OpenAI will begin automatically watermarking eligible text produced by ChatGPT and Codex in the European Union over the coming weeks, the company said on October 5 in a blog post, citing transparency requirements under the EU AI Act2,3. The EU AI Act's transparency rules, which took effect on August 2, require AI companies to mark AI-generated content in a way other systems can identify.
API customers worldwide can opt in to text watermarking on supported models starting the same day, with the feature remaining off by default1. OpenAI said it is working with cloud partners to make watermarking available for OpenAI model outputs accessed through their services in the coming weeks10.
Opt-in versus on-by-default
The design choice sets OpenAI apart from Anthropic. Text watermarking will remain off by default in OpenAI's API, letting customers decide how watermarking fits their transparency obligations, the company said. Anthropic, by contrast, said it would apply watermarking globally to supported Claude models at the model level, meaning it is present regardless of which Claude product or surface the text comes from. Anthropic does not describe an equivalent opt-out for API developers, according to The New Stack. Anthropic also said it did not yet have a reliable way to limit watermarking by region.
OpenAI said watermarking can be enabled at either the project or organization level, and no changes to individual API requests are required once it is turned on. Customers who opt into textGrain do not automatically gain the ability to detect its watermark, the company said. Detector access is initially limited to approved research and academic organizations studying text provenance and detection reliability. OpenAI also said the detector will report whether it detects an OpenAI watermark without identifying the user or revealing prompts or conversations.
Detection rates and fragility
OpenAI said its detector catches 95% of watermarked 400-token passages in domains such as psychology at a target false-positive rate of 1%. For shorter 200-token passages, detection drops to around 80% at the same false-positive rate. The company cautioned that shorter passages may contain too little material for reliable identification, and that detection is lower for more constrained material such as mathematics.
The watermark is also fragile under editing. In OpenAI's tests, replacing 10% of the words in a 400-token passage with synonyms reduced detection from around 92% to 66%, and replacing 25% brought it down to 17%. Source code is particularly difficult to watermark because there are fewer plausible word choices than in ordinary prose, the company said. OpenAI nonetheless plans to automatically watermark eligible Codex text output in the EU.
OpenAI said textGrain matched or exceeded SynthID's detection performance in its testing4. The company published a technical report co-written with researchers from the University of Pennsylvania and Yale. OpenAI tested its Astra model with and without watermarking against DeepSWE, AutomationBench, and Terminal-Bench, and said it found no meaningful difference in performance. OpenAI said it saw no meaningful change in its models' performance with watermarking switched on. OpenAI said textGrain gives it more control over the balance between watermark detectability and the variety of responses generated from the same prompt.
Broader provenance stack
The text watermark joins an existing provenance toolkit. OpenAI began adding Content Credentials, based on an open standard developed by the Coalition for Content Provenance and Authenticity, to generated images in 2024. It later added Google's SynthID watermarks to supported images in May 2026 and audio in July. OpenAI now offers a Content Provenance API for checking supported images and audio for those signals.
OpenAI said it plans to open-source textGrain.
ANALYSIS The opt-in default for API customers means the vast majority of API-generated text will ship without a watermark unless individual organizations choose otherwise. The EU-specific automatic rollout for ChatGPT and Codex ties the feature's mandatory deployment directly to the EU AI Act's transparency obligations, creating a two-tier regime: watermarked consumer output in Europe, unwatermarked API output everywhere else unless developers act.