Lots of misinformation in this thread. Yes they have it, it’s good but it’s probably nowhere close to 99.9% accuracy.
The primary way to detect AI is to inject a fingerprint into AI generation in the first place. This means only the model creators can do that. We don’t exactly know how the fingerprint works but it can be as simple as preferring 1 word synonym over the other. For example preferring word synonyms like “illustrate”, “peer” etc. quickly ads up to a statistical
These techniques pre-date chatgpt itself and do work! However there are a lot of caveats:
The fingerprint has to be trained for each model meaning each model version performs slightly differently and only owners know the fingerprint.
The fingerprint test can only work on longer bodies of text that are not modified further.
Extending model through more complex instructions (like character, tone) or RAG can significantly decrease the effectiveness.
The industry is understandably very secretive about it but your low effort chatgpt copy/paste can be detected by OpenAI and nobody else.
As for public release of the fingerprint: they can’t as it can be reverse engineered so it’s only valuable as an internal tool for now. Also if released it would serve no real purpose as detection can be easily defeated by remixing content to dilute the fingerprint.