Sentinel: Resident Autonomous Guardians for Critical Infrastructure — Authority, Partition and the Case Where the Site Is Gone

Critical infrastructure sites in Europe are acquiring permanent robotic residents. Quadruped and wheeled platforms already patrol substations, switchyards and process plants on fixed schedules; aerial systems conduct perimeter and post-event surveys; and the same class of hardware is being fielded at scale under fire in Ukraine. The engineering question is no longer whether a plant can host a resident multi-agent system. It is what that system is permitted to do, who decides, and what happens to those permissions when the network is cut, when one of the agents is compromised, or when the asset the system was commissioned to protect no longer exists. This paper argues that the binding constraint is authority rather than autonomy. Published analysis of SROS2, the security tooling for ROS 2, documents inadequate permission revocation that permits infected nodes to retain access after certificate revocation. Swarm research has shown that a single Byzantine agent holding a constant value can capture the consensus of an entire non-Byzantine population. Meanwhile the cryptography under all of it is scheduled for replacement on published national timelines. The paper proposes Sentinel, a reference architecture in which authority is bound to a mission mandate rather than to a site controller; every grant is short-lived, locally verifiable without contacting an issuer, and expressed against a shared site twin that both humans and agents read. It treats the loss of the protected object as a designed state rather than an undefined one, and specifies a degradation ladder that ends in escorted protection of credentialed persons. The construction is placed inside Swedish and European law as it stands in 2026, including the Swedish Cybersecurity Act, the Protective Security Act, the European Commission Action Plan on Drone and Counter-Drone Security of 11 February 2026, and the requirement for meaningful human control attaching to European Defence Fund research. Contribution: a specification that can be rehearsed in a national cyber range before it is relied upon, with a nine-month programme of work that produces a verified demonstrator rather than a study. Released as a draft for open review; corrections and counter-evidence are welcome and will be credited in the next version.