New data layer in OpenRouter and our API: Cost per Session
Cost per session is a way of assessing the real-world cost of using models, across tasks and harnesses.
Here's how Kimi K3 actually compares to Sol, Fable, and Opus variants on Hermes:
It's more useful than price per token, which doesn't incorporate how token-efficient a model is.
Cost per session encompasses all tasks across the market, as seen by each major harness, by looking at medians across 30 days.
Here's what the Pareto frontier looks like with cost per session:
You can make your own charts like this by simply querying our Dataset API.
The data is licensed under CC by 4.0.
View the rankings here: https://t.co/z5HcJalLKD
Or use the Dataset API: https://t.co/7oCDWKQPz4
agent harness — The scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.
token — The chunk of text a model reads and writes in — roughly three-quarters of a word — and the unit AI usage is billed in.
Why it matters
A cheap verbose model can cost more per finished task than a terse expensive one. Cost per session measures that directly, and the Dataset API lets you rebuild the frontier for your own agent harnessThe scaffolding around a model that turns it into a working agent — the loop, the tools it can call, and the rules for when to stop.Full definition →.
