Here’s how Claude secretly watermarks AI-written text

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Google and OpenAI have adopted SynthID watermarks for images generated by their AI models. This allows people to find out whether that shared image is the real deal. Generated text is a different story, though. However, Anthropic announced last week that Claude can add watermarks to generated text , and it’s now revealed more details about the system.

Anthropic explained in a blog post that Claude’s text watermark system is based on the SynthID-Text solution published by Google DeepMind. It adds that the watermark system isn’t visible to readers, doesn’t have a “practical” impact on content or quality of generated text, doesn’t have hidden characters, doesn’t require extra tokens, and can’t be traced to a specific person/organization/chat.

The company says AI models typically generate a word at a time and decide on the next word based on the preceding text. It uses the example of “The weather today was cold and…” It notes that the next word is unlikely to be “sugary” but likely to be “overcast” or “grey.”

Anthropic adds that the choice of “overcast” or “grey” doesn’t matter much to readers, so it uses a random number to choose which word will be generated:

It’s worth noting that Claude won’t lean towards a specific word, while the firm adds that the watermarking system won’t force the AI model to consider a word it wouldn’t have considered before.

The company also says its text watermarking system isn’t a silver bullet for detecting generated text:

In other words, this system doesn’t support other AI models but works best on longer text passages. Anthropic also confirmed that watermarking is reduced for factual passages, where there aren’t as many opportunities for low-stakes word choices (and therefore the insertion of watermark patterns). It uses the example of the sentence “Isaac Newton’s most famous work was called Principia…”. The only accurate choice here is “Mathematica.”