The short version
● Date — 3 November 2026
● Models — Mythos 5, Opus 4.7, Opus 4.6, Sonnet 4.6
● Method — a version of DeepMind SynthID-Text
● Opt-out — none published
On 3 November, Anthropic switched text watermarking on for Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6 and Claude Sonnet 4.6. Claude Opus 5.5 already carried it. The Amazon Bedrock rollout was expected to take a few additional days, and Anthropic's documentation says models launched before 2 August 2026 are being brought into line over the coming months.
The mechanism is a version of SynthID-Text, published by Google DeepMind. A secret key guides some word choices so they follow a subtle statistical pattern; Anthropic says it adds no characters or extra tokens and leaves quality unchanged. The detector needs Anthropic's key, and the detection API is in private preview for eligible organisations such as regulators, media outlets, fact-checkers and researchers rather than the general public. Images get signed C2PA provenance metadata instead.
The limits are stated plainly. Short passages offer too few word choices for a reliable reading, a full rewrite removes the mark, and a positive result shows only that Claude was likely involved, not whether it wrote, edited or translated the text. Anthropic cites the EU AI Act and applies the change worldwide because it cannot yet scope it by region. The practical contrast with xAI is the opt-out: Grok has one for team admins, and Anthropic has not published one.
More from 5 November
Full dated index: every AI release in November 2026.
Sources