OpenAI textGrain (5 Oct): invisible statistical watermark coming to ChatGPT and Codex text in the EU; API opt-in worldwide, detector limited to approved researchers
OpenAI said on 5 October 2026 how it will meet the EU AI Act requirement that generated text be machine-identifiable. Over the coming weeks it will add an invisible watermark, called textGrain, to eligible ChatGPT and Codex text output for users in the European Union across all plans; it is not a global default at launch. From 5 October, API customers anywhere can opt in to watermarked output for selected models, but it stays off by default in the API. textGrain nudges how the model samples between possible words so a keyed statistical pattern runs through a passage; it adds no hidden characters or odd spacing, and detection needs the passage plus OpenAI's secret key. Access to the detector starts with approved researchers and expert organisations only. OpenAI is candid about limits: in its 400-token tests, replacing 10% of words with synonyms cut detection from about 92% to 66%, and 25% replacement cut it to 17%; short, translated or heavily edited text may not be detected, and maths answers watermark poorly. A detected watermark does not identify the user, account or prompt, does not measure human contribution, and the absence of one does not prove human authorship. OpenAI says it plans to open-source textGrain. Primary: OpenAI post and technical report; wire: BleepingComputer 5 Oct.
- Product
- OpenAI ChatGPT and Codex (EU text output); OpenAI API (opt-in text watermarking for selected models)
- Versions
- n/a — provenance feature; no CVE
- Exploited in Australia?
- unknown
- Patch to
- Australian organisations with EU users or EU-facing content: check whether ChatGPT or Codex output you publish will carry the watermark, and decide whether to turn on API watermarking to support your own AI-transparency obligations. Do not use watermark detection, or its absence, as proof of who wrote something in misconduct, academic-integrity or fraud cases.
Primary: OpenAI — Our approach to EU text provenance rules (5 Oct 2026) · Vendor: OpenAI — textGrain technical report (5 Oct 2026) · BleepingComputer — OpenAI adding invisible watermarks to ChatGPT and Codex text in the EU (5 Oct 2026)