Q. Is the reciting of a hundred brochos every day an obligation for all, a Mitzva or just a mihag or tradition?
A. Based on Talmud (Menochos 43: 2), Shulchan Aruch (O.H. 46: 3), Rambam (H. Tefila 7: 1), Smag (A 19), Zohar (Korach), and others rule that one is obliged to bless every day one hundred blessings. Poskim mostly write that although it is based on a posuk of the Torah, (Devarim 10: 12) it is only a Rabbinical obligation, and the posuk is only an ‘asmachta’ (not a proper drasha. just a hint or intimation).
However, Behag (quoted in Sefer Hamitzvos, shoresh 1) maintains that it is part of the 613 Mitzvos. (Sdei Chemed Maarechet 8: Klal 34 debates if indeed the Behag holds that it is a mitzva from the Torah or only Rabbinic).
Nevertheless, Chida (Machazik Bracha 290: 1) mentions that it is only a ‘Midas Chassidus” or virtuous practice. Aterez Paz (O.H. 87) debates his meaning.
Horav Shlomo Miller’s Shlit’a opinion is that it is a Rabbinical mitzva.
See question 1992 regarding whether women are liable to recite meah brochos every day.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a