I've recently been able to use LLM on a large-ish internal project to find a bug. The prompt took the form of "here's the symptoms I observe, and some hypothesis, tell me where the code that handles this case is written" (it was a brand new repo that I hadn't looked at before - code written by a different team, that were claiming some weird race condition/ were not really willing to look into the bug). Basically I was asking the LLM to tell me where to look, and it actually found the issue itself.
Not 100x more productive, that's an exaggeration... not even 10x. But it helps. It is an extremely competent rubber duck [1].
I too did this (although on a small project), and I was incredibly impressed. My problem with it is that I first did it myself, and it was fairly quick and easy. The hard part was figuring out that there was a bug, and how exactly the bug behaved. The LLM helped with the easy part, but I don't know how to even explain the difficult part to it. There was no way to know which repo the problem is in, or that it wasn't a user error.
Not 100x more productive, that's an exaggeration... not even 10x. But it helps. It is an extremely competent rubber duck [1].
[1] https://en.wikipedia.org/wiki/Rubber_duck_debugging