Four older Claude models get a watermark on November 16

If your app calls Claude Opus 4.6, Opus 4.5, Sonnet 4.5 or Haiku 4.5 through the API, Anthropic will watermark that output starting November 16, and customers can’t turn it off, according to The Decoder. On all four, the mark applies only when temperature, top_p and top_k stay at their defaults. Anthropic tells teams that rely on it for compliance to leave them there.

Opus 4.6 was originally due on November 3 alongside Claude Mythos 5, Opus 4.7 and Sonnet 4.6, which likely got the mark on schedule. Anthropic confirmed the delay in a second email to API customers and cites the EU AI Act’s December deadline for the retrofit. On Amazon Bedrock, the rollout may take a few extra days.