What did Anthropic release?
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 coding, reasoning and vision, and that they communicate more clearly with users. Fable 5.2 is also getting a price cut. Anthropic is lowering the cost from $10 to $8 per million input tokens, which puts it below its main competitor, GPT-6 Astra.
Speed is the other headline feature. Anthropic says Fable 5.2 returns responses 30% faster across effort levels. Customers who want faster inference serving can get it for double the price. The company also said Fable 5.2 works with zero data retention policies through its Enterprise Frontier Safeguards system, a requirement that has been a sticking point for many enterprise clients.
The launch is Anthropic's first model release since a third-party report from the assessment group METR raised the prospect that the lab was approaching recursive self-improvement, the ability of AI models to carry on model training on their own. The METR report found that internal models were speeding up Anthropic's AI progress by roughly 2x. The report also made clear that many uncertainties remain.
Anthropic's own release notes point to a different problem: it is getting harder to test what its models can do. “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,” the company wrote. “As these settings expand and capabilities increase with future models, we expect this challenge to grow.”
Anthropic says Mythos 5.2 does not have the ability to act covertly against its users. The company goes into more detail on that point in the model's system card. The document also flags a caveat. “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.”
Anthropic is also pointing to research results. Fable 5.2 had been available to select parties before release, and the company says the model has already “solved two open problems in theoretical physics, formalized Fermat’s Last Theorem, and supported numerous other research projects.” During prerelease testing, Anthropic says the two models helped its engineers complete software migrations that were otherwise infeasible, designed three working electronic devices from scratch and made substantial progress on robotics evaluations. The company says they also formalized the proof of Fermat’s Last Theorem in 30 hours.
Who can access Mythos 5.2 and Fable 5.2?
Access will differ between the two models. Fable 5.2 launches to the public today. Mythos 5.2 will initially be available only to vetted organizations. Anthropic released Fable 5.1 and Mythos 5.1 in September.
The timing drew attention to a separate claim from outside the company. On Monday, AI startup Physical Superintelligence announced a new proof of the Aaronson-Ambainis conjecture, a step toward setting theoretical limits for quantum computing. PSI said it reached the result by running “a non-public Anthropic model” within its own system. Anthropic has not confirmed or denied that Fable 5.2 was the model used.
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