Claude Watermark Detector
Paste text below to see which patterns make it look AI-written.
How the detector works
- 1Paste text you suspect is AI-written (up to 500 words free, 1,000 words signed in).
- 2We score it using writing-pattern heuristics (sentence rhythm, stock phrasing, structure) plus an AI pass that reads the text for context-specific tells.
- 3You get an overall score, the text with suspect passages highlighted, and a plain-language reason for each flag.
Frequently asked questions
Does this detect Claude's official watermark?
No. Anthropic added a real invisible watermark to Claude's output in August 2026, but the official detection API is in private preview, limited to regulators, researchers, and media outlets. It isn't publicly available to anyone, including us. This tool analyzes general AI-writing patterns (stock phrasing, sentence rhythm, structure) as a practical stand-in, not Anthropic's cryptographic watermark.
How accurate is the score?
It's a heuristic signal, not a certainty. A high score means the text shows patterns commonly associated with AI writing; a low score means it doesn't. Treat it as one useful data point, not a definitive verdict. Human writing can occasionally trigger false positives, and a careful rewrite can occasionally avoid detection.
Does this only work on Claude's output?
No. The patterns it checks for (stock transition phrases, unusually uniform sentence length, heavy em-dash use) show up across most LLMs, not just Claude. It's useful for checking AI-generated text generally.
Is it free?
Yes. One free check without signing up (up to 500 words), and two more checks after a free signup (up to 1,000 words each). Paid plans with more usage are coming.
What are the hidden-character findings?
Separate from the AI-writing score, we also scan for invisible Unicode characters (zero-width spaces and similar) that don't appear in normal typed text and are sometimes used to embed hidden tracking data. This check is instant, free, and unlimited: it runs entirely in your browser as you type, with no signup or usage cost.