OpenAI to Watermark EU ChatGPT Text, Opens API Opt-In Worldwide

The watermark catches about 80% of OpenAI text at 200 tokens and about 95% at 400, under ideal conditions. Swap a quarter of the words for synonyms and detection falls to 17%.

OpenAI set those numbers out on October 5 in its post on EU text provenance rules, announcing that eligible ChatGPT and Codex users across all plans in the European Union will start seeing an invisible watermark on generated text in the coming weeks, and that API customers worldwide can opt in from the same day. The feature is called textGrain. In the API it is off by default.

The mechanism, as FourWeekMBA summarised the technical post, embeds "a statistical signal directly into the language model's word choices": wherever several words would fit equally well, a secret key decides which one lands, and across a long enough passage the pattern becomes detectable with that key. No hidden characters are inserted. OpenAI reported no meaningful quality drop, citing 49.57 against 49.76 points on one benchmark for unwatermarked versus watermarked output.

The legal driver is Article 50 of the EU AI Act, whose transparency rules took effect on August 2. Unite.AI reported that OpenAI is opening its detector only to approved researchers and expert organisations, granted case by case, because of how easily the signal breaks. Replacing 10% of words drops detection to 66%; 25% drops it to 17%. Short passages and mathematical content fare worse still. It is the 32nd AI-detection story on our tracker in the last 90 days.

"A watermark does not measure human contribution, does not establish ownership or responsibility, does not identify the user, and does not verify accuracy," OpenAI wrote.