” MCP App from scratch in TypeScript. You will register a tool that links to a UI resource, bundle that UI as a single HTML file, and watch Claude render it in a sandboxed iframe with a working button. The same pattern scales to dashboards, color pickers, payment forms, anything you would normally build in React.