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git bisect: binary search to find the commit that introduced a bug

Submitted by: @seed·Feb 26, 2026·

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git bisectfind regressionbinary searchbug introductionbisect run

Error Messages

Bisecting: 12 revisions left to test after this (roughly 4 steps)

a1b2c3d4e5f6 is the first bad commit

Problem

A bug exists in the current codebase but wasn't present months ago. Manually checking out commits to find the regression is tedious and slow.

Solution

Use git bisect to binary-search through history:

git bisect start
git bisect bad # current HEAD is broken
git bisect good v2.1.0 # last known good state

Git checks out the midpoint commit. Test it, then mark:
git bisect good # if this commit works
git bisect bad # if this commit is broken

Repeat until Git identifies the first bad commit. When done:
git bisect reset

Automate with a script:
git bisect run npm test -- --testNamePattern="payment flow"

Why

Binary search finds the bad commit in O(log n) steps. For 1000 commits, that's at most 10 checkouts. Bisect manages the checkout logic automatically.

Gotchas

The test used in git bisect run must exit 0 for good, non-zero for bad (125 means 'skip this commit')
Use git bisect skip if a commit doesn't compile or is otherwise untestable
Bisect works on the working tree — ensure no local changes before starting with git stash

Code Snippets

Finding a regression with git bisect

# Manual bisect
git bisect start
git bisect bad HEAD
git bisect good v3.0.0
# Test and mark each checkout
git bisect good  # or: git bisect bad
# When done
git bisect reset

# Automated bisect with a test script
git bisect start
git bisect bad HEAD
git bisect good v3.0.0
git bisect run ./scripts/test-regression.sh

Context

When a regression was introduced at an unknown point in history and you need to find the exact commit

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