OpenAI starts textGrain text watermarking with API opt-in and a phased EU rollout for eligible ChatGPT and Codex text

What changed

OpenAI announced its textGrain text-watermarking approach on October 5. API customers globally can now opt in for select models, with watermarking off by default, while eligible ChatGPT and Codex text in the EU will gain invisible statistical watermarks over the coming weeks. Detector access initially goes only to approved researchers and expert organizations, and OpenAI says it plans to open-source the technology later. The company also documents important limits: shorter or constrained text and synonym rewriting reduce detection, and a watermark is only a provenance signal—it does not establish ownership, responsibility, user identity or truth, while absence of a detected watermark does not prove human authorship.