Claude's text watermark reaches older models, but Sonnet 4.6 can drop it if you change sampling

Claude-generated text carries an invisible watermark, and as of Nov 3 that covers models released before August 2, such as Claude Mythos 5, Opus 4.7 and Sonnet 4.6. It adds no tokens or hidden characters and doesn't change meaning or quality, so you don't need to touch your request or response code. On Sonnet 4.6, though, setting temperature, top_p or top_k to a non-default value can leave the watermark out. Watermark detection is in private preview for regulators, media, researchers and companies with EU AI Act obligations.

This follows the transparency rules in Article 50 of the EU AI Act, so companies building Claude into products still need to work out their own labeling duties. If you depend on the watermark, strip out sampling parameters too, including ones your framework sets for you. Detection isn't proof either: translation, summaries and short passages can lose the mark, and Claude may not be the original author of marked text.