> my first bet would be specifications and tests

You are missing another dimension how easy it would be to migrate if adding new feature hits a ceiling and LLM keeps breaking the system.

Imagine all tests are passing and code is confirming the spec, but everything is denormalized because LLM thought this was a nice idea at the beginning since no one mentioned that requirement in the spec. After a while you want to add a feature which requires normalized table and LLM keeps failing, but you also have no idea how this complex system works.

Don't forget that very very detailed spec is actually the code