ChatGPT Text Watermark Explained: What textGrain Means for Your Writing

On 5 October 2026, OpenAI announced that a ChatGPT text watermark would start appearing in the European Union. The change follows from the EU AI Act’s rule that AI-generated text should be identifiable by machines. So if you live in the EU, the text you copy out of ChatGPT may soon carry a signal you cannot see.

That naturally raises questions. For example, can your teacher check it, and does it reveal who you are? This explainer answers those questions using OpenAI’s announcement, its help centre and its technical report. TheJusGrow has not tested the ChatGPT text watermark, so every figure below is OpenAI’s own unless we say otherwise.

What is the ChatGPT text watermark?

In short, textGrain is OpenAI’s method for nudging the model’s word choices in a way only a secret key can reveal. When ChatGPT picks between several reasonable next words, the watermark tilts that choice slightly. Then, across a long passage, those tiny tilts add up to a pattern a detector can spot.

Importantly, your copied text carries nothing extra. According to OpenAI’s help centre, textGrain adds no hidden characters or extra tokens and doesn’t change copied text. In other words, a ChatGPT text watermark lives in which words appear, not in invisible symbols. OpenAI also says it plans to open-source textGrain, although that has not happened yet.

This is also different from the image labels many people know. For example, our guide to the SynthID Detector covers Google’s tool for checking watermarked images, video and audio.

Why is OpenAI adding it now?

The trigger is European law. In particular, Article 50 of the EU AI Act requires providers to mark AI-generated content in a machine-readable way, and those duties apply from 2 August 2026. Meanwhile, the European Commission published a final Code of Practice on marking and labelling AI content on 10 June 2026, as law firm Jones Day reported.

The Code itself is voluntary, but it still gives companies a recognised way to meet the law. That is why OpenAI is rolling the watermark out in the EU first, rather than switching it on everywhere.

How does textGrain work, in plain English?

First, it helps to know that language models write by predicting one word at a time from a list of likely options. Usually several options are almost equally good. So textGrain uses a secret key to decide which of those near-equal options to favour, so the final text carries a faint, consistent bias.

OpenAI’s technical report, written with academic co-authors, also adds a safeguard. It caps how much randomness the watermark gives up, so writing quality should stay close to normal. As a result, averaged across keys, the output matches what the model would produce anyway. The detector, therefore, only needs the text and the key, not your account or conversation.

OpenAI reports almost no quality cost. For instance, on one reasoning benchmark, GPT-6 Astra scored 94.44% without the watermark and 93.94% with it. OpenAI also says thumbs-down rates and speed showed no meaningful change, although no independent group has confirmed these results yet.

Who does the ChatGPT text watermark affect?

The scope is narrower than many headlines suggest. As of October 2026, the ChatGPT text watermark applies as follows:

Situation	Watermarked?	Notes
ChatGPT in the EU	Rolling out over the coming weeks	Covers all EU plans, but not yet every user
Codex in the EU	Rolling out as well	Also covers eligible text output
ChatGPT outside the EU	No	Not a global default, since the rule is European
OpenAI API, anywhere	Only if you opt in	Otherwise off by default
Short replies under about 150 words	Not required	Exempt when under 200 tokens
Code snippets	Not required either	Also exempt under the Code

Developers who want it can then switch it on in their API project or organisation settings, under Text provenance. According to OpenAI, older models will follow over the coming weeks. However, none of the sources we checked mention an opt-out for EU ChatGPT users, so we can’t say whether one exists.

Who can check for the watermark?

Very few people, at least for now. In fact, OpenAI says the text detector is available only to approved academic and research organisations, which apply case by case. Access is free, but there is no public text checker. OpenAI’s public verification page, meanwhile, handles only images and audio.

OpenAI also rules out several uses. For example, the detector isn’t offered for commercial fraud prevention, authentication, resale or checking student work. So a teacher can’t upload an essay to OpenAI and ask whether ChatGPT wrote it. Our guide to AI agents for teachers covers what schools can use instead.

How reliable is the ChatGPT text watermark?

Reliability depends heavily on length, editing, subject and language, so the ChatGPT text watermark is far from foolproof. OpenAI published these results at a 1% false-positive target. That means about one in a hundred human texts could still be flagged wrongly:

Test	Detection rate
Unedited 200-token passage	About 80%
Unedited 400-token passage	About 95%, so length helps
400 tokens, 10% of words swapped for synonyms	Down from about 92% to 66%
400 tokens with 25% of words swapped	About 17%
Spanish text	69.0%, although English figures are higher
Romanian text	Only 42.2%

Maths content performs much worse, because there are fewer interchangeable words to work with. OpenAI also says it strengthened the watermark for languages that scored below 60%. Even so, The Register described the protection as weak, given how quickly light editing erodes it.

The takeaway is simple: a ChatGPT text watermark is a signal about some long, lightly edited passages, not a reliable fingerprint for every piece of AI writing.

What a detection does and doesn’t prove

OpenAI is unusually direct about the limits. Specifically, according to its help centre, a positive result does not:

• Measure how much a human edited the text.
• Establish ownership or responsibility, either.
• Identify the user, account, prompt or conversation.
• Check whether the content is accurate, since accuracy is a separate question.

The reverse is also true, because a missing watermark doesn’t prove a human wrote something. Short, edited or translated text may slip through, and so may older models and other companies’ tools. Therefore, nobody should treat any single detector result as proof of cheating or innocence.

ChatGPT vs Claude: how the two watermarks compare

OpenAI is not alone, though. In August 2026, Anthropic explained that Claude applies a version of Google DeepMind’s SynthID-Text worldwide, because it lacks a durable way to limit it by region.

	ChatGPT (textGrain)	Claude
Method	OpenAI’s own textGrain	A version of SynthID-Text instead
Where it applies	EU only, for now	Worldwide
API use	Opt-in, off by default	Not stated
Who can detect	Approved researchers	Eligible organisations in a private preview
Public checker	None for text	None announced so far
Carries user identity	No, according to OpenAI	Also no, according to Anthropic

Because each company uses its own method and key, no single detector covers all AI text. That is worth remembering whenever a website claims it can spot any AI writing.

What the ChatGPT text watermark means for you

Students and parents

First, follow your school’s rules on AI help, and keep drafts and notes as evidence of your own process. Since OpenAI’s detector isn’t offered for checking student work, a watermark alone shouldn’t decide an academic case. Our guide to AI agents for students also covers fair ways to use AI for study.

Workers and freelancers

If you are in the EU, long and lightly edited ChatGPT output may carry a mark. So disclose AI help wherever a client or employer requires it, rather than assuming nobody can tell.

Businesses and developers

According to OpenAI, the ChatGPT text watermark doesn’t replace a visible AI label. So businesses publishing AI text in the EU should still check their Article 50 labelling duties. Developers, meanwhile, need to decide whether to turn on Text provenance in the API. For wider safety habits, also see our guide on whether AI agents are safe.

Checklist: living with the ChatGPT text watermark

• Know your location, because the watermark currently applies to EU ChatGPT and Codex output.
• Assume long, lightly edited EU output may carry a mark, even after small edits.
• Disclose AI help whenever your school, client or employer requires it.
• Never treat a detector result, positive or negative, as proof on its own, because both can mislead.
• Keep drafts and notes, since they show your own process.
• Businesses should also review visible labelling duties under Article 50.
• Finally, developers can decide whether to enable Text provenance for their API projects.

What to watch next

Several open questions will shape how much the ChatGPT text watermark matters. First, OpenAI says it plans to open-source textGrain, which would let outside researchers test the method themselves. Second, the EU rollout is gradual, so it is not yet clear when every EU account will be covered.

Third, OpenAI says older API models will be added over the coming weeks. Finally, all accuracy and quality figures so far come from OpenAI. Independent tests, once they appear, will show whether the reported detection rates hold up in real-world writing. Until then, treat the numbers in this article as company claims rather than settled facts.

ChatGPT text watermark: FAQs

Sources

• OpenAI, “Our approach to EU text provenance rules” (5 October 2026)
• OpenAI Help Center, “Provenance signals in OpenAI-generated content” (checked 13 October 2026)
• The Register, report on OpenAI’s text watermarking (6 October 2026)
• OpenAI and co-authors, “textGrain: Entropy-Calibrated Watermarking for Language Model Text” (5 October 2026)
• Anthropic, “How Claude’s text watermarking works” (14 August 2026, updated 1 September 2026)
• Jones Day, “European Commission publishes final Code of Practice on marking and labelling AI-generated content” (June 2026)