git-github-advanced: note the PR-head-checkout CI gotcha - #146
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On pull_request, actions/checkout defaults to the synthetic refs/pull/N/merge commit — an ephemeral merge of the head into the base — so HEAD is a merge commit, not the branch tip. Any CI job that inspects commit shape/history misfires on it; e.g. a "no merge commits" check false-positives on a linear branch. Land the gotcha next to the sibling actions/checkout-default trap in the git-github-advanced skill (GitHub Actions gotchas are its scope), framed portably rather than tied to this repo's own checks. Captured from the #141/#142 session, where this cost a CI round-trip. Closes #145 Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_019mW2ZDXbTgZxwsyrPcUrwy
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What
A lessons-learned capture from the #141/#142 session (squash-merge-history work-scoping).
That change made
squash-merge-historyinspect HEAD's commit shape (mergeBase..HEADfor merge commits). Onpull_requestevents,actions/checkoutdefaults to the syntheticrefs/pull/N/mergecommit as HEAD — an ephemeral merge of the branch into the base — so the check false-positived in CI even though the branch was linear. It cost a full CI round-trip on PR #142 before diagnosis; the fix there wasref: ${{ github.event.pull_request.head.sha || github.sha }}inchecks-ci.yml, with a co-located comment.Routing (corrected)
The portable lesson is a GitHub Actions gotcha, so per
growth/item-routing.mdit belongs in the practice group's git/GitHub owner — thegit-github-advancedskill, whose scope explicitly includes "GitHub Actions gotchas." It lands right next to the siblingactions/checkout-default trap ("A CI job that reads submodule files must fetch submodules in its checkout"), framed portably (any commit-shape/history job, any repo) rather than tied to this repo's own checks.(An earlier revision of this PR wrongly put it in
checks/README.md, which is operational doc, not a lesson owner. Reverted.)Why prose, not a check
Mechanism triage lands on rung-5: a check would need to detect that a CI job "inspects commit shape," which has no reliable static signature — fragile and disproportionate. The actionable knowledge is platform behavior (
actions/checkout's default), which prose carries.Closes #145
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