OpenAI’s textGrain system — an invisible statistical watermark woven into the word choices of ChatGPT and Codex output, rolling out EU-first and offered as an opt-in to API developers, so AI-written text can be identified.
Last verified · 2026-10-05 · by Moe Ameen
ChatGPT text watermarking is OpenAI’s system, branded textGrain, for marking text its models generate. Announced on October 5, 2026 with an accompanying technical report, it does not add a visible symbol. Instead it subtly shapes the model’s word choices as it writes, using a secret key to sort next-word predictions across many small nudges, leaving a statistical pattern a reader cannot perceive but a detector can recognize. Because the signal is carried in the words themselves rather than in file metadata, it survives copy-paste. The report was co-authored with researchers from the University of Pennsylvania and Yale, and OpenAI says it plans to release textGrain as open source.
The rollout is scoped. For ChatGPT and Codex, the watermark is being added for eligible users in the European Union, on all plans, over the weeks after launch. Separately, developers using OpenAI’s API anywhere in the world can turn it on for select models from day one — it is off by default. OpenAI also opened applications for access to its text-watermark detector, which at launch is limited to approved researchers and expert organizations while the company evaluates and improves it.
The driver is regulation. The EU AI Act’s Article 50 transparency rules took effect on August 2, 2026 and require providers of generative AI to mark machine-generated content so other systems can identify it. textGrain is OpenAI’s answer, launched EU-first for the consumer apps with a global opt-in for the API.
The honest framing before you rely on it: this is a provenance layer, not a content tool, and it is early. OpenAI itself notes that editing erodes the signal — replacing about 10% of words with synonyms cut detection from roughly 92% to 66%, and 25% cut it to 17% — and that short passages, math answers, and translated text are harder to detect. It stresses the mark does not measure human contribution, establish ownership, identify a user, or verify accuracy, and that a missing mark does not prove a human wrote something. Confirm current behavior and coverage in OpenAI’s own documentation before building a compliance process on it.
The most useful way to work with textGrain is to know exactly where the mark lives across your stack — and that is a question a full content engine makes concrete. textGrain is a text watermark, EU-first for ChatGPT and Codex and off-by-default on the API, so in practice most of your pipeline is either marked or unmarked depending on which engine wrote each asset. [Kompozy](/) is a full AI content generation and multi-platform publishing engine, not a chat window, so the map is clean: Kompozy generates copy with OpenAI alongside Claude, so text produced through a watermark-enabled OpenAI model can carry the signal, while its images come from gpt-image and Gemini composited through [HyperFrames](/glossary/hyperframes) — and because textGrain only marks text, it does not touch those visuals at all. Knowing that split is the difference between a clean provenance story and a guessed one.
From there, Kompozy does the job the watermark does not: it turns a draft into finished, published content. One source fans out into [18 output formats](/glossary/output-buckets) — [Blog Articles](/glossary/output-buckets), newsletters, and text posts on the copy side, plus [Carousel Posts](/glossary/output-buckets), quote cards, and avatar [Persona Shorts](/glossary/persona-shorts) — all held to your voice by the [Persona Brief](/glossary/persona-brief) so the text reads like you rather than median-prompt AI, which is what actually protects reach as detectors proliferate. The per-post review gate is where a developer or creator bakes a consistent AI-disclosure line into everything before [Autopilot](/glossary/autopilot) schedules and publishes it across the eight social platforms plus blog and email. If you build on OpenAI’s API, the opt-in watermark and a publishing engine are complementary: flip the mark on for compliance, then let Kompozy make the output on-brand and actually ship it. For the wider picture, compare it with [Claude content watermarking](/ai-tools/claude-content-watermarking).
It is textGrain, OpenAI’s invisible watermark for text, announced October 5, 2026. It shapes the model’s word choices with a secret key so ChatGPT and Codex output carries a pattern a reader cannot see but a detector can identify, and because the signal is in the words, it survives copy-paste. It is rolling out to EU users first and is available as an opt-in setting for select API models worldwide.
No. For ChatGPT and Codex it is being added for eligible EU users on all plans over the weeks after launch. On OpenAI’s API it is off by default and developers choose to enable it for select models. Outside the EU, most consumer output carries no mark unless the API toggle is turned on.
Editing weakens it. OpenAI says replacing about 10% of words with synonyms cut detection from roughly 92% to 66%, and 25% cut it to 17%; short, math, or translated text is harder to detect too. That fragility means a missing mark does not prove a human wrote the text, and no single detector should be treated as definitive.
It can, on the copy side. Kompozy generates text with OpenAI and Claude, so copy produced through a watermark-enabled OpenAI model can carry the textGrain signal. Because textGrain marks text only, it does not attach to Kompozy’s images, which come from gpt-image and Gemini via HyperFrames. The practical approach is consistent disclosure rather than trying to strip marks.
To meet the EU AI Act’s Article 50 transparency rules, which took effect on August 2, 2026 and require generative-AI output to carry machine-readable marking. OpenAI launched textGrain EU-first for ChatGPT and Codex, offered it as a global API opt-in, and says it will publish the method as open source.