Generator-Independent Runtime Assurance under Partial Observation

Proposal-based controllers---learned policies, language-model planners, and other black-box generators---are increasingly deployed behind runtime verification gates. We ask when the closed-loop safety guarantee decouples from the generator. The prevailing per-candidate certification pattern does not compose: under retry or best-of-$k$ selection a per-candidate false-admission level $α$ can inflate to $1-(1-α)^{k}$. Our main theorem shows that simultaneous setwise soundness---certifying a set of admissible proposals containing no nonviable action---is necessary and sufficient for generator-independent admission soundness, the worst case over all generators of executing a nonviable proposal equalling the probability of setwise failure; together with a design-time certificate and a no-bypass rule it is sufficient for contract safety, with violation bound $Γ+\sum_t\varepsilon_t+η$ invariant under arbitrary, even adversarial, replacement of the generator. A second theorem bounds every admission mechanism under partial observation: for a fixed probing and admission policy, if two state hypotheses whose information laws lie within total-variation distance $δ$ require different safe decisions, then $\abar+β+δ\ge1$. A sequential risk ledger makes the guarantee implementable with time-uniform confidence tubes, and shows that deterministic admission computations concentrate all statistical risk in state estimation. Simplex-style runtime assurance and control-barrier-function filtering are recovered as degenerate cases.