This is a tangential point (this post is not really about TUIs; sort of the opposite) and I think lots of people know it already but I only figured it out last week and so can't resist sharing it: agents are good at driving tmux, and with tmux as a "browser", can verify TUI layouts.

So you can draw layouts like this and prompt Claude or Gemini with them, and get back working versions, which to me is space alien technology.

It's actually really easy in Claude Code. Get a TUI to the point where it renders something, and get Claude to the point where it knows what you want to render (draw it in ASCII like this post proposes, for instance).

Then just prompt Claude to "use tmux to interact with and test the TUI rendering", prompt it through anything it gets hung up on (for instance, you might remind Claude that it can create a tmux pane with fixed size, or that tmux has a capture-pane feature to dump the contents of a view). Claude already knows a bunch about tmux.

Once it gets anything useful done, ask it to "write a subagent definition for a TUI tester that uses tmux to exercise a TUI and test its rendering, layout, and interaction behavior".

Save that subagent definition, and now Claude can do closed-loop visual and interactive testing of its own TUI development.