Claude watermark
Anthropic marks Claude’s text with a version of SynthID-Text, biasing which word the model picks next. Nothing is inserted into the text, so no character scanner can find it.
In short
Claude marks its text on models launched from August 2026. Anthropic biases which word the model picks next, with a version of SynthID-Text, and inserts no character. Older models are in a transition period, and code is not marked.
Two things you can do with your own text
Free, and it runs in your browser: nothing is sent. It finds characters, not a watermark in the word choices.
Try it without an account: three rewrites, up to 3,000 characters each, and a trial result is not saved. We show how much of your draft changed and promise no detector result.
What the evidence says
Anthropic marks the text of Claude models launched from 2 August 2026, the date the EU AI Act transparency rules took effect. Marking applies worldwide rather than only in Europe, and covers the API, Claude, Claude Code and the cloud platforms that resell the models. Models released before that date are in a transition period: Anthropic says it is adding marking to them, so the answer for a given passage depends on which model produced it.
The method is a version of SynthID-Text, the scheme Google DeepMind published and open-sourced — the same family of watermark that marks Gemini. Instead of using an arbitrary random number generator to pick the next word, the model uses a key and the few words that come before to settle the choice, so the sequence of choices itself carries a pattern that a holder of the key can measure.
That mechanism decides what any tool can do about it. Nothing is added to the text and there are no hidden characters, so scanning a document for zero-width spaces, byte-order marks or unusual spacing tells you nothing about whether Claude wrote it. Anthropic also signs C2PA provenance metadata on generated image files, which is a separate mechanism and does not apply to prose.
The stated limits are specific. Detection works poorly on short samples, where there are fewer word choices to measure. It is sparser on factual passages, and where an exact output is required — code, or a term that would be wrong if it were swapped — the watermark is not applied at all. Light editing probably will not remove it; a complete rewrite where every word is replaced will. When Claude edits someone else’s writing, most of the words are the person’s and there is very little for the mark to attach to.
Detection is not open to the public. The detection API is in private preview for organisations eligible under EU law, such as regulators, law enforcement, media, fact-checkers, independent researchers, educational institutions and EU civil society groups. Anthropic documents no way to switch marking off. It also cautions that a detected mark is a signal rather than proof, and that the absence of one does not mean a text was not AI-generated.
A common misreading
Sites that sell “Claude watermark removers” strip invisible Unicode characters. Anthropic’s watermark is not made of characters, so removing them does not touch it — those characters are worth cleaning up, but not for that reason. What does reach the mark is replacing the word choices it is made of, which is what Anthropic itself describes above: light editing probably will not remove it, a complete rewrite where every word is replaced will.
Sources
- How Claude’s text watermarking worksAnthropic · · Provider documentation
- How Claude marks AI-generated contentAnthropic Help Center · · Provider documentation
- Anthropic shares more details about how Claude’s new watermarks will workTechCrunch · · Independent report
Frequently asked questions
Can I detect Claude’s watermark myself?
No. Detection needs the key, and the detection API is in private preview for organisations eligible under EU law. No public tool, including ours, can read this signal from text alone.
Do invisible characters in my text mean Claude wrote it?
No. Invisible characters come from editors, clipboards, PDFs and web pages as readily as from any model, and Claude’s watermark does not use them. Finding one points to a text-handling step, not to an author.
Is every Claude response watermarked?
No. Marking covers models launched from August 2026, and it is not applied where an exact output is required, such as code. Older models are in a transition period while Anthropic adds marking to them.
Related tools
Related reading
Other assistants in the database
Working on your own draft?
GenPolish rewrites text you already have, showing every change before you accept it. A watermark lives in the word choices, so replacing the words is what reaches it. We show what share of your words changed, and make no claim about detectors.