Bisect
A way to find the commit that introduced a bug by halving the range. You name a good commit and a bad one, git checks out the middle, you say good or bad, and it halves again. A thousand commits takes about ten tests.
When an agent is stuck guessing at a bug, bisect stops the guessing, because its answer is one commit. It can run unattended too: give it a script, and it finds the commit while you do something else. It depends on every commit building, and on the commit it names being small enough to read.
“v1.2.0 worked and HEAD is broken. Write scripts/repro.sh so it exits 0 without the bug, 1 with it, and 125 if the commit won't build. Then run git bisect start HEAD v1.2.0 and git bisect run scripts/repro.sh, and show me the commit it names.”