The main thing is to call the cafe in advance and clarify whether they allow outside cakes at all. Many cafes prohibit this outright because they lose dessert sales or for sanitation reasons. If they allow it, they often charge for a knife, plates, forks, or just a symbolic service fee - usually peanuts, but you need to check. Sometimes there's a condition: if you bring a cake from outside, you don't order any other desserts there.
When you sort it out, clarify a few details: what time you're planning to cut the cake, whether the staff can help serve it or if you need to do it yourself, whether you need separate place settings. It's better to bring the cake shortly before it's served - if you leave it in the cafe's kitchen in advance, it can get damaged, dry out, or just go missing. If the place is small, there might be fridge issues, especially if the cake is cream-based.
The pitfalls: some cafes just silently agree, and then show staff resentment or deliberately cause problems - they cut it crooked, serve it wrong. Best to confirm the arrangement in writing, either through a message or their social media. And don't bring a huge cake to a tiny cafe - it always looks weird and gets in the way. Your friend's birthday is worth celebrating, but it's not worth a conflict with the establishment.