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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. For a useful review, record the full commit identifier and the conditions that produced it, then assess the patch against the project’s declared API and release policy as a separate step.
What a bisect result establishes—and what it does not
Git describes git bisect as a binary search through project history to find which commit introduced a bug. You give it a known bad revision and one or more known good revisions; Git selects commits between them for you to test. Marking each candidate good or bad narrows the range until the investigation identifies a commit associated with the tested change. See the Git Project’s git-bisect documentation.
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The result is evidence about the behavior your test measured. It does not, by itself, show that a patch is correct, safe to merge, or compatible with users’ code. Those questions require inspecting the patch and the project’s API promises.
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Define the behavior first
Write down what “good” and “bad” mean before starting. Describe the observable behavior and the test conditions clearly enough that another contributor can apply the same test to each candidate revision. If the test changes between revisions, the labels may no longer describe a consistent property.
#1 Best Overall
Bisect between known endpoints
Start with a revision where the behavior is known to be bad and at least one where it is known to be good. Test each revision Git selects using the same procedure, then mark it accordingly. If a candidate cannot be tested, record that fact and how it was handled; skipped revisions can affect how confidently another reviewer interprets the finding.
Preserve the investigation record
In the review, capture the full commit identifier reported by the investigation, the good and bad endpoint revisions, the test instructions and relevant environment or setup details, and any revisions that could not be tested. Git does not mandate a particular SHA length or review-note format; recording enough context is a reproducibility practice, not a Git requirement.
Rank #2
Verify that the patch under review is the one found
Before drawing conclusions, compare the recorded commit with the patch being reviewed. Confirm the commit identity and inspect its changes in context. A bisect associates a commit with the tested behavior; it does not establish that every change in that commit caused the behavior, nor that the candidate patch is equivalent to the investigated change.
Identify the public API the project promises
Do not assume that every exported-looking or widely used internal symbol is part of the supported API. Check the project’s own declarations and commitments: documentation, API stability statements, release policy, and any code-level mechanisms that mark supported interfaces. The Semantic Versioning 2.0.0 specification says software using SemVer must declare a public API, which may be defined by documentation or enforced by the code, and that the API should be clear and precise. Read the Semantic Versioning 2.0.0 specification alongside the project’s actual policy.
Assess compatibility and versioning separately
First determine whether the changed surface is part of the declared public API. Then ask whether existing users can continue to rely on it in the same way. For projects that adopt SemVer 2.0.0, the specification classifies release increments as follows:
| Change under the project’s declared API | SemVer increment |
|---|---|
| Backward-incompatible change to public API | Major |
| Backward-compatible new public functionality, or deprecation of public functionality | Minor |
| Backward-compatible bug fix, for versions after 1.0.0 | Patch |
These rules apply to projects following SemVer; they do not override a project’s different documented release policy. SemVer also treats major version zero as initial development, when the public API should not be considered stable. That caveat does not make compatibility consequences irrelevant: explain them to reviewers and users in the terms of the project’s own policy.
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Keep the evidence and decision distinct. State what behavior the bisect demonstrated and under what test conditions, identify the public API surface the patch changes, and explain the compatibility impact under the project’s release policy. Where relevant, note tests needed to verify the change and migration guidance users may need. The commit identifier anchors the investigation; it cannot substitute for the API analysis.
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Review checklist
- Are “good” and “bad” defined by the same observable test?
- Are the full identified commit and the good and bad endpoints recorded?
- Can another reviewer reproduce the test setup and understand any skipped revisions?
- Does the patch under review match the investigated commit or change?
- Is the affected surface part of the project’s declared public API?
- Is the change backward-compatible, and what release policy does the project follow?
- Are additional tests or migration notes needed to make the decision clear?
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