Model routing is simple—until you measure real cost, not sticker price

Model Routing Is Simple. Until It Isn’t.

IBM Research found that routing by model sticker price backfired in agent workloads. Across 417 AppWorld tasks, Claude Sonnet 4.6 cost $79 total vs. GPT-4.1's $155—nearly double—because Sonnet's lower cache-read pricing exploited high context reuse across steps. The post argues real cost, latency, and complexity all depend on workload-infrastructure interaction, making routing a systems optimization problem, not a classification one.

Why it matters: IBM ran 417 AppWorld tasks and found that routing by list price alone fails—Sonnet 4.6 cost $79 total while GPT-4.1 cost $155, nearly double. The core insight: when agents reuse the same context repeatedly, cache-read pricing dominates the total bill. Concrete numbers, counter...