Failure-Band Authorization for Filtered Retrieval

Filtered vector search in private and on-prem retrieval-augmented generation (RAG) systems must control tail risk under fixed latency, RAM, and SSD budgets, not merely pick a fast backend. Our measurements isolate a recurring ambiguity regime concentrated in the 5-15% failure band, where small shifts in vector-metadata alignment flip which familiar P1-P4 action is safe and create 3-6 P95 spread. We study that regime as authorization over fixed actions rather than as generic planning. The paper contributes a measured failure-band stress test, an auditable authorize/veto rule over fixed P1-P4 actions, and PACER as one concrete controller that combines recall-floor adjustment, calibrated tail intervals, and regime-local fallback. The measured failure band exposes a different runtime decision boundary: when the provisional winner is separated enough to trust and when uncertainty should refuse it. On Medium+EntDoc, evaluated on a dual-AMD EPYC 7543 server with 128GB RAM at matched recall in the 5-15% failure band, PACER reduces P95 by 46%\ and P99 by 45%\ relative to the strongest baseline while maintaining 0.94-0.98\ Recall@20. The gain concentrates where plan rankings are unstable and weakens when the authorization logic is stripped back, while feasibility, transfer, drift, and hierarchy-aligned slices remain supporting diagnostics. For recurring fixed-budget filtered retrieval, these results show that an auditable authorize/veto layer over fixed actions can materially stabilize tail latency in the measured failure band.