A boundary is a decision about your own behaviour. An ultimatum is a demand about someone else’s. They can sound almost identical out loud, which is why so many conversations that were meant to create safety end up creating a standoff instead.
Here is the test I give clients. Say the sentence out loud and ask: can I keep this promise entirely on my own? “I am not going to keep discussing this after ten at night” passes – it needs nobody’s cooperation. “You need to stop bringing this up late at night” fails, because the outcome now depends on someone else changing.
The second sentence is not wrong to want. It is simply not a boundary, and calling it one sets you up to feel betrayed when it is not honoured.
What this changes in practice
Once a boundary is something you enforce rather than something you request, the conversation gets quieter. You are no longer negotiating. You are informing someone of what you will do, and then doing it, consistently, until it stops being a surprise.
That consistency is the hard part, and it is where most of the work in a session actually goes.