AI makes tech debt more expensive

> However, in ‘high-debt’ environments with subtle control flow, long-range dependencies, and unexpected patterns, they struggle to generate a useful response

I'd argue that a lot of this is not "tech debt" but just signs of maturity in a codebase. Real world business requirements don't often map cleanly onto any given pattern. Over time codebases develop these "scars", little patches of weirdness. It's often tempting for the younger, less experienced engineer to declare this as tech debt or cruft or whatever, and that a full re-write is needed. Only to re-learn the lessons those scars taught in the first place.

I recently watched a team speedrun this phenomenon in rather dramatic fashion. They released a ground-up rewrite of an existing service to much fanfare, talking about how much simpler it was than the old version. Only to spend the next year systematically restoring most of those pieces of complexity as whoever was on pager duty that week got to experience a high-pressure object lesson in why some design quirk of the original existed in the first place.

Fast forward to now and we're basically back to where we started. Only now they're working on code that was written in a different language, which I suppose is (to misappropriate a Royce quote) "worth something, but not much."

That said, this is also a great example of why I get so irritated with colleagues who believe it's possible for code to be "self-documenting" on anything larger than a micro-scale. That's what the original code tried to do, and it meant that its current maintainers were left without any frickin' clue why all those epicycles were in there. Sure, documentation can go stale, but even a slightly inaccurate accounting for the reason would have, at the very least, served as a clear reminder that a reason did indeed exist. Without that, there wasn't much to prevent them from falling into the perennially popular assumption that one's esteemed predecessors were idiots who had no clue what they were doing.

> Only to spend the next year systematically restoring most of those pieces of complexity as whoever was on pager duty that week got to experience a high-pressure object lesson in why some design quirk of the original existed in the first place.

Just to emphasize the point: even if it's not obvious why there is a line of code, it should at least be obvious that the line of code does something. It's important to find out what that something is and remember it for a refactor. At the very least, the knowledge could help you figure out a bug a day or two before you decide to pore over every line in the diff.

In my refactoring I always refer to that as Chesterton's Fence. Never remove something until you know why it was put in in the first place. Plenty of times it's because you were trying to support Python 3.8 or something else obsolete, and a whole lot of the time it's because you thought that the next project was going to be X so you tried to make that easy but X never got done so you have code to nowhere. Then feel free to refactor it, but a lot of the time it's because of good reasons that are NOT obsolete or overtaken by events, and when refactoring you need to be able to tell the difference.

https://www.chesterton.org/taking-a-fence-down/ has the full cite on the names.