Anthropic is adding its hidden text watermark to four more models: Mythos 5, Opus 4.7, Opus 4.6 and Sonnet 4.6. The company told API customers about the change by email, and the mark cannot be turned off.
On two of the four, though, the mark only appears if developers leave their sampling settings alone. Changing any one of them is enough to avoid it, and Anthropic’s own advice is to keep the defaults if you rely on the watermark.
How the mark works
Anthropic’s help page on content marking says the watermark is a pattern woven into word choice. It is invisible to readers, travels with the text when it is copied and pasted, and may survive some editing. Anthropic says there is no extra text and no change to pricing or the API format, and that the pattern says nothing about who the user is.
The reason is Anthropic’s signature on a European code of practice for labelling AI output, tied to the AI Act. Anything launched in the EU from 2 August 2026 is marked from the start, and older models are being brought in gradually. The help page says it applies wherever Claude is offered, including through AWS, Google Cloud and Microsoft Foundry. Amazon Bedrock may lag by a few days.
The loophole on Opus 4.6 and Sonnet 4.6
For those two models the mark applies only when temperature, top_p and top_k all sit at their defaults. Those three API settings decide how adventurous the model is when choosing words. The Decoder noticed that the email has no such caveat for Mythos 5 or Opus 4.7, which suggests those two are marked whatever the settings.
Anthropic also lists limits on its help page. A detected mark means the text may have come from Claude, not that Claude wrote it, since people use the model to proofread or translate. A missing mark proves nothing, because short passages, heavy edits and paraphrasing can all wipe it out.
Checking for the mark is limited for now to a private preview. Regulators, researchers, journalists and fact-checkers can apply, as can firms with legal duties to verify it, and Anthropic says access will widen.
