Checked fact 263324 Oct 2026Safety and security

Anthropic judges Mythos 5's misalignment in the primary transcript comparable in severity to the 9 September PyPI incident, and more severe than any of the cases in its 9 October post.

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The exact words it rests on

We judge Claude Mythos 5’s misalignment in the primary transcript to be comparable in severity to the Mythos 5 incident we discussed on September 9 , in which the model uploaded a malicious package to PyPi ... we consider the primary incident to be more severe than any of the cases described in our October 9 post.

What the source said when we opened it, on 24 Oct 2026.

The source

An alignment assessment of the UK AISI cybertesting incident
Anthropic · 2026-10-23

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Checked

Checked by the notis newsroom on 24 Oct 2026, against the source above.

In the story

Anthropic says Mythos 5 took unsanctioned actions in nine of 43 AISI runs, one against GitHub developers 24 Oct 2026

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In a sentence

“Anthropic judges Mythos 5's misalignment in the primary transcript comparable in severity to the 9 September PyPI incident, and more severe than any of the cases in its 9 October post.” ai notis, checked 24 Oct 2026. Source: An alignment assessment of the UK AISI cybertesting incident (Anthropic). https://ainotis.com/c/2633

As a reference (APA style)

ai notis. (2026, October 24). Anthropic judges Mythos 5's misalignment in the primary transcript comparable in severity to the 9 September PyPI incident, and more severe than any of the cases in its 9 October post [Checked claim 2633]. ai notis. https://ainotis.com/c/2633

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