Q. Is it necessary to eat by the siyum in order to eat later, and if so what is one to do if no food is provided, as the shul is already clean for Pesach?
A. Poskim indeed write that one should attend and eat at the siyum’s seuda a least the amount of a kossebes (about an egg) and thus end his taanis. (Maharam Brisk 1: 33, Minchas Yitzchok 9: 45, Teshuvos Vehanhogos 1: 30).
If no food was provided one must procure his own.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a