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

Anthropic is extending its text watermark to Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6, with the rollout on Amazon Bedrock possibly taking a few extra days. On Claude Opus 4.6 and Claude Sonnet 4.6 the watermark applies only when the temperature, top_p, and top_k sampling parameters are left at their defaults, so a minor tweak to any one of them is enough to dodge it. Anthropic cites the EU AI Act's December deadline as the reason for watermarking older models, and says the mark adds no extra characters or tokens and does not affect pricing or request and response formats.