A boundary said calmly lands harder than one that’s screamed.

Volume gets treated as the point, which lets everybody avoid the actual content. Shout it and the conversation becomes about how you said it, and the thing you needed gets lost behind an argument about tone. Said evenly, the same sentence has nowhere to be filed except as a fact about what you will not do.

Practice the flat version before you need it: I am not doing that, I will not be talking about this. No rise at the end, no justification stacked behind it. Say it once, then let the silence sit rather than filling it with reasons.