https://arstechnica.com/features/2025/06/study-metas-llama-3-1-can-recall-42-percent-of-the-first-harry-potter-book/

The claim was "Yet most AI models can recite entire Harry Potter books if prompted the right way, so that’s all bullshit."

In this test they did not get a model to produce an entire book with the right prompt.

Their measurement was considered successful if it could reproduce 50 tokens (so, less than 50 words) at a time.

The study authors took 36 books and divided each of them into overlapping 100-token passages. Using the first 50 tokens as a prompt, they calculated the probability that the next 50 tokens would be identical to the original passage. They counted a passage as “memorized” if the model had a greater than 50 percent chance of reproducing it word for word.

Even then, they didn't ACTUALLY generate these, they even admit that it would not be feasible to generate some of these 50 token (which is, at most 50 words, by the way) sequences:

the authors estimated that it would take more than 10 quadrillion samples to exactly reproduce some 50-token sequences from some books. Obviously, it wouldn’t be feasible to actually generate that many outputs.