Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1
On September 1, 2026 Anthropic released Claude Fable 5.1 and Claude Mythos 5.1. They are the same underlying model with different safeguards: Fable 5.1 is generally available, while Mythos 5.1 is for verified cyber and life-science users. It roughly doubles Fable 5's Terminal-Bench-Science score, cuts cache-read prices by 75% and typical costs by ~25%, and adds anti-distillation blocks. It is Anthropic's most intelligent generally available model.
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.
Changelog
- 2026-09-29: added post link(s) (posts-as-events pass)
- 2026-09-29: created
Videos (28)
Introducing Claude Fable 5.1
Anthropic · 2026-09-01 · officialAlex Albert from Anthropic’s Research Product Management presents the release announcement for Claude Fable 5.1. The video outlines the model’s focus on complex, multi-step problem solving, including software engineering, analysis, and scientific research workflows. [00:00] Alex Albert introduces the model in a…
Debugging across the whole stack with Claude Fable 5.1
Claude · 2026-09-01 · demoThis promotional demonstration video from Anthropic showcases Claude Code operating with the Claude Fable 5.1 model (1M context) to troubleshoot an automotive software bug. Without spoken voiceover, the video illustrates an engineer handing off a complex, multi-system vehicle climate failure ticket to Claude Code…
Claude Fable 5.1 runs the forecast overnight
Claude · 2026-09-01 · demoThis promotional demonstration video by Anthropic showcases an automated enterprise forecasting workflow powered by Claude Fable 5.1. It illustrates how the model handles a "night shift" task, analyzing tens of thousands of customer accounts, running cohort simulations, and updating morning executive reports that can…
Claude Fable 5.1 builds the ops review in Slack
Claude · 2026-09-01 · demoThis is a promotional product demo from Anthropic highlighting agentic project management capabilities for Claude Fable 5.1. It shows Claude acting as an autonomous workplace agent inside Slack, collecting disparate files, synthesizing an executive review presentation, cross-referencing team channels, catching data…
Claude designs proteins that bind in the lab
Claude · 2026-09-01 · demoThis video is a promotional showcase highlighting de novo protein binder designs and reported experimental hit rates across twelve biological and therapeutic targets. Presented with 3D molecular visualizations and background synth music, it concludes with Anthropic's Claude branding. - [00:00] 15-PGDH: 3D structural…
Building Enterprise Frontier Safeguards with our customers
Claude · 2026-09-01 · officialThis video is an official promotional testimonial from Anthropic highlighting their "Enterprise Frontier Safeguards." It features executives from Uber, Visa, KPMG, and Salesforce discussing their collaboration with Anthropic to deploy frontier AI models securely within strict enterprise data privacy and security…
More videos (22)
- alignment — Claude Fable 5.1 uncanny-fyi · 2026-09-28
- I Made Opus 5.5, Fable 5.1 & GPT-6 Build the Same App (RAW RESULTS) Pat Simmons · 2026-09-25
- Opus 5.5 vs Fable 5.1 vs GPT-6 Astra Code Minecraft Plugin (Advanced Test) Matej (kangarko) · 2026-09-24
- I Tested Opus 5.5 vs Fable 5.1 on 7 Real Use Cases (Not Even Close) Ben AI · 2026-09-23
- like-an-asteroid — Claude Fable 5.1 uncanny-fyi · 2026-09-15
- Everyone's Testing Claude Fable 5.1 On Code. It Made Me A 37-Second Film. AI News & Strategy Daily | Nate B Jones · 2026-09-08
- Claude Fable 5.1 Recreates 5 Popular Games AI PILLED · 2026-09-08
- Claude Fable 5.1 + MCP = New king of Algo-trading! Algo-trading with Saleh · 2026-09-08
- Claude Fable 5.1 | First impressions Arena AI · 2026-09-08
- Claude Fable 5.1 Is INSANE – Hands-On With the BEST Model Yet! Bijan Bowen · 2026-09-08
- Spending $5,000 Vibe Coding With Claude Fable 5.1 BridgeMind · 2026-09-08
- Vibe Coding With Claude Fable 5.1 BridgeMind · 2026-09-08
- I Tested Fable 5.1 vs Fable 5 vs Opus 5 (Cost/Speed/Design) Brock Mesarich | AI for Non Techies · 2026-09-08
- I Tried To Make GTA 6 Using Fable 5.1 Claude Knows My API Key · 2026-09-08
- Claude Fable 5.1 is Ridiculous. Cole · 2026-09-08
- We Tested Anthropic's Fable 5.1 for a Week Every · 2026-09-08
- Claude Fable 5.1 - Huge Upgrade in App and Web Design Jason Lee · 2026-09-08
- Fable 5.1 Is Absurd. LanceyPoo · 2026-09-08
- 10 INSANE Things Created With Claude FABLE 5.1 (Fable 5.1 Use Cases) TheAIGRID · 2026-09-08
- Claude Just Built A Full 3D House In Blender From One Prompt (Fable 5.1) Vaibhav Sisinty · 2026-09-08
- Claude Fable 5.1 Is WILD (we're cooked) Viral Echoes · 2026-09-08
- Claude Fable 5.1 Should Not Be This Good (way better than Fable 5) Zo · 2026-09-08
All videos with Gemini's descriptions: videos · llms-full.txt
Related posts (2)
- The Overhang Ethan Mollick @emollick · substack · 2026-09-18
The most widely read mainstream take after Astra and the pacing week: models like GPT-6 Astra and Fable 5.1 already outrun what almost anyone does with them. - Introducing Claude Fable 5.1 and Claude Mythos 5.1 Claude @claudeai · x · 2026-09-01
Launch post for Anthropic's September 2026 frontier models, billed as the world's most advanced for coding and knowledge work.
Related events
- Anthropic releases Claude Fable 5 and Claude Mythos 5 — first generally available Mythos-class model ★★★★★
- Anthropic releases Claude Opus 5.5 — Fable-5.1-level performance at $4/$20, first model of the Claude 5.5 family ★★★★★
- Claude-written Lean proof claims the dying percolation conjecture θ(p_c)=0 in every dimension ★★★★★
- Claude produces the first complete machine-checked proof of Fermat's Last Theorem in Lean, in 11 days ★★★★★
- White House asks OpenAI and Anthropic to hold new models back from the UK AI Security Institute until the US reviews them ★★★★
Sources (9)
- officialIntroducing Claude Fable 5.1 and Claude Mythos 5.1 (Anthropic)
- paperClaude Fable 5.1 / Mythos 5.1 System Card
- officialDeveloping Enterprise Frontier Safeguards with our customers
- officialImproving Fable 5's biology safeguards (Aug 7, 2026)
- pressMacRumors: Fable 5.1 with lower costs and fewer false positives
- pressMarkTechPost: Fable 5.1 and Mythos 5.1 — 52.6% on Terminal-Bench-Science
- pressYahoo Tech: Anthropic launches Claude Fable 5.1 — can it stop AI copycats?
- videoIntroducing Claude Fable 5.1 (official video)
- officialClaude on X: Introducing Claude Fable 5.1 and Claude Mythos 5.1
id: 2026-09-01-claude-fable-5-1-mythos-5-1 · updated 2026-09-29 · open in the interactive timeline