What Claude's AI text watermark actually does

Anthropic has begun building a watermark into text generated by future Claude models, a change the company says is meant to help identify whether a given piece of writing was likely produced by its AI. This new feature, implemented to comply with EU rules, is meant to be indistinguishable to the human eye, without changing Claude's normal writing output. The company laid out the mechanics and rationale behind the feature in a post published to its website. According to Anthropic, the watermark exploits the countless small, low-stakes decisions a language model makes as it generates text. Rather than using a truly arbitrary random number to make that pick, the watermarked version of Claude bases the decision on a cryptographic key combined with the preceding text. The result, Anthropic says, is a subtle statistical pattern spread across a response that's invisible to a human reader but detectable to anyone with the matching key, which allows them to estimate the probability that Claude generated the text. Anthropic was clear in that the change carries no cost to output quality. The company said internal testing turned up no measurable difference in the creativity, accuracy, or reada