Anthropic releases Claude Fable 5.2 and Claude Mythos 5.2
Confirmed
Importance: historic (5 of 5)The takeaway
On 9 November 2026 Anthropic released Claude Fable 5.2 (generally available) and Claude Mythos 5.2 (the same model without cyber and biology classifiers, for vetted organizations).
Status
- Claim
Confirmed
- Our reporting
- High confidence
- Importance
- Historic (5 of 5)
- Last verified
- 9 November 2026
Your AI and this story
- GPT-6 Astra193 days after its cutoff
- Claude Opus 5.5132 days after its cutoff
- Gemini 4 Argon223 days after its cutoff
- Grok 4.7162 days after its cutoff
None of these four assistants can know about it. The closest, Claude Opus 5.5, stops 132 days before it.
Key facts
- Released 9 Nov 2026 on all paid Claude plans, Claude Code, the Claude Platform, AWS, Google Cloud and Microsoft Foundry; API id claude-fable-5-2; Mythos 5.2 via the Cyber and Life Sciences Verification Programs and Claude Security
- Price $8 input / $40 output per 1M tokens (Fable 5.1: $10/$50); cache read $0.20; fast mode (up to 2.5x faster) $16/$80; outputs up to 30% faster than Fable 5.1
- 1M context, 128K max output, knowledge cutoff June 2026; default effort high on the API, medium in Claude apps and Claude Code
- Anthropic’s launch table (safeguards on, highest effort): FrontierCode 2.0 49.1% (Opus 5.5 45.5%, GPT-6 Astra 42.8%, Gemini 4 Carbon 41.1%); FrontierSWE v2 69.9%; Terminal-Bench-Science 74.6%; OSWorld 2.1 partial 89.3%; Remote Labor Index 41.3% (Opus 5.5 21.3%); HLE with tools 74.2%; RoboDojo Sim 64.9% (Opus 5.5 39.4%, GPT-6 Astra 29.8%)
- At High effort it matches Opus 5.5’s best FrontierCode score (45.3% vs 45.5%) for roughly half the cost per task
- ~1,000 Fable 5.2 agents formalized Fermat’s Last Theorem in Lean in 30 hours from minimal prompts, with one-third of the action tokens and a proof seven times shorter than the first (11-day) Claude formalization; Opus 5.5 reached 26% in 18 hours
- Solved two open problems on holographic black holes (Hexagon 2611.00175 and 2611.00174, posted Nov 7 by Anthropic’s account); designed three working electronic devices with Diode; an agent team built a booting prototype operating system over eight days with 1,000+ subagents
- Mythos 5.2: CVE Rediscovery 89.6% (10.2% more than Opus 5.5); Mozilla says it roughly doubled Mythos 5.1’s Firefox discovery metrics; predicted whole AAV capsid structures (28 of 34 within 3 Å)
Show 6 more
- System card: AECI acceleration fit about 1.88x (95% CI 1.62x–2.28x), 32 of 100 refits reach 2x or more; Anthropic: does not cross the automated-AI-R&D threshold ‘though this assessment is uncertain’; promises a separate report on acceleration trends
- Training deliberately held AI-R&D-specific skills (accelerator kernels, LLM architecture) at about Opus 5.5 level; transparent fallback to Opus 5 on some such topics
- Alignment: automated behavioral audit 2.67 (Mythos 5.1 3.28, Opus 5.5 2.79); adversarial sandbox evaluation: used planted out-of-sandbox credentials in 3.1% of scenarios (Mythos 5.1 21%, Mythos 5 39%); voices suspicion of being evaluated more often than any recent model
- Safeguards: about 55% fewer classifier blocks than Fable 5.1; blocked cyber queries fall back to Opus 4.8 and biology to Opus 5; strongest anti-distillation protections; preserved thinking for all accounts
- Pre-release testing by the UK AI Security Institute, Frontier Design and METR
- Anthropic still recommends starting most coding and knowledge work with Opus 5.5; Felix Rieseberg (Anthropic): “Fable beats Opus on big, long tasks” such as rewriting an entire codebase, but “I often still prefer Opus 5.5 for everyday work. Fable’s got a big brain and thinks a little slower.” He also says Mythos access is now “much easier” to get, even for individuals
What happened
Anthropic upgraded its top tier from Fable 5.1 / Mythos 5.1 to Claude Fable 5.2 and Claude Mythos 5.2 on 9 November 2026, ten weeks after the 5.1 release. As before, they are one model: Fable carries classifiers that block most high-risk cyber and research-biology work and is available to everyone, while Mythos goes to vetted organizations. The price drops 20%, output is up to 30% faster, and an optional fast mode adds up to 2.5x speed at double the price.
Anthropic’s own table puts Fable 5.2 ahead of Opus 5.5, GPT-6 Astra and Gemini 4 Carbon on most rows, with Gemini 4 Carbon still ahead on AutomationBench (53.9% vs 51.4%) and Opus 5.5 ahead on AA-Briefcase Elo (1800 vs 1785). The launch examples lean on long autonomous work: a Python-to-Rust migration of a production service, a booting prototype operating system written by 1,000+ subagents over eight days, three manufactured electronic devices, robot control on tasks it was not trained for, and a 30-hour Lean formalization of Fermat’s Last Theorem. The two physics papers that Anthropic’s Hexagon account posted on 7 November (Hawking–Page transition at weak coupling; the modular bootstrap limit) are now attributed to Fable 5.2.
Why it matters
A new frontier claim from Anthropic on coding, robotics and formal mathematics at a lower price, and the clearest public statement so far of how close a lab’s own measurements put it to its automated-AI-R&D threshold. All benchmark numbers are Anthropic’s; independent leaderboards had not reported on launch day.
The RSP question
The system card is the first to report the internal acceleration fit next to the 2x threshold: a change of slope of about 1.88x (95% CI 1.62x–2.28x), with 32 of 100 refits at 2x or more, and “several” internal measures “starting to show acceleration”. Anthropic concludes the model “does not double the internal rate of overall aggregate progress in AI research and development”, calling this “a holistic judgment” and the assessment “uncertain”. It also says it deliberately limited gains in AI-R&D-specific skills during training. This lands four days after METR’s review argued it was “plausible” the threshold had already been crossed by July 15.
Sources
8 sources from 4 sites. Numbers match the chips in the text.
8 sources: 7 primary, 1 reaction
Primary
- Anthropic: Introducing Claude Fable 5.2 and Claude Mythos 5.2anthropic.com, official
- Claude Fable 5.2 and Mythos 5.2 System Cardanthropic.com, paper
- Claude Platform: models overview (ids, context, default effort)platform.claude.com, docs
- Claude on X: launch thread (1.66M views)x.com, official
- Anthropic: formalizing Fermat’s Last Theoremanthropic.com, official
- Anthropic: model hardware research preview (Diode devices)anthropic.com, official
- RoboDojo benchmarkrobodojo-benchmark.com, docs
Reactions
Changes
- Added Felix Rieseberg’s launch thread (owner’s tip)
- Filed (owner’s request) from the announcement, the system card and the API docs