import difflib


def generate_diff(old: str, new: str) -> str:
    """Return a unified diff between two prompt strings, or empty string if identical."""
    old_lines = old.splitlines(keepends=True)
    new_lines = new.splitlines(keepends=True)
    diff = difflib.unified_diff(old_lines, new_lines, fromfile="current", tofile="proposed")
    return "".join(diff)