Model routing is deciding, per request, which model handles it, instead of sending everything to one expensive frontier model. The goal is to match each request to the cheapest model that can do it well. That cuts cost and latency without hurting quality where it counts.
A cheap-first cascade is the simplest routing pattern. You try a small, fast, cheap model first. If its answer is good enough, you return it.
The savings depend on current provider prices and traffic mix. Measure how often the small model succeeds, then include the cost and delay of requests that must be retried on a larger model.
Two things to watch. First, the escalation signal is the hard part; a bad one either escalates everything (no savings) or nothing (quality drops).
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