In a circuitous way, you can rather successfully have one agent write a specification and another one execute the code changes. Claude code has a planning mode that lets you work with the model to create a robust specification that can then be executed, asking the sort of leading questions for which it already seems to know it could make an incorrect assumption. I say 'agent' but I'm really just talking about separate model contexts, nothing fancy.
Cursor's planning functionality is very similar and I have found that I can even use "cheap" models like their Composer-1 and get great results in the planning phase, and then turn on Sonnet or Opus to actually produce the plan. 90% of the stuff I need to argue about is during the planning phase, so I save a ton of tokens and rework just making a really good spec.
It turns out that Waterfall was always the correct method, it's just really slow ;)
