When a bot opens a PR against main and there's already another PR on
the same head branch targeting staging, GitHub's PATCH /pulls returns
422 with:
"A pull request already exists for base branch 'staging' and
head branch '<branch>'"
Pre-fix: the retarget Action exited 1 with no further action. The
target-main PR sat there as a duplicate, the workflow run showed
red, and someone had to manually close the duplicate. Today's case
(#1881 duplicate of #1820) had to be closed manually.
Fix: catch that specific 422 message and close the main-PR as
redundant instead of failing. Any OTHER 422 (or other error) still
fails loud — the grep matches the specific duplicate-base text, not
a blanket "any 422 means duplicate".
Behaviour matrix:
PATCH succeeds → retargeted, explainer
comment posted
PATCH 422 "already exists for staging" → close main-PR with
explainer (NEW)
PATCH any other failure → workflow fails (preserves
loud-fail for real bugs)
Tests: GitHub Actions don't have an inline unit-test framework here.
The workflow YAML parses (validated locally) and the bash logic is
straightforward. Real verification will be the next duplicate-PR
scenario in production.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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