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Claude adds a text watermark to four older models starting November 3
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Claude adds a text watermark to four older models starting November 3

Anthropic is extending Claude's text watermark to older models to meet the EU AI Act, though two models still have an easy way around the mark.

Ammar Rahimi · 2 November 2026 · 3 min read

Starting November 3, 2026, four older Claude models will begin producing text with an invisible watermark: Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6 and Claude Sonnet 4.6. The news comes from an Anthropic email to API customers that circulated online, and Anthropic has confirmed the watermark exists to comply with the EU AI Act.

1. What the Claude text watermark is

The watermark is a pattern in word choice that readers cannot see. When the model picks its next word, several candidates mean roughly the same thing, like "overcast" or "grey". The watermark nudges those choices along a hidden pattern that computers can detect.

Anthropic says the mark adds no characters or tokens, does not change pricing, and carries no information about the user or their organization. It is embedded at the model level, so it works on every platform offering Claude, and users cannot turn it off.

2. Why now

The reason is the EU AI Act. Article 50 requires AI providers to mark outputs so they can be detected as AI generated. Models launched after August 2, 2026 have carried the watermark since day one.

Older models got a transition period. According to The Decoder, Anthropic cited a December deadline as the reason it is retrofitting older models now, so November 3 puts these four well inside that window. Still older models, Opus 4.5, Sonnet 4.5 and Haiku 4.5, have no announced date yet.

3. A loophole on two models

The Decoder flagged one interesting catch. On Claude Opus 4.6 and Claude Sonnet 4.6, the watermark only applies when three sampling parameters stay at their defaults: temperature, top_p and top_k. These control how much randomness the model uses when picking words.

A small tweak to any one of them is enough to dodge the mark. Anthropic warned in its email: if you rely on the watermark for your own compliance, keep these settings at their defaults. For Mythos 5 and Opus 4.7, the report does not mention such a condition.

4. Who can actually check for it

Ordinary users cannot test whether text carries the mark. Anthropic's detection API is still in private preview, open to organizations eligible under EU law such as regulators, law enforcement, media, fact-checkers, independent researchers, educational organizations and EU civil society groups.

Some customers, quoted in the r/ClaudeAI subreddit, are unhappy with the setup: their text carries a mark they cannot verify themselves. According to the customer email, rollout on Amazon Bedrock may take a few additional days.

What it means for you

If you use Claude to write content, expect text from nearly all current models to carry a hidden mark from this month. For publishers and readers in Malaysia, this is part of a global wave of AI content labeling that started in the EU but is likely to become common practice everywhere.

  1. Everyday users: nothing to change. Claude quality and pricing are said to stay the same.
  2. API developers: check your temperature, top_p and top_k settings. Changing any one of them on Opus 4.6 or Sonnet 4.6 may leave outputs unmarked.
  3. Content publishers: EU law requires disclosure of AI generated text for public interest news. Good practice everywhere: state clearly when content was AI assisted.
  4. Fact-checkers and media: the detection API is still in private preview. Wait for broader access before relying on it.

Sources