You missed out the input to the LLM, which would presumably be a requirements spec with all behaviour specified in exact detail, including all the tricky corner

You missed out the input to the LLM, which would presumably be a requirements spec with all behaviour specified in exact detail, including all the tricky corner cases were someone has to think hard about which solution is most useful and least confusing to the customer. Natural language isn't great for expressing such things. A formal notation would be easier. Perhaps something that makes it easy to express if-this-then-that kinds of things. I wonder if a programming language would be good for that.