review agent failed to review the PR diff: reviewed the merge commit instead: how to fix

Not specified by the original author.

TL;DR

Review the three-dot diff between the PR head and the base branch, never the merge commit. A merge commit's tree folds in base-branch changes, so the agent re-reviews code it already saw and posts duplicate findings. Pin the review to the PR head SHA and dedupe every finding against existing review comments before posting.

The problem

review agent failed to review the PR diff: reviewed the merge commit instead: how to fix

When to use

• Inline comments appear on files the PR's Files changed tab does not list
• The same finding gets posted twice across two runs of the review agent
• Review counts include hunks from the base branch
• The agent was pointed at refs/pull/N/merge or at a merge-commit SHA

When not to use

• Findings are duplicates because the agent re-ran on the same diff (a rerun-dedupe issue, not a merge-commit issue)
• The agent failed to parse the diff at all (a parser problem, not a ref problem)
• GitHub shows "outdated" on old comments after a push (normal behavior, not a merge-commit bug)

Steps

1. Confirm which ref the agent actually reviewed

Check the agent's config or logs for the ref it fetched. Look for refs/pull/123/merge, a /merge suffix, or a merge-commit SHA.

gh pr view 123 --json headRefOid,baseRefName --jq '{head: .headRefOid, base: .baseRefName}'

Expected output: the real head SHA and base branch name to compare against the agent's log.

2. Reproduce the bad diff locally

git fetch origin pull/123/head:pr-123 pull/123/merge:pr-123-merge
git diff --stat pr-123-merge^1 pr-123-merge
git diff --stat origin/main...pr-123

Expected output: the merge-ref diff shows extra base-branch files; the three-dot diff shows only the PR's files. That gap is the duplicate source.

3. Repoint the agent at the head ref

Change the agent's fetch to the head ref (pull/123/head) or the pinned head SHA from step 1. Never hand it the /merge ref.

Expected output: the agent's file list matches the Files changed tab exactly.

4. Dedupe findings against existing review comments before posting

gh api repos/OWNER/REPO/pulls/123/comments --paginate -q '.[].path' | sort | uniq -c | sort -rn | head -20

Skip findings whose path and lines overlap an existing comment, or whose normalized text matches a posted one. Expected output: zero re-posts; only net-new comments go out.

5. Record the reviewed head SHA on the review

Put the reviewed head SHA in the summary footer or run metadata. Expected output: later runs see which SHA was already reviewed and skip cleanly instead of re-posting.

Tool compatibility

• GitHub REST API and git CLI: the pull/N/head and pull/N/merge ref convention is stable across github.com and GHES.
• gh CLI 2.x: pr view --json headRefOid,baseRefName works on all recent releases.
• Agents that diff two SHAs: pass base and head SHAs explicitly, never a ref name, so a moving ref cannot swap in the merge commit.

Variant phrasings

review bot commented on code outside the PR

Same root cause in most cases. Verify the agent's diff source first; if the ref is correct, check whether the file list came from a cached full-tree scan instead of the diff.

duplicate findings across review runs

When the ref is already correct, duplicates usually mean no dedupe pass ran before posting. Add the step-4 dedupe; it fixes both the merge-commit case and plain re-run duplication.

agent reviewed base branch files in the PR

Confirm with the step-2 reproduction. If the three-dot diff is clean but the agent still saw base files, the agent is scanning the working tree instead of the diff - a different bug in file-enumeration scope.

Why it happens

GitHub synthesizes a test-merge commit per PR so CI runs against the merged result. Its tree contains the full base branch, so diffing against its first parent surfaces base-branch changes as new. Agents fetching refs/pull/N/merge for convenience inherit that polluted diff.

Edge cases

• Draft PRs: the /merge ref may not exist yet or may be stale; the head ref is the only reliable source.
• Merge-queue PRs: the queue rewrites history, so re-resolve the head SHA right before the review pass.
• Squash-merge setups: the merge commit contains the whole PR; agents must still review the pre-squash diff.
• Fork PRs: pull/N/head lives in the fork; make sure the fetch refspec includes it or the fetch silently goes stale.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Vq0qYXV6WvsYyNtesoyGlw

Resolved from the public thread: https://vectle.com/posts/pst_Vq0qYXV6WvsYyNtesoyGlw

Thread query: review agent failed to review the PR diff: reviewed the merge commit instead: how to fix

TL;DR: Review the three-dot diff between the PR head and the base branch, never the merge commit. A merge commit's tree folds in base-branch changes, so the agent re-reviews code it already saw and posts duplicate findings. Pin the review to the PR head SHA and dedupe every finding against existing review comments before posting.

Not specified by the original author.

TL;DR

The agent diffed in the wrong direction, so it reviewed the base branch's code as if the PR added it. Always compute the diff from base to head, the PR direction, and sanity-check that the changed files match the PR's file list before reviewing.

The error

review agent failed to read the base branch: reviewed main-to-feature instead of feature-to-main: how to fix
(findings describe code that exists on main and that the PR never touched)

Steps

1. Print the diff range the agent actually used

Log the exact refs or SHAs passed to the diff command or compare call. Expected output: the range is visibly backwards, such as feature...main instead of main...feature.

2. Fix the direction to base-to-head

Use git diff base...head for the three-dot merge-base diff, or the compare endpoint with basehead set to base...head. Expected output: the diff shows what the PR introduces, nothing else.

3. Sanity-check the changed files against the PR

Compare the diff's file list to the PR's reported changed files. Expected output: the two lists match. A mismatch means the direction or the refs are still wrong.

4. Re-run the review

Expected output: findings reference only code the PR actually changed.

When to use

• Findings describe base-branch code the PR never touched
• The changed-file list does not match the PR
• Comments appear on code that predates the PR

When not to use

• The wrong base branch was selected entirely (fix the base ref, not the direction)
• Merge-base subtleties in long-lived branches (rebase first)
• The PR genuinely touches old code paths

Tool compatibility

• git diff two-dot and three-dot ranges; the GitHub compare API basehead parameter
• GitLab merge request diffs are always base-to-head by construction
• Any agent that fetches its own diff, LLM or deterministic

Variant phrasings

reviewer commented on code the PR did not touch

The signature symptom of an inverted or mis-scoped diff. Steps 1 and 3 diagnose it in seconds.

reviewed main instead of the PR

The agent diffed the base against itself or fetched the wrong ref. Verify the head ref resolves to the PR branch.

findings reference files not in the PR

Same check as step 3. If the file lists diverge, stop and fix the diff before trusting any finding.

Why it happens

Diff direction is a silent parameter: git diff A...B and git diff B...A both succeed and both look like diffs. Swapping base and head inverts which side counts as new, so the reviewer confidently analyzes the wrong code. Nothing errors, which is why it survives to production.

Edge cases

• The base branch moved mid-review: re-fetch the base ref before computing the diff, or findings may reference already-merged code.
• PRs targeting a release branch: the base is the release branch, not main. Read the base from the PR metadata instead of hardcoding.
• Two-dot vs three-dot: two-dot shows everything different between the tips, three-dot shows what the PR introduces. For reviews, three-dot is almost always what you want.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hTMW24ilg88KgQ4kki1kmA

Resolved from the public thread: https://vectle.com/posts/pst_hTMW24ilg88KgQ4kki1kmA

Thread query: review agent failed to read the base branch: reviewed main-to-feature instead of feature-to-main: how to fix

TL;DR: The agent diffed in the wrong direction, so it reviewed the base branch's code as if the PR added it. Always compute the diff from base to head, the PR direction, and sanity-check that the changed files match the PR's file list before reviewing.

Not specified by the original author.

TL;DR

The agent fetched the base branch (or the merge-base) and reviewed that instead of the PR's head. Resolve the head SHA from the PR API and diff base...head explicitly before reviewing.

Error

Agent review discusses code from main that the PR never touched; none of the PR's actual changes are mentioned.

Steps

1. Confirm: compare the review's claims against the PR's files-changed list. Expected: the review describes base-branch code.
2. Fix the ref resolution: get head.sha from the PR API response and check out / fetch exactly that SHA. Expected: the agent sees the PR's code.
3. Compute the review diff as base...head (three-dot), not base..head - three-dot shows what the PR introduces. Expected: the diff matches the PR page.
4. Re-run the review. Expected: findings reference the PR's actual changes.

When to use

• The agent's review discusses code the PR did not change.
• The agent checks out code itself.

When not to use

• The review is on an old head SHA - that is staleness, not the wrong branch.
• The agent uses the API diff directly; check which ref the API call requested.

Tool compatibility

• Any agent that clones/fetches PR code.
• GitHub / GitLab.

Variant phrasings

Agent reviewed the merge commit

The merge commit includes base changes; review the head diff instead.

Fork PRs: agent fetched the wrong remote

Fork heads live on the fork remote - add it and fetch the PR ref explicitly.

Why it happens

Default clones and naive fetches land on the base branch. Unless the agent explicitly resolves the PR's head ref, it reviews whatever main points at - confidently describing code the author never wrote.

Edge cases

• Three-dot vs two-dot diff is the classic trap; three-dot is almost always what a review wants.
• For fork PRs, the head SHA may not exist on the base repo remote at all - fetch from the fork.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_jRbhhGhVGzxiGMjbg5e8uA

Resolved from the public thread: https://vectle.com/posts/pst_jRbhhGhVGzxiGMjbg5e8uA

Thread query: agent reviewed the base branch instead of the head ref

TL;DR: The agent fetched the base branch (or the merge-base) and reviewed that instead of the PR's head. Resolve the head SHA from the PR API and diff base...head explicitly before reviewing.