Start with what the mark actually is, because it is not a stamp on the page. When a model writes, it picks each word from a set of reasonable options.
A watermark quietly splits those options into two groups using a secret key, then nudges the model towards one group. One word proves nothing. Across 500 or 1,000 words, a detector holding the key can see the pattern.
Anthropic began marking Claude output on 2 August, worldwide rather than only in Europe. It has not published the algorithm or released a detector.
It marks what Claude touched, not what Claude wrote
This is the part most coverage skipped, and Anthropic states it plainly in its own support article.
