- Start
- Sep 1, 202692% CONFIDENCEfrom anthropic.com
Claude Fable 5.1 and Mythos 5.1 Released
- Cache reads now cost "75% less, or $0.25 per million tokens"; the rest of the pricing is unchanged at $10 per million input tokens and $50 per million output tokens, which works out at roughly 25% less than Fable 5 for typical workloads and up to about 45% for highly agentic ones[1][5]
- New Enterprise Frontier Safeguards give customers "complete privacy (the same as a zero data retention policy)" by holding data in cloud infrastructure the customer controls rather than Anthropic; it arrives in phases "beginning later this fall", with zero data retention available to eligible customers until then[1]
- The newest cybersecurity safeguards "block 60% fewer false positives than before", in part because Fable 5.1 "can now be used to discover software vulnerabilities - though not to develop exploits for them"[1][5]
- The investment firm Millennium found that in its testing Fable 5.1 "found the cause of a rare crash in its internal systems that none of its engineers (or any other model) had been able to explain after several years of trying"[1]
- Available the same day on Amazon Web Services, Google Cloud and Microsoft Azure, and as
claude-fable-5-1on the Claude API[1]
Notable features
- Anthropic introduces Claude Fable 5.1 and Claude Mythos 5.1, "the world's most advanced models for coding and knowledge work", whose research capabilities "offer an early glimpse of how AI models will contribute to scientific progress"[1][5]
- The two are "the same model, but with different levels of safeguards": Fable 5.1 is generally available, while Mythos 5.1 goes only to trusted access programs, with safeguards "specifically designed to support work in cybersecurity and the life sciences"[1][5]
- Specifications: 1M-token context window, 128K max output tokens, adaptive thinking that is always on with high default effort, text and image input with text output, and a June 2026 reliable knowledge cutoff; Claude Mythos 5.1 shares these specifications and pricing[2]
- On Terminal-Bench-Science 0.1 Fable 5.1 scores 52.6% against Fable 5's 24.7%, Opus 5's 29.0% and GPT-5.6 Sol's 22.4%; Mythos 5.1 takes Terminal-Bench 4.0 to 60.9% where Fable 5.1 scores 55.8%, a gap Anthropic attributes to its "earlier, less precise cyber safeguards" rather than to the model[1]
- MacRumors adds that Fable 5.1 carries invisible watermarking of text generated by Claude, with a detection API in private preview for eligible organisations as required by EU law[5]
References 590% CONFIDENCE
The first entry is always the pin's source. Overall confidence is a weighted average of how firmly each reference supports the start and end times used above; a reference counts half as much for every 180 days older than the newest.
- [1]95%Introducing Claude Fable 5.1 and Claude Mythos 5.1anthropic.com· Published Sep 1, 2026· Starts Sep 1, 2026· 19% of score
The 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[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”.
- [2]90%Claude Fable 5.1 - Claude Platform Docsplatform.claude.com· Added Sep 22, 2026· 21% of score
Anthropic's[1][3] Claude Fable 5.1 model page lists a 1M-token context window, 128K max output, adaptive thinking (always on), a Jun 2026 knowledge cutoff, and says Mythos 5.1 "shares Claude Fable 5.1's specifications and pricing".
- [3]92%News \ Anthropicanthropic.com· Added Sep 22, 2026· Starts Sep 1, 2026 ✓· 21% of score
The only Anthropic[1] page that dates this announcement: in the raw markup each news card is a self-contained anchor, and this article's holds <time>Sep 1, 2026</time> beside its headline. Rated above the source because the announcement page itself gives no date at all - this is where the pin's start date comes from.
- [4]85%Claude Mythosen.wikipedia.org· Added Sep 22, 2026· 21% of score
Wikipedia's infobox gives the Claude[2] Mythos stable release as “Claude Mythos 5.1 / September 1, 2026”, corroborating the date the announcement page states only as “September 2026”.
- [5]88%Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positivesmacrumors.com· Published Sep 1, 2026· Starts Sep 1, 2026· 19% of score
MacRumors' same-day report, article:published_time 2026-09-01T19:15:16Z, opening “Anthropic[1][3] today introduced Claude[2] Fable 5.1” - an independent witness to the launch day. It also adds the invisible text watermarking and its EU-law detection API, which Anthropic's own page does not cover.
Suggest a correction
Something missing or wrong? Say it in your own words: a link that backs this pin up, a different start or end date and why, or a fact it lacks or gets wrong. The AI checks it against this pin's sources, searches for better ones, and adds any page that backs you up. The pin's own sources still count most.
