MarkLLM

Anthropic Rolls Out Invisible Statistical Watermarks for Claude Models to Comply with EU AI Act

Anthropic has embedded invisible statistical watermarks into all outputs from its Claude models starting August 2, 2026, to meet Article 50 of the EU AI Act. The two-layer system applies a token-level bias using a secret key for text and C2PA metadata for images and files. Open-source projects appeared within 24 hours promising to strip the marks, yet none have demonstrated verifiable success against the statistical layer because Anthropic has not released a public detector. The technique, first described by Kirchenbauer et al. in 2023 and deployed by Google as SynthID, works by subtly biasing token selection toward “green” lists during generation. Editing, translation, or full paraphrasing rapidly degrades detectability, while short or rigidly formatted text such as code offers little room for the signal. The move affects every Claude deployment worldwide, not only EU users, to avoid maintaining dual model versions.