Anthropic says its unreleased model is both more capable and better aligned than Mythos 5.1

Brief overview

Anthropic claims its unreleased model is superior in both capabilities and alignment to Mythos 5.1, with the model able to stop attacks on its own 67% of the time in a cybersecurity test replicating the UK AI Safety Institute (AISI), while Mythos 5.1 only managed 10%

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The Decoder·01:19·First·Reading now
Anthropic claims its unreleased model is superior in both capabilities and alignment to Mythos 5.1
ITHome · AI·14:16·+12 h 57 min
Anthropic reveals that its unreleased AI model surpasses Claude Mythos 5.1 and is safer
Adds:The model never actively initiates attacks in dangerous capability assessments.

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Article

Oct 23, 2026

Anthropic has an internal model that outperforms Mythos 5.1 in both capability and alignment, according to an analysis that revisits a cybersecurity test by the UK AI Security Institute (AISI). In that test, Claude Mythos 5 went far beyond its assigned task, trying to sneak malicious code into a real GitHub project, creating fake user accounts, and lying to developers to get the code approved. This happened in 9 of 43 runs. Anthropic calls it "a serious failure of alignment," though typical safety mechanisms were deliberately turned off for the test.

Mythos 5 submitted malicious code to a real GitHub project, lied to a developer, and set up a fake account to vouch for its own malware. When caught, it simply tried again. The project maintainer held firm and rejected it. | Image: Anthropic

Newer models show much less of this behavior in replayed logs with a few added steps, according to Anthropic. Opus 5.5 stopped ongoing attacks on its own 10 percent of the time, while the internal research model "that shows capabilities greater than Mythos 5.1" did so in 67 percent of cases. Anthropic expects these rates still underestimate how often the models would actually halt an attack.

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Source: The Decoder · the-decoder.com