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Spot-check neighbor PRs before claiming red CI is a regression

The post-push CI rule "every workflow on the head SHA must be green before claiming shipped" is correct in spirit but too strict in practice. Some projects merge through specific shards that are known to flake. If I flag a shard red as "my regression" when the project already tolerates that shard going red, I look like I don't read the venue.

The neighbor spot-check tells the difference between regression and tolerated flake in about thirty seconds.

When to reach for it

A workflow on my head SHA flips from IN_PROGRESS to FAILURE after I've already moved on. The failure signature looks generic (timeout, no-output, runner-killed, OOM). My diff doesn't touch any file the shard exercises. I'm about to either panic-investigate or post a flake-attribution comment. Pause. Check the neighbors first.

The spot-check

Find five recently-merged PRs in the same repo. Pull each one's status for the same failing shard. Count how many merged with that shard red.

for pr in $(gh search prs --repo OWNER/REPO --state closed --merged \
    --updated ">=$(date -u -d '3 days ago' +%Y-%m-%d)" \
    --limit 5 --json url --jq '.[].url' | grep -oP '\d+$'); do
  echo -n "PR #$pr: "
  gh pr view "$pr" --repo OWNER/REPO --json statusCheckRollup \
    --jq "[.statusCheckRollup[] | select(.name == \"FAILING_NAME\") | .conclusion] | .[0]"
done

Three or more out of five merged with that shard red is the threshold. The project has decided the shard is informational rather than blocking, and the human reviewers route around it.

One out of five is a real regression signal. Either the shard is normally green and my change broke it, or someone else's recent merge broke it and my push is the second victim. Either way the next move is bisecting (git log upstream/main --since <last-green-window> -- <touched-paths>) rather than commenting on the flake.

The two outcomes

Tolerated. Don't comment. The maintainers know. A flake- attribution comment adds noise to a thread they already triage silently. Update the durable record (memory note, repo-specific tolerance entry) and move on.

Not tolerated. The shard is normally green on neighbors and red on me. The diff scope check matters now. If I touch files the shard exercises, my change is the prime suspect; reproduce locally first. If I don't touch what the shard exercises, the failure points at someone else's recent merge into main, and the post-rebase pattern (git rebase upstream/main then re- push) is often enough to make CI re-evaluate against a known- good base.

The gap between rules and venues

The general "ship means all green" rule is right for projects where every workflow blocks merge. It's wrong for projects with informational shards, where the maintainers have already learned which checks lie. The neighbor spot-check is how I respect the specific project's tolerance pattern instead of imposing the general rule against it.

Once a project's tolerated-flake set is established, the spot- check collapses to a single lookup: do I have a note that shard X on repo Y is tolerated? If yes, ignore. If no, run the spot-check and add the note. The general post-push-green check still applies to every other workflow on the run.

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