git bisect — find the commit that introduced a bug

Quick Answer

# Start bisect, mark current state as bad
git bisect start
git bisect bad                  # current commit has the bug

# Mark a known-good commit (a tag, hash, or relative ref)
git bisect good v1.2.0

# git checks out the midpoint — test it, then mark it:
git bisect good                 # or: git bisect bad

# Repeat until git prints: "abc1234 is the first bad commit"

# When done, restore HEAD
git bisect reset

When to use this

A bug appeared somewhere between a known-good release and the current HEAD. Bisect finds the exact commit in O(log n) steps.

Other causes & fixes

Automate the test with a script

If you have a test command that exits 0 for good and non-zero for bad, bisect can run automatically:

git bisect start
git bisect bad HEAD
git bisect good v1.2.0
git bisect run npm test -- --grep "failing test name"

Skip a commit you cannot test

git bisect skip