The CoT change is due to a new technique called recurrent depth, which essentially moves some reasoning to hidden states, allowing the "output" (or traditional CoT) to be more controlled by the model.

Some are calling it "neuralese" as reported by The Information[0][1], but I'm not seeing any sources from OpenAI beyond this tweet[2] attempting to quell the fear-mongering.

[0]https://www.theinformation.com/articles/secret-technique-beh...

[1]https://x.com/MTSlive/status/2095227056040919202

[2]https://x.com/merettm/status/2095023204993490967