Claude · Anthropic · SynthID-Text
Claude’s watermark is not hidden ink. It is which word came next.
Anthropic published the method. Future Claude models pick each next word with a secret key and the words so far. The sentence looks ordinary because it is. A key holder can ask whether Claude was likely involved. Nothing is added to the text.
Reviewed by Can Balkaya, CTO · Published
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Choose the surface, then the tool
| Surface | Gifi action | What can be verified |
|---|---|---|
| Ordinary words | Rewrite the prose | The words changed; no private detector certificate |
| C2PA, EXIF, XMP, properties | Strip named metadata | Action list plus an independent reinspection |
| Pixels, audio, or video | Not supported | No removal claim |
The dice are keyed
The method is SynthID-Text, from Google DeepMind. When two words would do, the key settles which one. Light edits usually keep the trail. A full rewrite does not. Short text, code and tight facts have fewer forks, so the mark is weaker there.
A hit means Claude was likely involved. It does not mean a person never touched the page. Anthropic will ship a detector. Older models are still in transition.
The file is only a note
Supported images and documents get a C2PA credential — a signed note in metadata, not a pattern in the words. We can strip the local label. Soft binding can survive.
What this cannot promise
- A rewrite does not prove the text was never AI-assisted.
- No one outside the lab can certify their detector. The key is private.
- Pixel watermarks, audio and video are out of scope.
Questions this usually raises
- Will a light edit remove it?
- Usually not. Anthropic said so. Replacing the wording does.
- Can it identify me?
- No. It tests whether Claude was involved, not who asked.
- Can you certify Anthropic’s detector?
- No. The key is private.
Primary sources
Claude
Statistical signal? Rewrite the prose.
A Claude mark, if it is in the words, needs different words. Hidden characters and C2PA need the file cleaner.