OpenAI's text watermark detects 80% of short samples

OpenAI says its textGrain watermark detector found watermarks in about 80% of 200-token psychology passages at a 1% false-positive rate, rising to 95% at 400 tokens, in its Oct. 5 EU text-provenance announcement. Eligible ChatGPT and Codex text in the EU will gain invisible marks over coming weeks, while API use is opt-in and off by default; the company says replacing 10% of words in a 400-token passage cut detection from 92% to 66%. For builders, that makes provenance one signal rather than proof of authorship, alongside the archive’s analysis of Claude’s watermarking limits and provenance role.