RouteFM is a pretrained in-context model router. Given an anonymous candidate pool, behavioral observations of candidate quality and relative cost, and a new target query, it predicts the target-specific quality and relative cost of every candidate. A frozen RouteFM adapts to new routing environments through context alone, without target-domain parameter updates. On MMR-Bench, which is excluded from pretraining, RouteFM outperforms the strongest non-RouteFM baseline by 2.23 quality points with only eight observations per candidate. 90-second introductions in English and Chinese, with matching narration, on-screen text, and captions. Concept footage and narration are AI-generated; method…