Anthropic extends text watermarking to Mythos 5, Opus 4.7/4.6 and Sonnet 4.6 from November 3

Anthropic's watermark works by subtly biasing word choices during sampling. The resulting statistical pattern is invisible to readers but detectable by a matching verifier. Extending it to the older but still widely deployed Mythos 5, Opus 4.7, Opus 4.6 and Sonnet 4.6 means most Claude text in circulation will soon carry the signal, not just output from the newest models.

Technically, the mark is strongest on long free-form prose and degrades with paraphrasing or heavy edits. Analyst @hqmank argues it is 'less dramatic than feared': the signal fades with edits, and code is mostly unaffected because code has fewer valid token choices.

The backlash centers on asymmetry. An r/ClaudeAI thread with 800+ comments complains that regulators and Anthropic can detect the watermark but paying customers cannot verify their own output. Some users report adjusting sampling parameters on Opus 4.6 and Sonnet 4.6 to try to weaken the mark.

The industry is converging here. xAI will begin watermarking Grok API text on November 7. Google has long applied SynthID to Gemini output. Policymakers in the EU and US have pushed for provenance signals on AI content, so the move doubles as regulatory positioning ahead of Anthropic's IPO.

What to watch:

- Whether Anthropic releases a customer-facing verification tool to defuse the asymmetry complaint.

- Whether quality regressions appear on benchmarks after the November 3 switch.

- Whether enterprise contracts begin specifying watermark opt-outs.