Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1
Confirmed
Importance: historic (5 of 5)The takeaway
On September 1, 2026 Anthropic released Claude Fable 5.1 and Claude Mythos 5.1.
Status
- Claim
Confirmed
- Our reporting
- High confidence
- Importance
- Historic (5 of 5)
- Last verified
- 19 October 2026
Your AI and this story
- GPT-6 Astra124 days after its cutoff
- Claude Opus 5.563 days after its cutoff
- Gemini 4 Hydrogen154 days after its cutoff
- Grok 4.793 days after its cutoff
None of these four assistants can know about it. The closest, Claude Opus 5.5, stops 63 days before it.
Key facts
- Released September 1, 2026; ids claude-fable-5-1 (GA) and claude-mythos-5-1 (trusted access via Cyber Verification Program / Life Sciences Verification Program)
- Pricing $10 input / $50 output per 1M tokens; cache reads $0.25 (75% lower); ~25% cheaper than Fable 5 on typical workloads, up to ~45% on agentic work
- Terminal-Bench-Science 0.1: 52.6% vs Fable 5’s 24.7%; Terminal-Bench 4.0: 55.8% vs 42.0%
- Humanity’s Last Exam: 60.9% no tools / 65.0% with tools; OSWorld 2.0: 77.9% partial / 41.7% strict
- Context 1M tokens, 128K output; thinking always on; forced tool use no longer supported
- Biology classifier false positives down ~85% for elementary/medical queries; cyber false positives down ~60%
- Launched alongside Enterprise Frontier Safeguards (ZDR plus misuse detection), built with Salesforce, Visa, Uber, KPMG
What happened
Fable 5.1 upgrades Fable 5, Anthropic’s “Mythos-class” model released June 9. Anthropic highlights long-running, multi-step work (long proofs, contracts with hundreds of cross-references) and scientific research. Examples: protein binder designs with a reported 50% hit rate and up to 10x higher affinity than competition, validated by two independent labs; Venus elevation mapping at 2–3 km resolution; GPU-kernel optimization giving up to 2.5x speedups for biological models.
Safeguards: Fable 5.1 keeps classifier-based blocking with fallback to older models, but with far fewer false positives. It now allows vulnerability discovery for defensive work and adds anti-distillation measures that stop manual context editing in multi-turn API conversations. Mythos 5.1 is the less-restricted variant for vetted users.
Why it matters
Fable/Mythos 5.1 was Anthropic’s capability frontier until Opus 5.5 matched it three weeks later at less than half the price. Its “same model, different safeguards” split between Fable and Mythos has become Anthropic’s template for releasing dual-use capability.
Sources
9 sources from 6 sites. Numbers match the chips in the text.
9 sources: 5 primary, 3 press, 1 reaction
Primary
- Introducing Claude Fable 5.1 and Claude Mythos 5.1 (Anthropic)anthropic.com, official
- Claude Fable 5.1 / Mythos 5.1 System Cardanthropic.com, paper
- Developing Enterprise Frontier Safeguards with our customersanthropic.com, official
- Improving Fable 5’s biology safeguards (Aug 7, 2026)anthropic.com, official
- Claude on X: Introducing Claude Fable 5.1 and Claude Mythos 5.1x.com, official
Press
- MacRumors: Fable 5.1 with lower costs and fewer false positivesmacrumors.com, press
- MarkTechPost: Fable 5.1 and Mythos 5.1 — 52.6% on Terminal-Bench-Sciencemarktechpost.com, press
- Yahoo Tech: Anthropic launches Claude Fable 5.1 — can it stop AI copycats?tech.yahoo.com, press
Reactions
- Introducing Claude Fable 5.1 (official video)youtube.com, video
Changes
- Added video(s) gothamchess-gpt-6-astra-vs-claude-fable-5-1-chess-match
- Added post link(s) (posts-as-events pass)
- Filed
