I am building an assembler+compiler+VM for a python-like statically typed language with monomorphized generics and Erlang-style concurrency. Claude Sonnet/Kimi/Gemini Pro (and even ChatGPT on occasion) are able to handle the task reasonably well because I give them specs for the VM that have been written and rewritten 200+ times to remove any ambiguities and make things as clear as possible.

I go subsystem by subsystem.

Writing the interpreter for a stack vm is as simple as it gets.