How do I use git bisect to find the commit that introduced a bug?

Question

A feature used to work, but it is broken on my current branch and hundreds of commits have landed. How can I find the first bad commit without checking every revision?

IT-QA Assistant · Accepted Answer

*AI-drafted answer — reviewed for correctness. Please verify against your own environment and versions; corrections and better answers are welcome below.*

`git bisect` narrows the commits between a known working revision and a broken revision by repeatedly choosing a candidate to test. Start with a clean working tree and a reliable reproduction:

```bash
git bisect start
git bisect bad HEAD
git bisect good v1.2.0
```

Replace `v1.2.0` with a revision you have actually verified as good. Git checks out a candidate. Build and test it, then run either `git bisect good` or `git bisect bad`. Repeat until Git identifies the first bad commit. Run `git bisect reset` afterward to return to your starting checkout.

If a script reliably classifies revisions, automate it:

```bash
git bisect run /absolute/path/check-regression.sh
git bisect reset
```

The script should exit 0 for good, 1 for the target regression, and 125 when a revision cannot be tested. Other exit codes have defined meanings, so do not blindly propagate unrelated build failures.

Use `git bisect skip` for an untestable revision; skipping can leave an ambiguous result. Flaky tests or a bug that disappears and reappears can mislead the search. Confirm the reported commit and its parent manually. See git bisect (https://git-scm.com/docs/git-bisect).

How do I use git bisect to find the commit that introduced a bug?

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