There are two basic halachic precedents and one major distinction regarding the halachot that apply in this case. Poskim maintain differing opinions regarding in-between cases.
Rav Pappa presents[1] rules as to which foods need their own berachot and at which stages of a meal. The Rashba and others[2] infer from these rules that foods brought to the table during a meal are generally covered by the original beracha. The Beit Yosef[3] sees this as a source that berachot, including those recited outside the framework of a full meal, generally cover even foods that were not present at the time that the beracha was recited.
The Taz[4] rejects broadening this rule to include any type of eating, maintaining that specifically a meal has the special ability to subsume many foods under one beracha. Both the Taz and the Magen Avraham[5] cite a different precedent for the halacha regarding a beracha covering foods that are brought later. In the context of the discussion regarding a shochet who made a beracha before slaughtering several animals, and additional animals are then brought to him at a later point, the Tur[6] cites several opinions as to whether a new beracha is needed on the slaughter of these new animals. Some maintain that it depends if the animals are of the same species as those on which the shochet initially recited the beracha (Itur). Others conclude that it depends if the new animals were brought before all of the original animals were shechted (S’mak). Still others rule that it depends if the shochet had in mind when reciting the beracha to cover animals that were not yet present. The Shulchan Aruch[7] adopts the S’mak’s distinction, whereas the Rama[8] follows the Itur. Presumably, claim the Taz and the Magen Avraham, the parallel rules should apply to berachot on food.
However, in the context of the laws of berachot, the Shulchan Aruch and the Rama do not make such distinctions. Given that the Shulchan Aruch[9] mentions no distinctions, he apparently rules that one never makes a beracha on a newly brought food. The Rama,[10] while not necessarily disagreeing, states that one should preferably have in mind that the beracha includes all the food that will be brought. Sephardi poskim agree that the Rama’s suggestion is a good practice to follow, considering that the Beit Yosef cites differing opinions on the matter.[11]
Let us now put things in perspective. All opinions agree that if one had in mind to eat only certain foods or only certain amounts of the foods (e.g., those with dietary goals), the beracha is then limited to what he had in mind.[12] All also agree that explicit intention to cover additional foods works in all but exceptional cases. The machloket between the Taz/Magen Avraham and the accepted reading of the Shulchan Aruch applies specifically to cases in which one did not give the matter thought or did not come to a clear decision when reciting the beracha.
Based primarily on distinctions in the context of shechita, Ashkenazi poskim rule that one must say a new beracha for foods regarding which he did not originally have clear intention, unless one of the following conditions exists: 1) The new food was in front of the person when he made the beracha.[13] 2) The new food is of the exact same type as that upon which the person made the beracha.[14] 3) The person had not finished eating the original food when he decided to eat the new food.[15] 4) He originally sat down to eat a significant amount of this type of food.[16] 5) He is dependent on others to determine what he will be offered to eat (e.g., a guest).[17]
Some of these factors do not help to exempt from a beracha in certain cases:[18] 1) When the new food came from an unexpected place (e.g., a guest brought it after the beracha was made).[19] 2) If, according to the rules of the order of berachot, the beracha should have been made on the second food, then only explicit intention allows the first beracha to cover it.[20]
Since the detailed halachot are complex and difficult to remember, and many of them are subject to machloket, it is best to follow the view of the Rama and to have clear intention whenever making a beracha on one food to cover a wide variety of foods. If one has a broad intention on a regular basis, he does not need clear intention each time. (It is possible that not all poskim agree with this statement,[21] but we believe it to be the correct ruling).