Last Tuesday I lost about three hours to a regression in our checkout service. The cart total was off by a cent on certain promo combinations, and the only signal was a Slack ping from finance with a screenshot. No stack trace. No exception. Just wrong numbers.

I did what I always do first. I opened the diff for the last deploy, scrolled, squinted, and tried to feel my way to the bug. Forty minutes in, I had a mental shortlist of suspects and zero proof. The code I was sure caused it had been merged a week earlier and was already reverted in a different branch for unrelated reasons. I was, as my old tech lead used to say, debugging the file I wanted the bug to be in.

Then I remembered git bisect run .

I keep relearning this tool. It's been around forever. I've recommended it to juniors more times than I can count.…