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.

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.