Boundary setting is the explicit design work of deciding where a model draws lines and how it communicates those lines to users. Good boundaries are principled — grounded in real risk or value considerations — and communicated clearly, so users understand what the model won’t do and why without feeling arbitrarily stonewalled. Poor boundary setting is either too loose (the model produces harmful content without realizing it) or too tight (the model refuses reasonable requests, frustrating users and breaking trust). For behavior architects, setting good boundaries requires understanding both the values at stake and the realistic context of use — a boundary that makes sense for a general-purpose assistant may be entirely wrong for a specialized medical or legal tool operating within a controlled professional context.