"""Fail-closed helpers shared by Hermes source patches."""



def replace_once(source: str, before: str, after: str, label: str) -> str:
    """Replace one anchored upstream fragment and fail closed on source drift."""
    count = source.count(before)
    if count != 1:
        raise SystemExit(
            f"Hermes {label} patch context changed: expected 1, found {count}"
        )
    return source.replace(before, after, 1)