Yes, you need some kind of other source of truth. I think the best way to get that is to do clean room development with a different agent, but ultimately if you give them the wrong idea they'll do the wrong thing.
The other thing I do, not as much as I should, but it's very powerful, is to generate spikes and deliberately throw them away to understand how to prompt better. Like I generated a swift version of the react native app I'm working on, and Alloy provers for the state transitions. None of it is production quality but getting great results that way is useful to scope future work.
If you've managed people, these are all familiar problems. I found you need much more than a functional specification, you also need motivation, background, related work, ideas tried, etc., because those help disambiguate the right path in the inevitable situation where your original task description is unclear or conflicts with itself.
Yes I'm using the entire consultancy stack - define values, etc, and work your way down the "where do these not match reality on the ground and need change", but for little robot people instead of (arguably less messy) humans.
> I think the best way to get that is to do clean room development with a different agent
What is this about? Could you give an example?