Find the commit that broke something with git bisect

Binary-search the history between a known good commit and a known bad one until git names the commit that introduced the problem.

git bisect start
git bisect bad                 # the commit you're on is broken
git bisect good v1.4           # a tag, branch or sha that worked

Git checks out the commit halfway between them. Build, run the failing thing, then tell git what you saw:

git bisect good    # or: git bisect bad

Repeat until it prints <sha> is the first bad commit. Then put your branch back:

git bisect reset

Let a script do the marking

If the failure is a command that exits non-zero, hand it to bisect and walk away:

git bisect start HEAD v1.4
git bisect run npm test -- --run src/auth.test.ts

run marks each step from the exit code: 0 is good, 1 to 127 is bad, and 125 tells bisect to skip that commit, which is what to return when the build itself fails there.

Gotchas

• A commit that won’t build is neither good nor bad. Mark it git bisect skip and git picks a neighbour.
• Bisect moves HEAD around under you. Commit or stash before you start, or the checkout is refused halfway through.
• Ten steps cover about a thousand commits, so a wide range is fine. What costs time is a slow reproduction, which is why run is worth setting up.
• git bisect log prints what you’ve marked so far. If you mark one wrong, git bisect reset, edit that log, and replay it with git bisect replay.