# OpenAI adds optional watermarking to ChatGPT and Codex in EU

Digest AI · Policy & Regulation · published 2026-10-05T17:32:42Z

Canonical: https://digestai.news/story/openai-adds-optional-watermarking-to-chatgpt-and-codex-in-eu

## Summary

OpenAI will embed invisible watermarks in ChatGPT and Codex responses for users in the European Union, starting in the coming weeks. The move complies with the EU AI Act, which reportedly requires machine-readable labels for generated text. Unlike Anthropic, OpenAI will make watermarking optional for API users worldwide, including those on cloud partners like Microsoft Azure, which will offer it in the coming weeks as well.

The technology, called textGrain, works by embedding a statistical signal in the model’s word choices. OpenAI claims it matches or exceeds other detection methods, including Google’s SynthID, in internal tests. However, detection rates vary significantly by content length and subject matter. For example, 95% of 400-token passages about psychology were detected at a 1% false-positive rate, while math content saw substantially lower rates. Editing text—even replacing just 10% of words—reduces detection rates sharply, from about 92% to 66%. OpenAI also notes that watermarking does not affect output quality in its tests across benchmarks like GPQA Diamond or BrowseComp, though critics question whether it impacts writing quality. The company will release textGrain as open source and initially restrict detector access to researchers and specialist organizations under the EU’s Code of Practice.

## Key points

- OpenAI will watermark ChatGPT and Codex responses in the EU starting in the coming weeks
- API users worldwide can opt out of watermarking, unlike Anthropic’s mandatory global approach
- Detection rates vary by content length and subject, with edits reducing effectiveness significantly

## Why it matters

The EU’s AI Act forces compliance, and OpenAI’s optional global API watermarking could influence how other providers handle detection. Lower detection rates for edited or math-heavy text may limit its effectiveness, but open sourcing the tech could spur broader adoption and debate.

## Sources

1. [OpenAI will watermark ChatGPT text in the EU but makes it optional for API users worldwide](https://the-decoder.com/openai-will-watermark-chatgpt-text-in-the-eu-but-makes-it-optional-for-api-users-worldwide) (The Decoder, 2026-10-05)

Part of the developing story: [AI Giants Adopt Invisible Text Watermarks](https://digestai.news/thread/anthropic-s-invisible-text-watermarking-in-claude-ai) (5 stories)

## Cite

Digest AI, "OpenAI adds optional watermarking to ChatGPT and Codex in EU", 5 October 2026, https://digestai.news/story/openai-adds-optional-watermarking-to-chatgpt-and-codex-in-eu

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