# How Claude’s text watermark works

Claude’s watermark is not hidden ink. It is a trail of choices.

Anthropic will watermark future Claude models so a key holder can test whether Claude was involved — a requirement of the EU AI Act. The method is SynthID-Text. Nothing is added to the text.

Interactive demonstrations of the mechanism live on the [HTML page](https://gifi.ai/how-ai-text-watermarks-work). They do not reproduce Anthropic’s key or detector.

## What the watermark does, and does not, mean

### Will a light edit remove it?

Usually not. Anthropic says light editing probably keeps the signal. Replacing every word does not — though at that point the text is no longer the original generation.

### What about short text?

Fewer forks mean less evidence. Detection is weak on short samples and grows more confident as the passage lengthens.

### What about code?

Code has fewer equally-good next tokens, so the mark is sparser. Comments and other arbitrary wording can still carry it; a required identifier cannot.

### What if Claude only proofread?

The watermark only attaches to words Claude chose. Light grammar edits may be too few to register. Heavier rewriting leaves more of a trail.

### Do translations carry it?

Yes. A translation produced by Claude is a sequence of words Claude chose, so the watermark has a full passage to live in.

### Can it identify me?

No. Nothing in the watermark or its key points to a person, organisation, or chat. It tests whether Claude was involved, not who asked.

### What about other models?

Other providers who signed the same EU Code of Practice will use their own keys, and possibly their own methods. A Claude check says nothing about them.

### What does a hit prove?

That Claude was likely involved at some point. It cannot tell “Claude wrote this” from “Claude heavily edited this,” and it does not prove human authorship.

### What about images and files?

Those get a C2PA content credential in metadata — a signed note, not SynthID-Text. The file cleaner can strip that label. Soft binding can survive and re-link to a remote manifest.

## Choose the right tool

Statistical signal? Rewrite the prose. Hidden characters or C2PA? Clean the file.

Gifi keeps those jobs separate because they have different costs, different risks, and different standards of proof.

- [Rewrite the words](https://gifi.ai/login?next=/rewrite)
- [Clean hidden marks](https://gifi.ai/clean)

Use rewriting only on content you own or are authorised to process. A changed statistical signal does not prove human authorship, and no third-party tool can guarantee the result of a private detector. [Read Anthropic’s explanation](https://www.anthropic.com/news/claude-text-watermark).