# OpenAI’s textGrain Watermarks ChatGPT and Codex Text in the EU — API Opt-In Worldwide

Times of AI Desk · 2026-10-05 · Policy

[https://timesof.ai/2026/10/openai-textgrain-eu-text-watermark](https://timesof.ai/2026/10/openai-textgrain-eu-text-watermark)

> OpenAI will roll out textGrain, an invisible statistical text watermark, to eligible ChatGPT and Codex users on all plans in the EU over coming weeks to meet AI Act transparency rules. API customers worldwide can enable it per project or organization now; it stays off by default — unlike Anthropic’s global Claude default. Light paraphrase defeats detection; a missing watermark proves nothing. The desk does not claim generated code is watermarked.

Brussels’ transparency rule is changing how frontier chat products behave in Europe — and OpenAI chose a narrower default than Anthropic.

On **October 5, 2026**, OpenAI said it will roll out **textGrain** — an invisible statistical watermark built into the model’s word choices — to **eligible ChatGPT and Codex** users on **all plans in the EU** over the coming weeks, to comply with [EU AI Act](https://timesof.ai/2026/08/eu-ai-act-enforcement-begins-august-2) transparency rules that took effect **Aug. 2**. **API** customers **worldwide** can enable it per project or organization for select models **starting now**; it stays **off by default**. That is the opposite of [Anthropic’s Claude text watermark](https://timesof.ai/2026/08/anthropic-claude-text-watermark), which Anthropic applied **globally** because it said it could not yet scope by region.

## How it works — and where it fails

textGrain nudges next-token choices under a secret key so a detector can spot a pattern in longer passages. OpenAI says it does **not** identify the user, adds **no** hidden characters, and showed **no meaningful** quality or speed change in its tests (including on Astra). Detection access is limited to approved researchers (named partners include Cornell’s John Thickstun, ETH Zurich/INSAIT’s Martin Vechev, and KInIT). OpenAI plans to **open-source** textGrain and published a technical report with Penn and Yale co-authors.

OpenAI’s own figures (via its Help Center charts as read by TechCrunch and The New Stack): about **80%** detection of **200**-token and **95%** of **400**-token passages at a **1%** false-positive rate in domains like psychology. Replacing **10%** of words cut detection from ~**92%** to **66%**; **25%** replacement: **17%**. Short text, math, code and translations are harder. Across official EU languages at 1% FPR, detection ranged from **69.0%** (Spanish) to **42.2%** (Romanian) before strength tuning.

OpenAI itself says a **missing** watermark **does not prove** human authorship — the text may be too short, heavily edited, or from another model. Light paraphrase is enough to degrade the signal.

## Code caveat

OpenAI’s Help Center states code is **harder to watermark** because there are fewer plausible next tokens, and the EU Code of Practice does not require watermarks for code snippets or outputs under ~200 tokens. The New Stack notes OpenAI has not clarified what “eligible” Codex output means or whether **generated source code** itself carries the mark. **This piece does not claim code is watermarked.**

## Limits

- Primary materials inspected: OpenAI Help Center provenance FAQ and TechCrunch / The New Stack secondaries. OpenAI’s separate “Our approach to EU text provenance rules” blog was not fetched as a standalone page.
- Detection rates and edit-sensitivity figures are OpenAI’s (charts summarized by TNS/TechCrunch).
- EU ChatGPT/Codex rollout is “coming weeks,” not a dated GA day.
- API watermarking is opt-in and model-selective; detector access is gated separately.

## Sources

- [OpenAI Help Center: Provenance signals in OpenAI-generated content](https://help.openai.com/en/articles/8912793-provenance-signals-in-openai-generated-content)
- [TechCrunch: OpenAI will start watermarking ChatGPT’s text in the EU (October 5, 2026)](https://techcrunch.com/2026/10/05/openai-will-start-watermarking-chatgpts-text-in-the-eu/)
- [The New Stack: OpenAI brings text watermarking to its API — and unlike Anthropic, it’s off by default (October 5, 2026)](https://thenewstack.io/openai-api-text-watermarking/)