A boundary needs behavior behind it
A useful boundary does not try to win an argument. It names the line, the action you will take if the line is crossed, and the follow-up that keeps the relationship from becoming a courtroom.
This tool keeps the script short enough to remember when the room is charged. If safety is uncertain, it points you toward safety-first planning instead of a dramatic announcement.
What usually goes wrong when you say it
Most boundaries fail in the ten seconds after they are stated, not in the wording. The other person pushes back — reasonably, sometimes warmly — and the instinct is to soften, explain, or add a qualifier that quietly withdraws the line. A boundary that arrives with three reasons attached invites a negotiation of the reasons.
So say it once and stop. You do not owe an argument, and repeating yourself in the same calm sentence is more effective than finding a better one. Expect the first attempt to feel abrupt: a line that has been unstated for years will feel disproportionate the first time it is said out loud, to both of you. That feeling is not evidence you were wrong.
One thing this tool will not do is help you enforce a boundary with someone who has no interest in respecting it. If the script keeps needing to be repeated to the same person about the same thing, the useful question stops being how to phrase it.