This. Waterfall never worked for a reason. Humans and agents both need to develop a first draft, then re-evaluate with the lessons learned and the structure that has evolved. It’s very very time consuming to plan a complex, working system up front. NASA has done it, for the moon landing. But we don’t have those resources, so we plan, build, evaluate, and repeat.
So, rollback and try again with the insight.
AI makes it cheap to implement complex first drafts and iterations.
I'm building a CRM system for my business; first time it took about 2 weeks to get a working prototype. V4 from scratch took about 5 hours.
That "first draft" still has to start with a spec. Your only real choice is whether the spec is an actual part of project documentation with a human in the loop, or it's improvised on the spot within the AI's hidden thinking tokens. One of these choices is preferable to the other.