Model Routing

Model routing is the practice of directing different steps of a workflow to different models. Providers offer tiers spanning roughly an order of magnitude in price and latency, so a system that sends every step to the largest model is usually overpaying, and one that sends every step to the smallest is usually unreliable.

Routing is an architectural decision because capability differences are not uniform. Cheaper models are often perfectly good at bounded, well-specified work and specifically bad at things that need judgement across a wide space. Two failure modes recur:

Cheap models under-produce what they are merely permitted to produce. Given an output shape where a component is optional, a smaller model will systematically omit it, and no amount of instruction fixes this. Making the component required in the schema fixes it immediately, because the grammar enforces requirements and nothing else. See Constrained Decoding.

Cheap models have tighter structural ceilings. A schema that compiles for a large model may be rejected or time out for a small one, so the tightest-routed model is the one to probe against.

Routing also interacts with testing: an environment configured for a cheap tier will produce different results from production, and a verification that never reached the interesting comparison because it bailed on a model-identity check has proved nothing.