OpenAI will watermark ChatGPT and Codex text in the EU: what changes and what it can't show
OpenAI says that over the coming weeks, text written by ChatGPT and Codex for users in the European Union will carry an invisible watermark, on every plan. API customers anywhere can switch it on from today. Nobody outside a small group of approved researchers can check for it yet. Here's what the watermark is, how reliable OpenAI says it is, and what it doesn't tell anyone about you.
What happened
On October 5, OpenAI published its plan for marking AI-written text, its answer to the EU AI Act, which requires generative AI providers to make generated text identifiable by machines. The plan has three parts. API customers around the world can opt in to watermarked text for select models starting now; it stays off by default. Over the coming weeks, ChatGPT and Codex add an invisible watermark to eligible text for users in the European Union, on all plans, and only there. The detector stays private: only vetted researchers can apply.
OpenAI explains how it will watermark ChatGPT text to comply with EU provenance rules
How the watermark works
OpenAI calls the technology textGrain. It doesn't add hidden characters, invisible spaces or odd punctuation, so there's nothing to see or copy along with the text. It works on word choice instead. A model picks each next word with some randomness, and textGrain nudges those picks according to a secret key. Over a passage, those picks form a pattern. Only a detector with the same key can test for it. OpenAI says it saw no meaningful difference in the scores of Astra, its latest model, with and without the watermark.
How reliable it is, by OpenAI's own numbers
OpenAI is unusually direct about the limits, and they explain why the detector isn't public:
- Length matters. With false alarms held to 1%, the detector found the watermark in about 80% of 200-token passages and about 95% of 400-token ones, on loosely worded subjects like psychology.
- Precise answers carry little signal. Math and other answers with one right wording leave the model few choices to nudge, and detection there was much lower.
- Editing weakens it. In OpenAI's tests on 400-token passages, swapping 10% of the words for synonyms cut detection from about 92% to 66%. Heavy rewriting or translation can make the watermark undetectable.
What a watermark doesn't tell anyone
- It doesn't identify you. A detection result isn't tied to a person, account, prompt or conversation.
- It doesn't measure how much you contributed, or whether the model wrote the text or only edited yours.
- It doesn't settle ownership, whether disclosure was required, or who is responsible for the text.
- No watermark doesn't prove a person wrote it. The text may be short, edited, translated, older than the rollout, or from another company's model.
What it means for you
Outside the EU, nothing changes in ChatGPT for now. Inside it, text you generate will start carrying the signal over the coming weeks, and OpenAI doesn't mention a setting to turn it off. Since only approved researchers can run OpenAI's detector at launch, a teacher or client can't check your text with it; the AI detectors they might use are separate tools with their own error rates. If your school, employer or client asks you to disclose AI help, the honest answer is still disclosure. A watermark can't show the thinking and editing you did, so keep your drafts and notes if your work may be questioned.
If you build on the API and owe your own users transparency under the AI Act, the switch is in your project or organization settings. OpenAI says cloud partners that resell its models will offer the same option in the coming weeks. Our ChatGPT review and Codex review list what each plan includes.
Try it
Edit the draft below for clarity and grammar. Keep my argument, my examples and my voice. Return two things: 1. The edited text. 2. A change log: every sentence you changed, the original and your version side by side, with one reason each. Mark any sentence you added that wasn't in my draft as NEW. Don't add facts, quotes or numbers I didn't give you. If something is unclear, ask me instead of guessing. My draft: [paste your draft]
Sources
- OpenAI, Our approach to EU text provenance rules, October 5, 2026: the rollout, detection rates, editing tests, what a watermark doesn't show
- OpenAI Help Center, Provenance signals in OpenAI-generated content: how textGrain works, no hidden characters, the API setting
- OpenAI, textGrain technical report
- TechCrunch, OpenAI will start watermarking ChatGPT's text in the EU, October 5, 2026
- Reddit, r/OpenAI announcement thread, October 5, 2026