On 11 August Anthropic announced that every Claude model released after 2 August 2026 will watermark the text it produces. Not just in the chat app: the watermark lives at the model level, so it is there whether the text came through the API, Claude Code, Cowork, or anything else built on top. A follow-up post a few days later explained the mechanism, and the short version is that it’s a version of DeepMind’s SynthID-Text, which in turn descends from a scheme Scott Aaronson sketched out while at OpenAI in 2022.

Predictably it has caused a lot of confusion, especially in the academic circles (“is it zero-width characters?”, “can I strip it with a regex?”, “does this mean Turnitin finally works?”), most of which comes from not knowing how an LLM actually turns a probability distribution into words. So this post starts there, builds the watermark up from the sampler, and then tries to be honest about what it does and doesn’t mean for people who write papers and mark essays for a living.

Nothing here is hidden: the watermark is not metadata, not invisible Unicode, not a hidden token. It is a statistical pattern in which words were chosen, which is exactly why it survives copy-paste and exactly why it fades when you rewrite.