---
id: "2026-09-01-claude-fable-5-1-mythos-5-1"
url: "https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/"
as_of: "2026-11-13T13:49:00+01:00"
date: "2026-09-01"
date_precision: day
category: model-release
importance: 5
confidence: high
status: [Confirmed]
sources: 9
editor: human editor
human_review: null
version: "2026-10-19"
---

As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/

# 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.

## Your AI and this story

- GPT-6 Astra (training cutoff April 2026): 124 days after its cutoff
- Claude Opus 5.5 (training cutoff June 2026): 63 days after its cutoff
- Gemini 4 Hydrogen (training cutoff March 2026): 154 days after its cutoff
- Grok 4.7 (training cutoff May 2026): 93 days after its cutoff

## Sources

1. [Introducing Claude Fable 5.1 and Claude Mythos 5.1 (Anthropic)](https://www.anthropic.com/claude-fable-and-mythos-5-1) (anthropic.com, official)
2. [Claude Fable 5.1 / Mythos 5.1 System Card](https://www.anthropic.com/claude-fable-5-1-mythos-5-1-system-card) (anthropic.com, paper)
3. [Developing Enterprise Frontier Safeguards with our customers](https://www.anthropic.com/news/enterprise-frontier-safeguards) (anthropic.com, official)
4. [Improving Fable 5's biology safeguards (Aug 7, 2026)](https://www.anthropic.com/news/improving-fable-5-s-biology-safeguards) (anthropic.com, official)
5. [Claude on X: Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://x.com/claudeai/status/2094848572143407483) (x.com, official)
6. [MacRumors: Fable 5.1 with lower costs and fewer false positives](https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/) (macrumors.com, press)
7. [MarkTechPost: Fable 5.1 and Mythos 5.1 — 52.6% on Terminal-Bench-Science](https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/) (marktechpost.com, press)
8. [Yahoo Tech: Anthropic launches Claude Fable 5.1 — can it stop AI copycats?](https://tech.yahoo.com/ai/claude/articles/anthropic-launches-claude-fable-5-182403780.html) (tech.yahoo.com, press)
9. [Introducing Claude Fable 5.1 (official video)](https://www.youtube.com/watch?v=ROF2Nv_KjOM) (youtube.com, video)

## Changes

- 2026-10-19: Added video(s) gothamchess-gpt-6-astra-vs-claude-fable-5-1-chess-match
- 2026-09-29: Added post link(s) (posts-as-events pass)
- 2026-09-29 (filed): Created

## Related

- 2026-09-22: [Anthropic releases Claude Opus 5.5](https://postcutoff.com/e/2026-09-22-claude-opus-5-5/index.md)
- 2026-06-09: [Anthropic releases Claude Fable 5 and Claude Mythos 5](https://postcutoff.com/e/2026-06-09-claude-fable-5-mythos-5/index.md)
- Archived: 2 posts and 56 videos, listed in https://postcutoff.com/e/2026-09-01-claude-fable-5-1-mythos-5-1/index.json