After refreshing the browser I see that the old o3-mini-high has gone now so I continued my coding task conversation with o4-mini-high. In two separate conversations it butchered things in a way that I never saw o3-mini-high do. In one case it rewrote working code without reason, breaking it, in the other it took a function I asked it to apply a code fix to and it instead refactored it with a different and unrelated function that was part of an earlier bit of chat history.

I notice too that it employs a different style of code where it often puts assignment on a different line, which looks like it's trying to maintain an ~80 character line limit, but does so in places where the entire line of code is only about 40 characters.

Not saying it’s for sure the case but it might be that the model gets confused by OOD text from the other model whereas it expects its own text to be online from itself (particularly if the CoT is used as context for later conversations).