Claude’s Invisible Watermark Marks Even Your Own Writing

Anthropic is adding invisible watermarks to everything Claude produces - and to everything Claude merely edits.

Under the EU AI Act’s Article 50, providers of AI systems that generate text must mark their outputs so they can be detected as machine-made. Anthropic’s implementation, announced in August 2026, applies that mark at the token-selection level: every time Claude chooses a word, the choice is nudged to encode a hidden signal.

The consequence is that the technique cannot distinguish between text Claude authored from scratch and text a user wrote themselves but asked Claude to polish. If Claude touched it, it is marked.

The answer-first version: Anthropic is