One post tagged with "MCP Apps"

MCP Apps look simple in the spec. Your tool returns HTML, the host renders it in an iframe, the user sees a dashboard instead of a wall of JSON. Build one app, it works everywhere.

In practice, I've shipped MCP Apps to both ChatGPT and Claude over the past few months and learned that "works everywhere" requires handling a surprising number of sharp edges — iframe sandboxing, data format differences, a picky initialization handshake, and an interactive tool-calling pattern that's barely documented anywhere.

Here's everything I've learned, with the exact code for each one.