Anthropic's text watermark is easy to bypass on several 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 a second wave on November 16, 2026 adding Claude Opus 4.5, Claude Sonnet 4.5, and Claude Haiku 4.5. On the older models the watermark applies only when temperature, top_p, and top_k are left at their defaults, so changing any one sampling parameter avoids it. Anthropic cites the EU AI Act's December deadline and says the watermark, a variant of Google DeepMind's SynthID text method, is embedded at the model level and cannot be turned off.