What type of Casus Belli is a Joint War?

[beestar]

If I ask an AI to jointly declare on a third party, what sort of Casus Belli is it?

Formal War?
Surprise War?
The option with the lowest warmongering score?

Is it different for me and the AI?

Similarly, if I ask an AI to Join Ongoing War, which type of war declaration is triggered?

Apologies if this has been answered previously - I searched and couldn't find the answer

 

[DWilson]

Formal war

 

[beestar]

Is it Formal War for the AI as well? It seems to me I've been able to ask for a Joint War even if the AI hasn't denounced the victim, but I may be mistaken

 

[DWilson]

It is, and it doesn't require a denouncement.

 

[Roxlimn]

I do not know if the AI manages its Joint War, but you can manage your Casus Belli for going into a Joint War on the diplomacy screen. It is not always Formal War.

 

[Laurana Kanan]

It used to strictly be a "Formal War", but as of the Spring Update:

Joint Wars/Third Party War Update

• Players can now ask other players or AI to join wars they are already in.
• Trade screen allows Casus Belli to be chosen when declaring a Joint War.
• Joint War now requires one party to have denounced the enemy for 5 turns.
• Leader screen makes it clear that war declaration is part of a joint war.

 

[beestar]

It used to strictly be a "Formal War", but as of the Spring Update:

Thanks! I must have missed that UI element. Nice to see this flexibility.

Until now, to reduce warmongering penalties, I had been declaring war using a casus belli, then asking the AI to join in