Anthropic's watermark rollout leaves a loophole on two older Claude models

Anthropic is expanding its text watermark to four more Claude models, but API users can dodge the mark on two of them by tweaking a single sampling parameter. In an email to API customers, Anthropic said it will watermark Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6. Newer models like Claude Opus 5.5 already carry the mark, and the rollout on Amazon Bedrock may take a few extra days.

Anthropic calls the watermark an "imperceptible pattern in word choice" that doesn't change meaning, quality, or readability. The company says it adds no extra characters or tokens, doesn't affect pricing or request and response formats, and barely slows output speed. Anthropic also says the watermark "contains no information about the user, their organization, or their conversations with Claude." On Claude Opus 4.6 and Claude Sonnet 4.6, Anthropic says the watermark kicks in only when developers leave the three sampling…