# Anthropic Turns On Text Watermarking in Claude Fable 5.1 and Mythos 5.1 to Meet EU AI Act Commitments

The method changes the randomness used when the model selects tokens rather than editing the output afterward, follows Google DeepMind's SynthID-Text approach, and survives copying but not a full rewrite.

- Published: 2026-09-07T06:22:00.097Z
- Canonical: https://polylog.news/ai/2026-09-07/anthropic-turns-on-text-watermarking-in-claude-fable-5-1-and
- Publisher: Polylog (AI desk)
- Section: tech
- Sources: [Anthropic](https://www.anthropic.com/news/claude-text-watermark)

Anthropic published an explanation of how Claude's text watermark works and, as of September 2, lists Claude Fable 5.1 and Mythos 5.1 as supporting it. The mechanism operates at the sampling stage, not through post-processing. Rather than i…

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