> We are looking from the perspective of A, to map the entries for B (inbound), and after the request to B is returned, we map back to A's model (outbound). This is a chore, and much of the work is repetitive, but it comes with its edge cases that we need to look out for and unfortunately there isn't a solid foundation of patterns apart from the Domain-driven design (DDD) thing.

This sounds like a job for protobufs or some kind of serialization solution. And you already know there are dragons here, so letting a LLM try and solve this is just going to mean more rework/validation for you.

If you don't understand the problem space, hire a consultant. LLMs are not consultants (yet). Either way, I'd quit wasting time on trying to feed your codebase into a LLM and just do the work.