TheZvi breaks down the Claude Fable 5.1 system card: 200+ pages of safety evals
TheZvi · x · 2026-09-05
TheZvi published his analysis of the Claude Fable 5.1 and Mythos 5.1 system card. Fable 5.1 was, by a healthy margin, the most capable publicly available AI model at release, accompanied by a 200+ page model card, with reference to Anthropic's August 2026 Risk Report.
Key facts: Mythos 5.1 and Fable 5.1 are the same model under the hood, except Fable has classifiers superimposed on it, so most assessments apply to both. The post focuses on areas of change versus prior system cards (Mythos 5, Opus 5), covering safety, alignment and model welfare properties.
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