Yes, this is very much my experience too.

Switching to GPT5 Thinking helps a little, but it often misses things that it wouldn't when I was using o3 or o1.

As an example, I asked it if there were any incidents involving Botchan in an Onsen. This is a text that is readily available and must have been trained on; in the book, Botchan goes swimming in the onsen, and then is humiliated when the next time he comes back, there is a sign saying "No swimming in the Onsen".

According to GPT5 it gives me this, which is subtly wrong.

> In the novel, when Botchan goes to Dōgo Onsen, he notes the posted rules of the bath. One of them forbids things like: > “No swimming in the bath.” (泳ぐべからず) > “No roughhousing / rowdy behavior.” (無闇に騒ぐべからず) > Botchan finds these signs funny because he’s exactly the sort of hot-headed, restless character who might be tempted to splash around or make noise. He jokes in his narration that it seems as though the rules were written specifically to keep people like him out.

Incidentally, Dogo Onsen still has the "No swimming sign", or it did when I went 10 years ago.

I feel like the value of my plus subscription went down when they released GPT-5, it feels like a downgrade from o3. But of course OpenAI being not open, there is no way for me to know now.