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fix: use merge-base for accurate PR diff in code review example #64

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fix: use merge-base for accurate PR diff in code review example #64

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15 changes: 13 additions & 2 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -35,13 +35,24 @@ jobs:
with:
# Explicitly check out the PR's merge commit.
ref: refs/pull/${{ github.event.pull_request.number }}/merge
# Fetch full history so git merge-base can find the common ancestor
fetch-depth: 0

- name: Pre-fetch base and head refs for the PR
run: |
git fetch --no-tags origin \
${{ github.event.pull_request.base.ref }} \
+refs/pull/${{ github.event.pull_request.number }}/head

- name: Compute merge-base
id: merge_base
run: |
# Find the common ancestor between the PR branch and base branch.
# This ensures we only review changes introduced by the PR, not
# changes added to main after the PR branch was created.
MERGE_BASE=$(git merge-base origin/${{ github.event.pull_request.base.ref }} HEAD)
echo "sha=$MERGE_BASE" >> "$GITHUB_OUTPUT"

# If you want Codex to build and run code, install any dependencies that
# need to be downloaded before the "Run Codex" step because Codex's
# default sandbox disables network access.
Expand All @@ -54,8 +65,8 @@ jobs:
prompt: |
This is PR #${{ github.event.pull_request.number }} for ${{ github.repository }}.

Review ONLY the changes introduced by the PR, so consider:
git log --oneline ${{ github.event.pull_request.base.sha }}...${{ github.event.pull_request.head.sha }}
Review ONLY the changes introduced by the PR. Use this to see the diff:
git diff ${{ steps.merge_base.outputs.sha }}..HEAD

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