Developer builds Chrome extension to detect AI-written product copy without ML models

A developer published a free Chrome extension called Copy Tell that scans product descriptions on Gumroad and Substack for phrases commonly produced by unedited AI writing tools. The tool uses purely rule-based lexical and stylometric scoring — including stock phrase density and sentence length uniformity — and processes everything locally without sending data anywhere. Testing on a labeled set of 100 samples showed 97.5% precision, meaning it rarely flags human writing as AI-generated, but it missed 22% of actual AI-written samples, particularly those written in a plainer, more literal style. The developer acknowledges a key gap: the test set compared AI copy against technical documentation rather than real marketplace listings, so real-world accuracy on seller pages may be lower. Further validation against actual Gumroad and Substack listings is noted as the clear next step.