Anthropic will soon offer a watermark detection API that lets third-party developers plug AI text detection into their own apps.
The company uses a variant of the SynthID Text method that Google Deepmind published in Nature in 2024 for its Claude AI model. The watermark tweaks the randomness source during word selection, creating a traceable pattern. According to Anthropic, this has no effect "on the content, level of creativity, or readability of Claude's text." Google Deepmind visualized the process in 2024 with the following animation.
The watermark works less reliably on short texts or fact-heavy passages where there aren't many alternative phrasings. The same goes for code. Pure corrections, where a human chose every word, won't carry the watermark either. Translations are a different story, since Claude picks all the words there. Heavy rewriting can strip the watermark out, Anthropic says in a published FAQ.
The watermark can only flag that Claude was likely involved in creating a text. It can't tell whether Claude wrote the whole thing or just edited it heavily, and it can't determine whether a text came from a human or a different AI model.
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