1. Find a known-good commit and a known-bad one. Confirm both by actually checking out and testing. 2. `git bisect start; git bisect bad <bad>; git bisect good <good>`. 3. At each step, run the SMALLEST test that distinguishes good from bad. Mark `git bisect good/bad`. 4. When git names the first bad commit, read its diff. The bug is in those lines — don't guess elsewhere. 5. `git bisect reset`. Report: the commit, the line, the one-sentence cause. Automate it: `git bisect run ./repro.sh` if you have a script that exits non-zero on the bug.
