OpenAI to add invisible watermarks to EU ChatGPT text

OpenAI said on Monday, October 5, that it will start embedding an invisible watermark in eligible ChatGPT and Codex text produced in the European Union over the coming weeks. The company says the move is meant to meet the EU AI Act's requirement that AI-generated content carry machine-readable markings. The technology is called textGrain. It is the first time OpenAI has shipped a text watermark at scale, after building one in 2024 and choosing not to release it.citybiz+2

How textGrain works

In a blog post, OpenAI wrote that textGrain "adds an invisible statistical signal to the model's word choices". Readers see no label. Instead, a detector checks whether a passage follows the pattern set by a secret key. The EU watermark will apply to ChatGPT users on all plans. Starting the same day, API customers anywhere in the world can opt in for selected models, though the feature stays off by default.openai+2

The company said it is "not making text watermarking a global default at launch," according to excerpts of its announcement. Access to the detector will be limited at first to approved researchers and expert organizations, who can now apply. OpenAI also says it plans to release textGrain as open-source software.cellcog+1

Limits OpenAI acknowledges

OpenAI warned that detection can be wrong, and that editing or translating text can weaken the signal. In the company's own tests, as summarized by one analysis, swapping 10% of the words in a 400-token passage for synonyms cut the detection rate from about 92% to 66%. Swapping 25% cut it to 17%. OpenAI also said a watermark does not identify a user, reveal prompts, verify accuracy or establish ownership, and that a missing watermark does not prove a human wrote the text.citybiz+1

Critics have long argued that text watermarks are easy to remove. Software engineer Sean Goedecke wrote in July that text AI watermarks "will always be trivial to remove".seangoedecke

The regulatory clock

The AI Act's Article 50 has applied since August 2, 2026. It requires providers of generative AI to mark synthetic output, including text, so that it can be detected as artificially generated. Systems that were already on the market before that date have until December 2, 2026, to comply. That window was granted under the EU's Digital Omnibus package, and the European AI Board did not extend it at its September 17 meeting.cloudsecurityalliance

Companies that signed the Commission's transparency code of practice have also committed to offering an interoperable way to detect watermarks by February 2, 2027. Google already watermarks text through its SynthID tool, which, OpenAI says, textGrain matched or beat in internal testing.cellcog+2