Broad distaste around the use of Claude, ChatGPT, and other AI tools has long been a thing in creative communities, including the world of fanfiction. Readers and writers have passed around tips for spotting supposedly AI-generated works, citing anything from em dashes to the broad concept of purple prose. But on June 29th, an anonymous X account called @heatedrivalryai promised a seemingly more reliable solution. It posted a skin — similar to an extension — for the popular fanfic repository Archive of Our Own (AO3) that would purportedly identify coding artifacts left behind by Anthropic’s Claude bot.

“When a Claude-generated response is pasted directly into AO3 from Claude, the text is wrapped by a Claude-injected code ‘font-claude-response-body,’” said the @heatedrivalryai account. “Its presence indicates the use of Claude definitively.” When a user visits a page (like a work of fanfic) with this code, the skin turns the entire background red.

Several test posts have been published to AO3 that allow users to check if it works. The screen immediately turned red when I tested the skin against these examples myself, and I published a Claude-generated short story to run my own experiment just in case. The red screen appeared when I directly pasted from the chatbot into the editor and vanished if I pasted text (including the exact same generated story) that didn’t come straight from Claude.

The Claude detector post was accompanied by examples of fanfic where the artifacts were spotted, which the anonymous creator said was meant to demonstrate the system works, not “create an environment of mistrust or accuse particular users.” But fanfic communities have quickly mobilized to publicly name and shame writers whose published works were flagged by the tool, and its creator certainly doesn’t consider AI a positive thing. “Fandom is a uniquely connective, collaborative space. It thrives on the human element and the creative spark which drives it and feeds off it,” they said. “If we unknowingly allow AI to corrupt these spaces, what will be left of them?”