Claude Fable 5.1 and Mythos 5.1 Released
CONFIRMED90% CONFIDENCEThe announcement page does not date itself - it says only “September 2026” and “available today”, and the one ISO timestamp in its HTML is a page build time. The date comes from Anthropic's[1][3] own news index, whose card for this article carries <time>Sep 1, 2026</time>, and is confirmed independently by MacRumors[5]' article:published_time of 2026-09-01T19:15:16Z (“Anthropic today introduced Claude[2] Fable 5.1”) and by Wikipedia's[4] Claude Mythos infobox, “Claude Mythos 5.1 / September 1, 2026”.
Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1 - the same model behind different safeguards - and cuts cache reads by 75%, making typical workloads about 25% cheaper to run than Fable 5.
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