When to Search, Continue, or Fall Back: Risk-Controlled Routing for Rare-Regime Discovery

Scientific rare-regime discovery requires choosing a search method under limited information, uncertain performance, and costly evaluations. We formulate this choice as a sequential replacement decision: when should a learned selector replace an executable fallback, pay for another observation, or fall back? QC-PHAST Router combines a permutation-invariant history encoder, route-conditioned survival predictor, and source-bank-group calibration to assess fallback-relative p90 loss. Information access determines admissible solvers, observations are charged, and proposed hits require independent verification. Finite-stage protection assumes within-track exchangeability; latent-risk protection additionally requires joint inner-interval coverage. Neither statement guarantees lower uncapped cost or protection under family shift. A frozen external evaluation records 1,152 configurations and 147,456 paired trials across 6 information tracks. The recorded procedure passes 0 of 3 primary replacement tests and makes 0 specialist commitments. A post hoc diagnosis finds that calibration corrections exceed the largest expressible prediction gap in 3 tracks. A subsequent audit identifies inherited-history errors in continuation evidence and related deployment paths, limiting interpretation of the recorded cost comparisons. We retain these records without claiming corrected routing efficacy. The study identifies two prerequisites for useful protection: enough decision resolution to authorize a route, and faithful accounting of the observations on which that decision depends.