I am fan of using markdown to describe the UI.

It is simple, effective and feels more native to me than some rigid data structure designed for very specific use-cases that may not fit well into your own problem.

Honestly, we should think of Emacs when working with LLMs and kind of try to apply the same philosophy. I am not a fan of Emacs per-se but the parallels are there. Everything is a file and everything is a text in a buffer. The text can be rendered in various ways depending on the consumer.

This is also the philosophy that we use in our own product and it works remarkably well for diverse set of customers. I have not encountered anything that cannot be modelled in this way. It is simple, effective and it allows for a great degree of flexibility when things are not going as well as planned. It works well with streaming too (streaming parsers are not so difficult to do with simple text structures and we have been doing this for ages) and LLMs are trained very well how to produce this type of output - vs anything custom that has not been seen or adopted yet by anyone.

Besides, given that LLMs are getting good at coding and the browser can render iframes in seamless mode, a better and more flexible approach would be to use HTML, CSS and JavaScript instead of what Slack has been doing for ages with their block kit API which we know is very rigid and frustrating to work with. I get why you might want to have a data structures for UI in order to cover CLI tools as well but at the end of the day browsers and clis are completely different things and I don not believe you can meaningfully make it work for both of them unless you are also prepared to dumb it down and target only the lowest common dominator.