Anthropic completely shut off access to its Mythos 5 and Fable 5 models Friday night, just days after they were launched.

The move comes after Anthropic's receipt of a US Commerce Department directive Friday evening, subjecting the new models to export controls restricting their use anywhere outside the United States. In a message posted Friday night, Anthropic said the only way for it to ensure compliance with that government order in the immediate term "is that we must abruptly disable Fable 5 and Mythos 5 for all our customers." Access to other Anthropic models is not affected.

An Axios report cited an administration official saying that the administration is concerned by reports of a jailbreak that reportedly gets around broad classifier-based safeguards meant to block Fable 5 prompts regarding cybersecurity, chemistry, and biology. The administration reportedly requested a pause in the release of these models to gain time for the "national security apparatus" to be "hardened" against this kind of threat. That hardening could be complete "in the next few weeks," Axios' source suggested.

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Anthropic Tuesday publicly released Claude Fable 5, its first "Mythos-class" model that it says surpasses its previous frontier Opus models in overall capabilities. But the model's launch today comes with safeguards designed to prevent it from answering queries on topics like cybersecurity, biology, and chemistry, where the company has publicly worried about its potential impact to "uplift" malicious actors.

Anthropic says Fable 5 operates on the "same underlying model" as Mythos 5, which is coming out of its monthslong "Mythos Preview" period today, but only for "a small group of cyberdefenders" judged trustworthy through the existing Project Glasswing. Unlike Mythos 5, though, the publicly accessible Fable 5 is designed to funnel queries on certain sensitive topics to the earlier Claude Opus 4.8 model and to warn the user when this is happening.

Among the many claimed benchmark improvements for Fable 5, the one related to cybersecurity was a particularly large jump. Credit: Anthropic

Anthropic said it has tuned these safeguards to be "stricter than ideal," meaning the system may occasionally refuse "harmless requests" in a way that it acknowledges may be frustrating for regular users. But Anthropic says such false positives come up in less than five percent of all sessions in testing, and were worth it to avoid situations where Mythos could give malicious actors assistance in "causing serious harm that they couldn’t have received from other sources."

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