The AO3 Claude AI detector skin, explained
By Claude Watermark Research · Updated
The AO3 Claude detector is a site skin containing a single CSS rule that flags any work whose paragraphs carry a class name containing “claude”. It fires only when Claude's output is pasted directly from the Claude web interface into AO3's rich-text editor, which is the only way that class survives. It is not an AI detector in the statistical sense and it does not analyse writing style at all.
The actual rule
The published skin is one selector: it matches `p` elements inside a work body whose class attribute contains the string “claude”, and inserts a warning banner before the work.
Claude's web UI wraps response paragraphs in classes such as `font-claude-response-body`. Copying rendered output carries the HTML — including those classes — onto the clipboard. AO3's rich-text editor preserves a subset of pasted HTML, so the class lands in the posted work.
Why it has effectively no false positives
General-purpose AI detectors guess from style and are wrong constantly. This skin does not guess. The class it looks for cannot appear in hand-written HTML by accident — a human would have to type `class="font-claude-response-body"` deliberately.
That specificity cuts both ways: it also means the skin misses every fic where the author pasted through a plain-text editor first, and every fic that was edited in a word processor.
How the artifact is avoided
Pasting into any plain-text surface first — Notepad, TextEdit in plain mode, a code editor — strips all HTML, and with it the class. This is why community threads keep converging on “paste into Notepad++ first” as the answer.
Our checker reports the same artifact and removes it, along with zero-width characters and non-standard spaces that plain-text pasting does not always catch.
Questions
- Does the AO3 skin detect all AI writing?
- No. It detects one HTML artifact from one provider's web UI. Text written with any other tool, or pasted through a plain-text editor first, is invisible to it.
- Can the skin produce a false accusation?
- The artifact itself is reliable, but what people conclude from it may not be. Text run through Claude for proofreading or translation carries the same class as text Claude wrote from scratch.
Check your own text. Free, unlimited, no account, and it runs in your browser so nothing is uploaded.
Open the checker