Start by understanding what the pull request is supposed to change. Then read the diff in context, trace the affected behavior through success and failure cases, and check whether tests cover the risks you found. Leave specific, evidence-based comments and request changes only when a defect needs fixing before merge.
How do I review a pull request for bugs before it’s merged?
Use a repeatable sequence: establish the intended behavior, map the scope, follow the changed code paths, evaluate test evidence, inspect security-sensitive changes, and choose a review outcome. Tailor each check to the repository’s contracts and your team’s merge policy; not every prompt applies to every change.
1. Establish the intent and scope
Read the pull request title and description, linked issue, acceptance criteria, and any review notes from the author. Write down what behavior should change and what should remain stable. If the intended behavior is unclear or the PR combines unrelated work, ask for context instead of guessing whether the implementation is correct. GitHub recommends focused pull requests with useful context for reviewers in its guidance on draft pull requests. Google’s reviewer guidance emphasizes considering how a change affects users.
2. Map the diff before judging it
Scan the changed-file list, then review files one at a time. Check the surrounding source when a diff hunk does not make sense by itself; changed lines can depend on callers, shared state, or behavior outside the hunk. GitHub describes file-by-file review and progress tracking in its proposed-change review documentation.
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Pay particular attention when the PR touches public interfaces, configuration, schemas or migrations, dependency manifests or lockfiles, permissions, authentication, workflows, or generated files. Such changes can affect behavior beyond the most visible code lines.
3. Trace the changed behavior
For each meaningful change, follow inputs through the relevant logic to outputs and side effects. Compare what the code does with the behavior the issue or product contract requires. Consider the cases that make sense for that feature:
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- Empty, invalid, repeated, unusually large, or boundary inputs.
- Errors that could leave data lost, duplicated, partially updated, or inconsistent.
- Cleanup or resource handling on both success and failure paths.
- State changes, ordering, or concurrency assumptions.
- Compatibility with existing callers, stored data, deployments, migrations, or supported environments.
- Whether identity and permission checks apply to the requested action and resource before protected work occurs.
These are prompts, not a requirement to invent edge cases unrelated to the change. Google’s guidance on reviewing behavior also asks reviewers to consider effects on how users build, test, interact with, and release code.
4. Check test evidence
Locate the tests changed or added alongside the implementation. Ask whether a test would fail if the suspected defect were present, and whether important failure paths or edge cases are exercised rather than only the happy path. Review relevant build and CI results, but treat passing automation as evidence—not proof—that the change is correct. GitHub’s author guidance recommends self-review and checking relevant tests or builds before requesting review.
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5. Inspect security and dependency risk
Give additional scrutiny to changes involving authentication, authorization, permissions, workflows, sensitive data, user-controlled input, or dependencies. Check that access decisions match the operation and resource, and happen before protected actions. Review dependency diffs directly as well as automated alerts: GitHub notes that dependency review may not show every manifest or lockfile change, including dependencies it cannot parse, in its dependency review documentation. The OWASP Code Review Guide includes authorization and business-logic checks among its security review areas.
What should I look for in a code review?
Use questions that fit the feature and its risk profile, rather than mechanically applying every item:
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- Does the implementation match the stated requirement and user-visible behavior?
- What happens with missing, invalid, repeated, very large, or boundary input?
- Can an error cause data loss, duplication, a partial update, or inconsistent state?
- Are identity and permissions checked for the correct action and resource?
- Do error handling and cleanup work on both success and failure paths?
- Could a dependency, configuration, workflow, or schema change affect behavior beyond the obvious lines?
- Do tests cover the changed behavior and a plausible failure case?
- Could an existing caller, deployment, migration, or supported environment break?
For each relevant question, look for evidence in the code, tests, or stated contract. If the evidence is incomplete, ask a focused question rather than presenting an assumption as a confirmed bug.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should I write a useful review comment?
Anchor a finding to the smallest code range that makes it understandable. Explain the condition that triggers the problem, the behavior you observed, and its likely impact. Where possible, give a concrete direction for clarifying or fixing it. For example: “If this request is retried after the timeout, this branch can create a second record because the operation is not idempotent. Can we check for the existing request before inserting?” Use a question when you need the author to confirm an assumption.
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Keep correctness and security findings distinct from preferences about style or design. GitHub supports line-level comments and suggested edits as part of its review workflow. Raise a limitation in your own domain or security expertise on a high-risk change, and request appropriate specialist input rather than implying certainty.
Should I comment, approve, or request changes?
| Outcome | Use it when |
|---|---|
| Comment | You have feedback or questions, but are not explicitly approving the change or asking that it be blocked. |
| Approve | You judge the change ready under your team’s standards. |
| Request changes | You found a concern that should be addressed before merge. |
These are GitHub’s documented review decisions; teams may also have their own merge rules. An approval means the change is ready under the standards you applied, not that no bug can remain. See GitHub’s pull request review documentation.
How should I use automated pull request review?
Automated review can supplement a human review, not replace judgment about product intent, repository context, and team requirements. GitHub describes Copilot code review as a feature that can identify bugs and security issues and offer suggestions in its Copilot code review documentation. Treat each suggestion as a lead to verify against the code and tests, not as proof of a defect or proof that unflagged code is safe.
When deciding how much weight to give automation, consider what repository and code context the tool can access, whether a finding is reproducible, the risk of the changed area, and whether your team can discuss and resolve findings before merge. Human review brings product and system context; automated suggestions still need validation.
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