On the day of writing, friends comment on the cake and discuss how to reply.

If your friend mentioned "cake is negotiable" in the comments, you can reply something similar to the following:

-Ha ha, yes, the cake must be discussed carefully, and don't let the birthday party down!

Thank you for your advice. We will definitely discuss and prepare delicious cakes to celebrate our birthday!

Of course, cake is an essential element for birthdays, and we will carefully select it to satisfy everyone!

Ok, let's discuss it later and see what good cakes are recommended!