Claude Fable 5.1 and Mythos 5.1 Released
CONFIRMEDDate confidence: CONFIRMEDStated as firmHow firmly the source words the date: confirmed, scheduled, estimated, delayed or unverified.Show all CONFIRMED pins90% CONFIDENCEOverall confidence: 90%How well the pin's source and references back up its dates.Weighted average of how firmly 5 references, the source included, support the pin's start and end times; a reference counts half as much for every 180 days older than the newestShow all pins at 75% confidence or betterThe 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.