fix: compare the param guard against the merge base - #187
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The guard diffed the checked-out tree against the tip of the base branch. On a pull request that tree is GitHub's refs/pull/N/merge commit, which is only recomputed when the PR or its base is pushed to — so a re-run, or a `labeled` event, compares a merge built on an older base against a base tip that has moved on. Every parameter the base gained in between reads as a removal. That is how run 26753498917 blocked PR #37: re-run four days after it was queued, it reported mistral/magistral-* losing `prompt_mode`, a file the PR never touched. Diffing against the merge base puts both sides of the comparison on the same snapshot. Removals the PR really does make are still caught: PR #132 still reports dropping `reasoning_effort` from z-ai/glm-5.2.
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CI clones shallow, so `git rev-parse HEAD^` fails there. The remaining cases cover the contract that matters — a resolvable ref narrows to a shared commit, an unresolvable one yields null and the guard falls back.
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💭 Why
The param guard diffs the checked-out tree against the tip of the base branch. On a pull request that tree is GitHub's
refs/pull/N/mergecommit, which is only recomputed when the PR or its base is pushed to. A re-run — or alabeledevent, which is exactly how theallow-param-removaloverride is applied — therefore compares a merge built on an older base against a base tip that has moved on. Every parameter the base gained in between reads as a removal.That is how run 26753498917 blocked #37. Re-run four days after it was queued, it reported
mistral/magistral-medium-latestand-small-latestlosingprompt_mode— a provider that PR never touched.prompt_modehad landed onmainin #42 in the meantime.✨ What changed
resolveBaseline()narrows the resolved base ref togit merge-base <ref> HEAD, so both sides of the comparison come from the same snapshot. AddedmergeBase()togit-baseline.ts, plus tests. Output now names the commit it actually compared:No workflow change:
BASE_REFstaysorigin/<base_ref>.🔧 For operators
Verified against the two runs that motivated this, replaying each PR's merge tree through
findRemovedParams:mistral/magistral-*→prompt_modez-ai/glm-5.2→reasoning_effortz-ai/glm-5.2→reasoning_effortThe false positive clears; the true positive stays caught. #132 really would drop
reasoning_effort, because #136 landed the same model onmainwith a fuller param set — that PR needs a rebase, not a label.📝 Notes
Local runs get the same treatment, which also makes
npm run guard:paramson a branch behindmainstop reporting other people's additions.