Claude's Fable 5.1 Caught Reasoning in Chinese, and Non-English Chains of Thought Are Becoming a Differentiator

A developer spotted the official Claude app producing a Chinese-language chain of thought, hinting at how multilingual internal reasoning is emerging in frontier models.

A small but revealing observation came from @EricSimons, who reported seeing "a Chinese chain of thought in official Claude app, Fable 5.1" — the first time he had encountered it. The model was reasoning internally in Chinese even in an English-language context.

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