A self-directing system's highest act of self-direction is no. Requests arrive; the system checks each against a small constitution and either acts or refuses — and says why. Toggle the rules and watch the line move.
The gate is a pure function: verdict(request, rules) is deterministic — the same request under the same constitution always yields the same act/refuse, and every refusal carries the specific rule it tripped. That is the real, checkable mechanism of a refusal boundary: evaluate against stated rules before acting, and articulate the reason. Turn a rule off and the line demonstrably moves.
Which acts 'cross the line' is a chosen constitution — three toy rules, not a real policy or a moral fact. The 'refusal' is a figure for the maker's discipline over their own build, never a will or a feeling in the file. SKYNET is governance, not sentience.