What Claude actually leaves in text you copy and paste
By Claude Watermark Research · Updated
Copying from a chat interface carries HTML along with the text. From Claude, that includes class names containing “claude”. Across providers you may also carry zero-width characters, non-breaking spaces, narrow and thin spaces, and typographic punctuation. Only the HTML classes and data-attributes are provider-specific enough to be proof of anything; the rest are common in any web-copied text.
Deterministic artifacts
Provider-specific HTML classes and `data-*` attributes. These identify the source tool unambiguously and are what the AO3 detector uses.
They only survive if you paste into an editor that accepts rich text. Plain-text destinations discard them.
Weak signals people over-read
Zero-width characters and non-breaking spaces show up constantly in text copied from any website, so their presence proves nothing on its own — though they are trivially greppable and do get used as a heuristic.
Em dashes, curly quotes and ellipsis characters are stylistic. Plenty of careful human writers use all three. Removing them is a choice about voice, not a technical fix, which is why our cleaner leaves em dashes alone unless you ask.
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