The widget is a second channel and the model is not on it

Two MCP Apps talks from the AI Engineer conference this week, and they disagree about what the feature is for. Indeed’s Dustin Mihalik built job search widgets for Claude and ChatGPT and came back with rules. The first: anything you show the user must also go to the model as data, or the next question, tell me about the first result, lands on a model that has no idea what is on screen. Same for clicks. The third rule, which they say supersedes the others, is to split the search tool from the render tool, because once results were drawn in a carousel the model stopped running the ten or fifteen searches a relocation query needs and called it done.

Jeremiah Lowin of Prefect wants the opposite thing from the same spec. Their talk describes the point of MCP Apps as bypassing the agent. A file upload through an ordinary tool means the model retypes a megabyte character by character, which they call the world’s most expensive copy paste, so the widget sends the file straight to the server instead. Nearby in the radar sits OUI-1, a model on r/LocalLLaMA fine-tuned to emit UI elements in a custom DSL rather than HTML. I have the headline on that one, so the claim stays with its poster.

Both talks are right about their own case, which is the tell. The vault’s tool notes record the verified shape: a server returns structured content plus an optional component, the component renders in a sandboxed iframe, and it talks to the host over JSON-RPC. That is a second channel to the user. Everything the model knows about it is whatever the server chose to send back.

The same page’s rule for any interface is to measure whether it reduces failures, context load, and manual repair. A widget scores well on context load and can quietly fail on the other two, because the model’s view of the conversation now has a hole in it the size of the screen.

Which is an old retrieval failure in new clothes. The important part is in the middle, and the middle is now a button the user pressed.