Anthropic is extending its text watermarking system to four additional Claude models: Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6 and Claude Sonnet 4.6, The Decoder reports. Newer models such as Claude Opus 5.5 already carry the feature, which Anthropic first rolled out on October 2. The watermark is an imperceptible pattern in word choice — generated text reads normally while carrying a statistical signal that can be checked later, through a detection API Anthropic offers in private preview. The retrofit of older models is tied to transparency obligations under the EU AI Act.

On two of the newly covered models, Opus 4.6 and Sonnet 4.6, the watermark only applies while the sampling parameters stay at their defaults — temperature, top_p and top_k. Change a single one of those settings and the marking switches off on those models. Anthropic has explicitly warned developers who rely on the watermark for compliance purposes to keep the default settings in their applications.

With this expansion, watermark coverage becomes the standard across the current Claude lineup rather than a feature of only the newest models — but on the two older models, enforcement is not yet unconditional.