Release2026-09-01
Anthropic released Claude Fable 5.1 and the restricted-access Mythos 5.1, cutting the price of re-reading stored context by 75% while leaving base input and output rates unchanged. The company also introduced Enterprise Frontier Safeguards, which lets customers keep misuse-monitoring data in their own cloud accounts. Benchmark gains are vendor-reported, and the data-custody option only begins rolling out in phases this autumn.
What changed
Fable 5 charged $1.00 per million cached input tokens, 10% of its base input price.
What it unlocks
Running agents that reuse the same large context for hours at a much lower repeat-read cost.
- cache reads $1.00 to $0.25 per 1M
- input $10 / output $50 per 1M
- ~25% lower effective cost, ~45% agentic
What you need to act on it
- API access via Anthropic, AWS, Google Cloud or Azure
- vetted-access program for Mythos 5.1
- Enterprise Frontier Safeguards rolls out in phases this fall
Sources