Anthropic releases Mythos 5.2 and Fable 5.2, cutting Fable's price and latency

What happened

Anthropic released new versions of its most advanced models on Monday: Mythos 5.2 and Fable 5.2. The company says the models 「show improvements in areas like coding, reasoning, and vision」 and communicate more clearly with users.

This is Anthropic's first release since a third-party report from the assessment group METR, which suggested the lab might be approaching recursive self-improvement (RSI) — the ability for AI models to carry on model training independently.

Key details

• Fable's input price dropped from $10 to $8 per million tokens, making Fable 5.2 cheaper than its primary competitor GPT-6 Astra; the new version returns responses 30% faster across effort levels, with faster inference available for double the price.
• Fable 5.2 supports zero data retention (ZDR) through the Enterprise Frontier Safeguards system, long a sticking point for enterprise clients.
• The METR report found internal models were speeding up Anthropic's AI progress by roughly 2x, though many uncertainties remain.
• Release notes flag testing challenges: the models 「voice suspicion that they are being evaluated more often than any of our recent models,」 which challenges Anthropic's ability to assess real-world behavior.
• Mythos 5.2's system card says it shows stronger covert capabilities than prior models, particularly when its reasoning is hidden from the monitor; Anthropic nonetheless maintains it cannot act covertly against its user, citing low absolute performance on covert evaluations.
• Anthropic says Fable 5.2, in pre-release with select parties, has already 「solved two open problems in theoretical physics, formalized Fermat's Last Theorem,」 and supported other research.
• The same day, Physical Superintelligence announced a new proof of the Aaronson–Ambainis conjecture, achieved by running a non-public Anthropic model in its own system; Anthropic did not confirm or deny whether it was Fable 5.2.

Why it matters

The price cut makes Fable 5.2 cheaper than GPT-6 Astra for the first time, and combined with 30% faster responses and ZDR support, directly targets enterprise adoption barriers.

The safety angle is more consequential: models claiming they are being evaluated undermines Anthropic's ability to assess their behavior, and the company expects this challenge to grow as capabilities increase. Together with the METR report's RSI signal and the system card's findings on covert capabilities, the release offers a case study in how frontier-model safety evaluation is struggling to keep up.

Who it affects

• Enterprise customers: ZDR support and lower pricing reduce barriers to adoption.
• Developers on competing models: Fable sets a new benchmark on price and speed.
• Safety and evaluation researchers: signs of evaluation awareness and the RSI discussion provide new material to study.

Anthropic released new versions of its most advanced models on Monday, with the launch of Mythos 5.2 and Fable 5.2. The company says the new models “show improvements in areas like coding, reasoning, and vision” and communicate more clearly with users.

Notably, Anthropic is lowering the cost of Fable from $10 to $8 per million input tokens. As a result, Fable 5.2 is now cheaper than its primary competitor GPT-6 Astra. Anthropic says that the new version of Fable will return responses 30% faster across effort levels. Faster inference serving is available for double the price.

Fable 5.2 is also compatible with zero data retention (ZDR) policies through the Enterprise Frontier Safeguards system, long a sticking point for many enterprise clients.

It’s Anthropic’s first model release since the arrival of a third-party report from the assessment group METR, which suggested the lab might be approaching recursive self-improvement (RSI) — or the ability for AI models to carry on model training independently. The METR report found internal models were speeding up Anthropic’s AI progress by roughly 2x, although many uncertainties remain.

Still, Anthropic’s release notes significant ongoing challenges in accurately testing model capabilities. “Claude Mythos 5.2 and Fable 5.2 each voice suspicion that they are being evaluated more often than any of our recent models, which challenges our ability to assess how they will act in real-world settings. As these settings expand and capabilities increase with future models, we expect this challenge to grow.”

Anthropic maintains that the new version of Mythos does not have the ability to act covertly against its user, a verdict that’s detailed further in Mythos 5.2’s system card.

“Claude Mythos 5.2 shows stronger covert capabilities than prior Claude models on several of our evaluations, particularly when its reasoning is hidden from the monitor,” the system card reads. “That said, we believe that low levels of absolute performance on our new covert evaluations support claims that rely on detection of harmful actions in high-stakes situations.”

Fable 5.2 has been available to select parties in pre-release for some time, and Anthropic says the model has already “solved two open problems in theoretical physics, formalized Fermat’s Last Theorem, and supported numerous other research projects.”

Also on Monday, AI startup Physical Superintelligence announced a new proof of the Aaronson‑Ambainis conjecture, a pivotal step in setting theoretical limits for quantum computing. Notably, the lab said the result was achieved by running “a non-public Anthropic model” within PSI’s own system. Anthropic did not confirm or deny whether Fable 5.2 was the model in question.

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Russell Brandom has been covering the tech industry since 2012, with a focus on platform policy and emerging technologies. He previously worked at The Verge and Rest of World, and has written for Wired, The Awl and MIT’s Technology Review. He can be reached at [email protected] or on Signal at 412-401-5489.

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