OpenAI said Monday it will begin adding an invisible watermark to text generated by ChatGPT and Codex in the European Union, to comply with the EU AI Act's transparency rules, which took effect August 2. The watermark will roll out over the coming weeks to eligible ChatGPT and Codex users on all plans, but only in the EU — and OpenAI is not making it a global default at launch.

The technology, called textGrain, works not as a visible mark but as a statistical signal: it subtly shapes the model's word choices, leaving a pattern readers can't see but a detector can pick up. Because the signal lives in the words themselves, it survives copy and paste. API developers anywhere in the world can opt in to watermarking on select models starting today; it stays off by default.

OpenAI published a technical report on textGrain, co-written with researchers from the University of Pennsylvania and Yale. The company is candid about limits: replacing 10 percent of words with synonyms dropped detection from about 92 to 66 percent in its tests, and short passages, math answers, and translated text are harder to detect. Detector access will initially be limited to approved researchers and expert organizations.

The watermark indicates an OpenAI system generated or processed part of the text — not who used it, how much a person contributed, or who owns it. Notably, the absence of a watermark proves nothing either: it does not establish that a person wrote the text.