Co parenting at Birthday time

So my stepson is turning 5 next month and I would really love to start planning some sort of celebration for him but I know the baby mama gets super mad & jealous when I do anything that she feels is “her job” This woman got upset that I picked up the Halloween costume and was going to get mad when she thought I had bought him a Christmas stocking. It’s not like I’m trying to beat her to the punch, she just doesn’t do the stuff and then gets mad when she finds out that I’ve already done it and she was way too late...

My question is how can I plan a birthday celebration for this little boy and not have it turn in to a problem. Ideally I would like to just leave it up to my guy to plan something but we all know that some guys are just not planning people 😂