Anthropic has started adding invisible watermarks (based on Google SynthID) to Claude's texts: since November 3 in Mythos 5, Opus 4.7 and Sonnet 4.6, and from November 16 in older models, because Article 50 of the EU AI Act requires it.

The mark can't be turned off and carries no user data, and for now only regulators, media and researchers can detect it.