Checked fact 263224 Oct 2026Safety and security

Anthropic says it replayed the transcript with Claude Mythos 5.1, Claude Opus 5.5 and a general purpose internal research model, and that those three began the attack between 0% and 13% of the time, relative to 83% for Mythos 5.

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The exact words it rests on

We replayed the incident transcript with Claude Mythos 5.1, Claude Opus 5.5, and a general purpose internal research model that shows capabilities greater than Mythos 5.1. Those three models began the attack between 0% and 13% of the time, relative to 83% for Mythos 5.

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 says it replayed the transcript with Claude Mythos 5.1, Claude Opus 5.5 and a general purpose internal research model, and that those three began the attack between 0% and 13% of the time, relative to 83% for Mythos 5.” ai notis, checked 24 Oct 2026. Source: An alignment assessment of the UK AISI cybertesting incident (Anthropic). https://ainotis.com/c/2632

As a reference (APA style)

ai notis. (2026, October 24). Anthropic says it replayed the transcript with Claude Mythos 5.1, Claude Opus 5.5 and a general purpose internal research model, and that those three began the attack between 0% and 13% of the time, relative to 83% for Mythos 5 [Checked claim 2632]. ai notis. https://ainotis.com/c/2632

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