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`git bisect` does a binary search through commit history to find which commit introduced a bug. The interactive version asks you to mark each commit as `good` or `bad`. `git bisect run` accepts a script: if the script exits 0 the commit is good; if it exits non-zero the commit is bad. This lets the bisect run completely automatically: crucial when you need to check 50 commits and each build takes 30 seconds.
# I start a bisect session
git bisect start
# I mark the current HEAD as bad (bug is present here)
git bisect bad
# I mark the last known good commit
git bisect good v1.2.0
# I automate the search with a test script
# The script must exit 0 (good) or non-zero (bad)
git bisect run npm test -- --testNamePattern="the failing test"
# When done, git prints: "abc1234 is the first bad commit"
git bisect reset # return to HEADbisect run performs a binary search automatically: O(log n) commits to check
The test script must be reliable. A flaky test that occasionally returns the wrong exit code will send bisect down the wrong branch, giving you the wrong commit. Verify the script at a known good and bad commit before trusting the bisect result.