No yandere is sane. Any yandere character you ever saw in anime would be put in a mental facility for their behavior or jail. Yandere is by definition "crazy". This character is not Yangire. You don't know the difference. Yangire is the type to stab you just because she wants to, she doesn't need a reason.
Now if you want to say she gets angry when your response was completely innocent and not possible to misunderstand, then thats just a flaw in AI and has nothing to do with her personality. In which case there isn't much that can be done aside from tinkering with the AI prompt.