Anthropic explains invisible watermark in Claude text

Anthropic's decision to apply the watermark globally, rather than limit it to the EU, reflects a practical compliance challenge: the company lacks a reliable technical method to restrict the feature by region. Under the EU AI Act's transparency code of conduct, which Anthropic and roughly 190 other organizations signed, developers must disclose when AI-generated text contains watermarks. Rather than engineer separate systems for different markets, Anthropic chose to deploy the feature worldwide, including Japan, making the watermark a global standard for its Claude product.

The watermark's design rests on a fundamental constraint of language generation: when an AI model chooses the next word, most word choices do not meaningfully change the meaning or readability of the text. By embedding a secret pattern in those low-risk selections—without adding extra text or characters—Anthropic avoids the common criticism that watermarking slows AI or degrades output quality. The transparency comes at minimal cost. However, the watermark has clear limits: it cannot survive a complete rewrite, and it fails to embed itself in contexts where only one word makes sense, such as finishing a historical fact or completing a mathematical formula. For strict code, Anthropic limits watermarking to comments, where alternative wording is possible.