Arena says prompt caching cuts agent task costs by 260% across 26K sessions

Arena (@arena) analyzed 26,225 agent sessions from September 12 to October 23, 2026, across 30 agents and 13 labs, and reports that caching is now the main driver of agent costs.

Arena says 86% to 96% of agent tokens are re-reads of context already seen, and that without caching a task sent to one of the top-10 agents would cost 3.6 times as much, or 260% more.

Arena gives one task as an example: a typical Claude Fable 5.1 task costs $7.44 with caching and would cost $27 without it. It also reports that a single cache invalidation raised a typical task's bill by 5%. Arena says frontier labs now charge 5% of the input price for cache reads, down from full price in early 2024.