OpenAI said on October 5, 2026 that it will start adding an invisible watermark to text generated by ChatGPT and Codex in the European Union, TechCrunch, The Verge and Engadget report. OpenAI's own announcement is titled "Our approach to EU text provenance rules."
The details, per TechCrunch:
- The reason is the EU AI Act's transparency rules, which took effect August 2 and require AI companies to mark AI-generated content in a way other systems can identify.
- The watermark rolls out over the coming weeks to eligible ChatGPT and Codex users on all plans, in the EU only.
- API developers anywhere can turn it on for select models starting October 5. It is off by default.
- OpenAI is not making text watermarking a global default at launch.
- It works by subtly shaping the model's word choices, leaving a pattern a detector can pick up, so it travels with copied text. OpenAI says it does not identify the user and saw no meaningful change in model performance.
- The method is called textGrain. OpenAI published a technical report with researchers from the University of Pennsylvania and Yale.
The limits OpenAI states:
- Editing can weaken it. In one test, replacing 10% of words with synonyms dropped detection from about 92% to 66%.
- Short passages, math answers and translated text are harder to detect.
- Detector access is initially limited to approved researchers and expert organizations.
- A missing watermark "does not prove human authorship."
The Verge reports OpenAI says textGrain matched or exceeded other approaches such as Google DeepMind's SynthID for text, and that Anthropic made a similar move earlier. We could not open OpenAI's page directly, so these details come from the three outlets.