Remote Model Context Protocol (MCP) servers expose tools, but did you know those tools can return UI, not just text?

Using the MCP Apps extension to the MCP protocol, an MCP app can deliver interactive experiences directly inside MCP hosts like Claude.ai and ChatGPT.

We’ll break down what makes an MCP app, how it all comes together, then live code one from scratch using the chatgpt-app-typescript-template as a starting point.

We’ll tunnel it to a public URL, register it in both Claude.ai and ChatGPT, then lock down tool calling with Pomerium, an open source (Apache 2.0) MCP gateway that handles authentication, authorization, and fine-grained tool access control so you control who gets to use your MCP app and its tools.