OpenAI will watermark ChatGPT text in the EU but makes it optional for API users worldwide

OpenAI is adding invisible watermarks to ChatGPT text to comply with EU rules. Unlike Anthropic, it will let API customers worldwide turn them off.

The EU AI Act reportedly requires AI providers to label generated text in a machine-readable format. OpenAI's answer is textGrain, a technology that embeds an invisible statistical signal in the model's word choices. It works much like Claude's SynthID watermark, which uses Google's open-source technology.

OpenAI says it will turn on watermarking for ChatGPT and Codex users in the European Union over the coming weeks. API watermarking will be opt-in worldwide, unlike Anthropic's mandatory watermarking for Claude, which applies globally regardless of how users access the models. OpenAI's API watermarking will also be available through cloud partners such as Microsoft Azure in the coming weeks.

Text edits sharply reduce detection rates

OpenAI says textGrain matched or beat other approaches in internal tests, including Google's SynthID for text. The company also plans to release the technology as open source so others can build on it.

Detection rates depend heavily on text length and subject matter. With the detector set to a target false-positive rate of 1 percent, it identified watermarks in about 95 percent of 400-token passages about psychology. At 200 tokens, that fell to about 80 percent.

Detection rates were "substantially lower" for math content, where the model has less freedom to choose words, OpenAI says. Longer passages might strengthen the statistical signal and improve detection, though results could still depend on the content. OpenAI provides no data to test that assumption.

A technical report explains how textGrain works in depth. Anthropic hasn't published detection rates for Claude's watermark, though it says its system holds up well against edits.

OpenAI will limit detector access for now

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