OpenAI is rolling out invisible watermarks for text generated by ChatGPT and Codex to comply with the EU AI Act. The EU AI Act’s transparency requirements took effect on August 2, 2026, obligating developers to mark AI-generated content in a way that other systems can identify.
The watermarking process will begin in the coming weeks, affecting all users regardless of their subscription level.
What does this watermarking look like in practice? Unlike traditional visible watermarks, this approach embeds unique patterns directly into the text. These patterns will be invisible to readers and will persist even if the text is copied and pasted. Detecting such watermarks relies on a method called “textGrain,” detailed in a technical report, which indicates that detection isn’t foolproof.
The document, co-authored with researchers from the University of Pennsylvania and Yale, explains how a secret key can organize the predictions needed to complete sentences. By making numerous subtle modifications, the detector can identify AI-generated content using just the text and the key.
However, the implementation isn’t flawless. OpenAI acknowledges that certain editing techniques can remove or obscure the watermark. For example, internal tests showed that replacing 10% of the words with synonyms dropped the detection rate from 92% to 66%. Additionally, recognizing watermarks becomes more difficult with shorter text snippets, answers to math problems, and translated texts.
The company warns that the absence of a watermark doesn’t automatically mean a piece of text was written by a human. It could also result from very short or heavily edited content or even text generated by another AI service.
Initially, the detection tool will be available only to a limited group of approved researchers and expert organizations. Developers using the OpenAI API can voluntarily enable watermarking for specific models, but the default setting will be off. For now, the watermarking feature is limited to the EU and is not yet available on the global market.
