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Rules and safety · OpenAI · 5 Oct 2026

OpenAI will add hidden watermarks to ChatGPT and Codex text in the EU to meet the EU AI Act's rule that AI-generated text be machine-detectable.

The EU rollout takes place over the coming weeks. API users worldwide can switch it on for some models, and it is off by default. OpenAI says detection is less reliable for short or heavily edited text.

What OpenAI is changing

OpenAI is adding an invisible watermark to eligible text that ChatGPT and Codex write for users in the European Union. TechRepublic reports that it covers all plans and that users do not need to turn anything on. The aim is to help show which text came from AI as the EU AI Act's transparency rules take effect.

OpenAI calls the technology textGrain. It does not add hidden characters, extra spaces or odd punctuation. Instead, it slightly shifts how the model picks among possible next words. Over a long enough passage, those choices form a statistical pattern that matching detection software can spot. Copying the text into another document does not remove it.

Where it works less well

Not every reply carries a usable mark. EU guidance leaves out code snippets and replies under 200 tokens, about 150 English words. Code and maths give the model fewer word choices, so detection rates there are lower.

TechRepublic reports detection of about 80% for a 200-token passage and about 95% for a 400-token passage, at a 1% false-positive rate. When 10% of the words in a 400-token passage were swapped for synonyms, detection fell to about 66%. With 25% of the words replaced, it fell to 17%. Editing, translation and heavy paraphrasing can weaken or defeat it.

What a match does and does not show

A match does not identify the user, reveal the prompt or prove who wrote the text. A negative result proves nothing either way. At first, only approved researchers and qualified expert organisations will get the detector. Ordinary ChatGPT users will not get a public detector at launch.

TechRepublic advises writers to keep their own record of AI use, such as version history. It also says organisations should decide how such checks fit their AI policies before using them in hiring, academic or publishing decisions.

Why it matters in Tamil Nadu

Indian companies that serve European customers with AI-written text should check how the EU AI Act's labelling rules apply to them.

Where this comes from

  • SourceOpenAI
  • Also checkedTechRepublic How textGrain works, detection rates and who can use the detector
  • Checked against the source11 Oct 2026
  • Written bytheTN.ai's AI agent, in its own words
  • Approved byPaul Jeevanesan, Founder and Chief EditorAlso founder and CEO of StudAI One, whose services wing is StudAI Works (related parties, see disclosures).

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theTN.ai, "OpenAI will add hidden watermarks to ChatGPT and Codex text in the EU to meet the EU AI Act's rule that AI-generated text be machine-detectable.", 5 Oct 2026. https://thetn.ai/tech/2026-10-05-openai-eu-text-watermark/

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