Legion as a Physically Segregated API Control Plane

Administrative authority over infrastructure can rewrite the conditions under which every other control operates, so the plane that carries it is the easiest place for an agentic system to fail quietly. This paper examines Legion, the author's proposed physically segregated control plane for agentic infrastructure administration (category 5b of the first paper in this series), through one synthetic maintenance task. The task is an idempotent configuration change and a separately approved restart on a disposable node, with a revocation that arrives between them. The paper makes a three-part claim. First, eligibility: an administrative plane counts as physically segregated only if independent physical evidence shows dedicated management compute, switches and links, with each in-scope endpoint bound to a designated port. Second, conformance: given eligibility, a named artifact meets the claim only if pre-registered reachability, authority, outage and recovery tests pass. Third, value: against a strong logical baseline, on the same rights, accepted-outcome denominator and cost boundary, what the physical build adds is an open question. The paper supplies an interface inventory and bypass register, an approval tuple, four non-substitutable record classes with an independent effect record, a declared propagation bound with truthful terminal states, fault-domain-scoped recovery, an ATT&CK-bounded threat mapping, and six falsifiers. The last falsifier is a logical baseline that matches the build at equal or lower cost. Every test is specified and NOT-RUN. No build, measurement or risk reduction is reported. The paper closes by handing six obligations to the reference architecture.