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Freedom251 › Blog › AI detection

OpenAI’s Reported AI-Writing Watermark and the 99.9% Accuracy Claim

By Freedom251 Team · · 4 min read

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Short answer: The Wall Street Journal reported in August 2024 that OpenAI had developed a text-watermarking system for identifying likely ChatGPT-generated writing. The paper described an internal OpenAI document claiming 99.9% certainty. That figure has not been published as an independently reproducible accuracy benchmark, and OpenAI had not released the watermark in the public status information available from August 2024.

What OpenAI reportedly built

The reported system is a text watermark, not a conventional detector that simply learns stylistic patterns. OpenAI says the method subtly influences how a model selects successive words or tokens. Those choices create a statistical pattern that a separate test can look for later.

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The result would indicate the probability that a passage came from ChatGPT. It would not prove who typed, edited or submitted the text, and the Wall Street Journal described the reported watermark as applying to ChatGPT output rather than writing from competing AI providers.

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What “99.9% certainty” means

The 99.9% certainty figure comes from a Wall Street Journal report in 2024 describing an internal OpenAI document. Public materials do not disclose the test corpus, decision threshold, minimum passage length, language coverage, false-positive rate or independent replication for that method. It should therefore be reported as “99.9% certainty — The Wall Street Journal, 2024, describing an internal document,” not as a universally verified 99.9% accuracy rate for all AI writing.

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Why this is different from OpenAI’s old AI Classifier

OpenAI’s later watermark is often confused with its 2023 AI Classifier. They are separate systems with different evidence and failure modes.

Feature 2023 AI Classifier Reported text watermark
Signal A classifier’s judgment based on text patterns An embedded statistical pattern created during token selection
Scope Attempted to classify AI-written text from multiple systems Reportedly targeted text generated by ChatGPT
Public status Released experimentally, then discontinued on July 20, 2023 Reported as unreleased; OpenAI’s August 4, 2024 update said the company was still considering it
Published performance 26% true-positive rate on AI text and 9% false-positive rate on human text in OpenAI’s English challenge set 99.9% certainty reported by the Wall Street Journal from an internal document; matched public test details were not provided
Known weaknesses Very unreliable below 1,000 characters; weaker on code and languages other than English; vulnerable to edits and calibration errors OpenAI said global rewriting, translation and retranslation, or rewriting with another generative model could make circumvention trivial

Because the two systems were not tested on the same data or under the same definitions, their percentages cannot be compared as if they were competing benchmark scores.

Why OpenAI did not release the reported watermark

OpenAI’s August 2024 public update said the watermark was technically promising but that the company was researching alternatives and weighing important risks. The Wall Street Journal reported that the project had been debated internally for about two years and had been ready for release for roughly a year.

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Circumvention is easy after substantial rewriting

OpenAI said localized tampering may leave the watermark detectable, but broader transformations can defeat it. The company specifically cited global paraphrasing, translation followed by retranslation, rewriting with another generative model, and inserting or removing a character between words. That means a detector could work well on untouched ChatGPT output yet provide little evidence about a passage that has been substantially revised.

Risk of disproportionate effects

OpenAI also warned that a watermark could stigmatize people who use AI as a writing aid, including non-native English speakers. The company’s concern was not limited to cheating: a tool that labels text as AI-generated can affect students, job applicants, researchers and anyone whose writing is concise or formulaic.

The Wall Street Journal said an OpenAI survey found that nearly one-third of loyal ChatGPT users would be turned off by anti-cheating technology. That is an account of the company’s survey, not a population-wide estimate.

What the discontinued classifier showed about false accusations

OpenAI described its 2023 classifier as “not fully reliable” and said it should not be the primary basis for decisions. In its English challenge set, it correctly identified only 26% of AI-written text as likely AI-written while falsely labeling 9% of human-written text as AI-generated. OpenAI said performance improved with longer passages but was very unreliable below 1,000 characters.

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  • OpenAI recommended English-only use for that classifier.
  • Code and other languages performed worse.
  • Predictable writing could not be reliably attributed to a machine.
  • Editing could evade detection, and confident false labels could occur.
  • Calibration could fail when text differed from the data used to train the classifier.

Those figures describe the withdrawn classifier, not the later watermark. They nonetheless demonstrate why a detector score should be treated as a lead for review rather than proof of misconduct or authorship.

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Evidence that detector errors can be unfair

OpenAI’s educator guidance reported that its detector labeled human-written works, including Shakespeare and the Declaration of Independence, as AI-generated. It also noted possible disproportionate effects on students learning English and on people whose writing is formulaic or concise.

A 2023 study by Weixin Liang, Mert Yuksekgonul, Yining Mao, Eric Wu and James Zou evaluated seven detectors on 91 TOEFL essays by non-native English writers and 88 U.S. eighth-grade essays. Across the seven tools, the average false-positive rate on the TOEFL sample was 61.22%; the authors reported near-perfect accuracy on the eighth-grade sample. These results are specific to those detectors and datasets. They do not measure OpenAI’s later watermark and should not be generalized to every detector.

Can ChatGPT tell whether it wrote a passage?

No. OpenAI’s educator FAQ says ChatGPT has no “knowledge” of what content could be AI-generated or what it generated. Asking ChatGPT to identify its own text can produce an invented or overconfident answer, so that response is not an authorship test.

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What schools and employers should do with detector results

A watermark or classifier result can be one signal in a broader inquiry, but it should not be the sole basis for discipline, rejection or an accusation. A fair review should consider the assignment or document history, drafts, notes, version records, the writer’s explanation and whether the alleged tool has published validation for the relevant language and text length.

For the reported watermark specifically, the public record available from OpenAI’s August 4, 2024 update establishes development and continued evaluation, not a confirmed public launch. The Wall Street Journal’s 99.9% figure remains an attributed internal claim rather than an independently verified universal accuracy rate.

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AI detectionAI ethicsAI watermarkingChatGPTeducation technologyOpenAI

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