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A git bisect result tells you which commit is associated with a behavior change under a particular test. It does not tell you whether a proposed patch preserves the project’s public API or is safe to merge. Keep those decisions separate: make the test repeatable, record the identified commit and conditions, then review the changed API surface against the project’s compatibility policy.
What a bisect result establishes—and what it does not
Git describes git bisect as a binary search for the commit that introduced a bug. You mark a known-bad revision and one or more known-good revisions, test candidate commits between them, and report each result. The process narrows the range to a commit associated with the tested change. See the Git Project’s git-bisect documentation.
The finding is evidence about the behavior your test checks. It is not an independent judgment that the commit is correct, that the patch under review is identical to it, or that the change is compatible with users’ existing code. Those questions require a separate code and API review.
How to run a repeatable good/bad test
Define the behavior first
Write down the observable behavior that changed and decide what counts as “good” and “bad” before testing revisions. Use the same test, inputs, environment, and pass/fail rule at each candidate revision. If the criterion shifts during the search, the result no longer represents a consistent comparison.
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Set known endpoints and test candidates
Start with a revision where the behavior is known to be bad and at least one where it is known to be good. Follow Git’s documented bisection procedure to test selected commits and mark each result accordingly. If a revision cannot be tested, record that fact and use Git’s documented skip behavior rather than treating an unknown result as good or bad.
For command syntax and options, use the current git-bisect reference; the exact commands depend on how your repository and test are set up.
Rank #2
What to record with the identified commit
Preserve enough context that another reviewer can understand and reproduce the finding. Git’s documentation describes the bisection operation, not a required SHA length or review-note format; recording the full identifier and test context is reproducibility guidance, not a Git requirement.
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- The full commit identifier reported by the bisect.
- The known-good and known-bad endpoint revisions.
- The behavior under investigation and the test instructions, including relevant inputs or environment details.
- Any skipped revisions and why they could not be classified.
- The patch or diff being reviewed, so reviewers can confirm it is the change associated with the finding.
How to identify the public API under review
Do not assume every symbol in the repository is part of its public API. Check the project’s documented contract and code-level declarations, such as the interfaces or exports it explicitly exposes. Semantic Versioning 2.0.0 says software adopting the specification must declare a public API; that API can be defined by documentation or enforced by the code, and it should be clear and precise. Read the Semantic Versioning 2.0.0 specification alongside the project’s own policy.
Rank #3
Projects do not all adopt SemVer, and a repository’s release policy may define compatibility differently. Treat the project’s published policy as the governing guide for its users, rather than assuming SemVer applies automatically.
How to classify compatibility impact under SemVer
For a project that follows SemVer 2.0.0, the version increment depends on the effect on its declared public API. The specification’s guidance is:
| Change | SemVer increment |
|---|---|
| Backward-incompatible change to the declared public API | Major |
| Backward-compatible addition of public functionality, or marking existing public functionality deprecated | Minor |
| Backward-compatible bug fix | Patch, for versions after 1.0.0 |
SemVer also treats version 0.y.z as initial development: the public API should not be considered stable. That is a specific SemVer rule, not a guarantee that every project at version zero has no compatibility promises; check its stated policy.
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Make the patch-review decision separately
After confirming the patch matches the bisected commit, review what it changes and how users may be affected. A useful review record states the behavior demonstrated by the bisect, the declared API surface touched, the compatibility assessment under the project’s policy, and any tests or migration notes needed. A commit identifier is a locator for the finding, not a substitute for this judgment.
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Review checklist
- Is the good/bad behavior criterion explicit and consistently tested?
- Can another reviewer reproduce the finding from the recorded endpoints, test instructions, and skipped revisions?
- Does the patch under review match the commit identified by the bisect?
- Which changed declarations or documented promises belong to the project’s public API?
- Is the change backward-compatible under the project’s own release policy?
- If the project adopts SemVer, does the compatibility impact align with the proposed version increment?
- Do tests or user-facing migration notes need to accompany the change?
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