X_BREAKING_ON_MAIN
Means
a change to main carries a BREAKING — entry under [Unreleased] while the major window is closed
Typical cause
after v30, main ships minors and patches only; a break lands on next and reaches main when the window opens (Stability). Read from the pull request's base branch (GITHUB_BASE_REF) or the pushed branch, so a local run without either reads nothing
Fix
git rebase --onto next main, then retarget the pull request: gh pr edit --base next