We had enough tests to know that no other rule configuration worked. Heck, we had mathematical proof (and a small pile of other documentation too obsolete or cryptic to be of use), and still, the only thing that saved the project was noticing different stylistic conventions in different parts of the source, allowing the minor monolith to be broken down into "this is the core logic" and "these are the parts of a separate feature that had to be weaved into the core logic to avoid a circular dependency somewhere else", and finally letting us see enough of the design to make some sense out of the cryptic documentation. (Turns out the XML held metadata auxiliary to the core logic, but vital to the higher-level interactive system, the proprietary binary encoding was largely a compression scheme to avoid slowing down the core logic, and the system was actually 8-bit-clean from the start – but used its own character encoding instead of UTF-8, because it used to talk to systems that weren't.)

Test-driven development doesn't actually work. No paradigm does. Fundamentally, it all boils down to communication: and generative AI systems essentially strip away all the "non-verbal" communication channels, replacing them with the subtext equivalent of line noise. I have yet to work with anyone good enough at communicating that I can do without the side-channels.