As someone from the region: I also have read this a few times in different sources over my life, with no controversy about it at all. This is pretty much how it is written in the history books and it also makes sense.
I get that you want sources, but this is a veeeeery known fact, at least for europeans, I guess. That's one of the first things which gets mentioned about Switzerland if you ask others to describe it.
It's like asking for sources when someone talks about the US having been to war with [INSERT RANDOM COUNTRY].
o3 is known to use web search to reference documents directly and be perhaps a bit overly diligent when it comes to double checking itself. Of course it can still make mistakes, but for something like this that’s a lot less likely than with 4o. Besides, I didn’t reference it, I just shared the results of a query as a third party to the thread (who decided to take as much time checking the claims as the two comments above me). Although it seems that the info is all coming from Wikipedia anyway so I might as well have shared that.
As someone from the region: I also have read this a few times in different sources over my life, with no controversy about it at all. This is pretty much how it is written in the history books and it also makes sense.