[jaggederest]

I think one way is to take the existing system in something like a docker container or equivalent, some kind of black box, and write tests against it in pure HTTP calls or using browser automation to record (can drive it with AI). When you've reached a truly massive test suite that covers everything, you delete the container and use the test suite as an oracle for writing a new version (open book, the AI can look at the test suite but not change it).

This is a tactic based on things I have read in "Working Effectively with Legacy Code" by Michael Feathers - he discusses using cut points to build a testing firewall to bring code under test, then gradually expanding the test suite from that beachhead of confirmed interface.