What must be eaten in the sukkah?

All meals that involve bread or mezonos must be eaten in the sukkah. However, if one is only eating fruits and vegetables, or less than a k’beitza (volume of an egg) of mezonos then a sukkah is not required11. It is meritorious to eat and drink exclusively in the sukkah12. One only recites the blessing “leisheiv ba’sukkah” if there is an obligation (i.e. a meal of bread or mezonos). During a meal, one may not eat anything outside of the sukkah, even fruits and vegetables13.
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11. Shulchan Aruch (O.C. 639:2)
12. Shulchan Aruch (O.C. 639:2)
13. See Mishna Berurah in Sha’ar Ha’tziyun 639:29