Route by task, not by loyalty

The biggest cost lever in an agent stack right now is a router that knows when not to call the frontier model.

Anthropic shipped Sonnet 5 last week at $2 in / $10 out per million tokens intro pricing ($3/$15 from September). Vendor numbers put it just under Opus 4.8 on agentic coding and ahead on some knowledge work. Terminal-Bench went from 67.0 on Sonnet 4.6 to 80.4.

Mid-tier models now clear the bar for most production agent loops. That changes the architecture more than the bill. What I would do with it:

1. Route by task, not by loyalty. Tool-call planning, extraction, classification go mid-tier. Deep multi-file reasoning escalates. Two tiers beat a monolith on cost per task.

2. Meter cost per task, not per token. Token price tells you little once models think for variable lengths. Tag every run with its task class and its dollars. Alert on outliers.

3. Re-run your evals before flipping traffic. The gap to frontier is benchmark-thin, and your workload is not a benchmark. Golden set first, then migrate.

We run the same tiering on-prem with open weights. Same math, different price tags.

The trap: the effort dial is a budget, not a quality knob. Uncapped, the cheap model on a hard task can spend past Opus and hand you the same answer.

Cost per task is the only price that matters in an agent stack.