Anthropic extends Claude text watermarking to older models including Mythos 5, Opus 4.7/4.6 and Sonnet 4.6, reportedly from November 3
Anthropics help center article "How Claude marks AI-generated content" (updated Oct 28) now lists older models released before Aug 2, 2026, including Claude Mythos 5, Opus 4.7, Opus 4.6, Opus 4.5, Sonnet 4.6 and Sonnet 4.5, as carrying text watermarks on first-party surfaces, and says Anthropic is adding watermarks to these models to meet EU AI Act Article 50 transparency duties. According to a customer email screenshot reported by outlets such as MACCUS, the legacy rollout starts Nov 3, with Amazon Bedrock possibly a few days later; Anthropics public pages do not state that date. The watermark follows Google DeepMinds SynthID-Text approach, adds no hidden characters or extra tokens, and detection is in private preview only for regulators, media, researchers and similarly obligated organizations, which has drawn complaints on X that users cannot check their own text.
Key Takeaways
- Scope: in the official support table, older models such as Mythos 5, Opus 4.7/4.6/4.5 and Sonnet 4.6/4.5 are ticked only for first-party text watermarks; cloud-partner watermarks and C2PA file credentials remain limited to models launched after Aug 2 (see the help center)
- Timing: a customer email reported by media puts the start at Nov 3, with Bedrock possibly a few days later (MACCUS); Anthropics public pages give no date
- Cost and quality: Anthropic says the watermark adds no characters, no extra tokens and no extra cost, using a variant of SynthID-Text (see Anthropics explainer)
- Detection: the detection API is in private preview for regulators, law enforcement, media, fact-checkers, researchers and obligated enterprises; ordinary users cannot check text yet
- Reach: watermarks apply worldwide across Claude, the Claude Platform API, Claude Code, Cowork and Tag, because Anthropic says it has no durable way to scope them by region yet
