Claude Watermark (claudewatermark.xyz)

Claude Watermark is a free browser tool that checks pasted text for the traces Claude's chat interface leaves behind, such as hidden HTML class names, invisible Unicode characters and stylistic tells like curly quotes and repeated em dashes. It is aimed at people who copy Claude's output into another editor, including writers posting fiction on AO3 and anyone pasting into Google Docs, WordPress, Notion or similar platforms, and who want to avoid being flagged as having used AI.

Checking runs entirely in the browser, so no text is uploaded, and it is free and unlimited without an account. The tool looks for HTML class names containing claude, zero-width and invisible Unicode characters, byte-order marks, non-breaking and exotic spaces, plus stylistic signals such as curly quotes and em dash frequency. Deterministic findings come back with a count and a position within the passage; a paid rewrite pass then resamples every sentence through a different model.

Claude Watermark distinguishes deterministic findings, which are either present or not, from stylistic findings such as em dashes and curly quotes, which prove nothing on their own; measuring ten pre-computer novels found em-dash rates swinging from zero per thousand words in Austen and Stoker to twenty-six in Melville. Claude Watermark states plainly that no tool, including its own, can verify Anthropic's statistical watermark without its unreleased key, and keeps its detection engine open source so that claim can be checked.

Checking and cleaning are free, unlimited and need no account, while the rewrite engine runs on credits bought through monthly plans: Starter at $9/month for up to 100,000 words, Pro at $19/month for up to 300,000 words, and Max at $49/month for up to 1,000,000 words, alongside one-time top-up packs. Monthly credits stack rather than reset, one-time credits never expire, failed jobs are never charged, and text processed for a rewrite is not stored.

Use cases

• Avoiding an AO3 Claude-detector flag on fan fiction
• Cleaning Claude's HTML and Unicode traces before pasting into platforms that keep markup
• Changing wording so an AI detector scores it as less predictable