I never have LLMs work on 1000 LOC. I don't think that's the value-add. Instead I use it a the function and class level to accelerate my work. The thought of having any agent human or computer run amok in my code makes me uncomfortable. At the end of the day I'm still accountable for the work, and I have to read and comprehend everything. If do it piecewise I it makes tracking the work easier.
Big test time compute LLMs can easily handle 1k depending on logic density and prompt densitity.
Never an agent, every independent step an LLM takes is dangerous. My method is much more about taking the largest and safest single step at a time possible. If it can't do it in one step I narrow down until it can.