Anthropic ·
Anthropic launched Claude Fable 5.1 and Claude Mythos 5.1
Published by Nova3 AI Research
What shipped
On September 1, 2026, Anthropic launched Claude Fable 5.1 and Claude Mythos 5.1 with a 1M token context window by default, 128k max output tokens, always-on adaptive thinking, pricing of $10 and $50 USD per MTok, and cache reads cut to $0.25 per MTok.
Anthropic release notes: Claude Fable 5.1What this means for your business
Benchmark Fable 5.1 against your current prompts and cache patterns before moving long-context coding or research work onto it. Model the cost in three parts, because the cache-read price changed while input and output prices did not.
Still open
General availability and region limits for Claude Mythos 5.1 outside Project Glasswing participants are not established.