Add model routing when your workflow contains tasks with different complexity or quality requirements. First establish an eval set so the routing decision can be tested.

Separate routine work from judgment

Extraction, classification, and formatting may have different requirements from reasoning or exception handling. Define the task boundaries and expected output before adding a router.

Count the cost of the router too

Routing adds its own latency, logic, and failure modes. Compare the complete routed workflow against a simpler baseline. Keep escalation rules explicit and monitor retries and review effort.