Many people enjoy putting small pieces of bread into their soup, and it is common for restaurants and caterers to serve small pieces of bread together with soup. Generally, of course, before eating bread one must wash Netilat Yadayim and recite "Ha’mosi," and recite Birkat Ha’mazon afterward. Does this apply also to bread in soup, or does the change in the bread’s consistency after it is placed in soup change its status regarding the Beracha, "downgrading" it to the category of "Mezonot"?
There is a fundamental rule in Halacha that hot liquid has the capacity to cook food placed in it only when the liquid is contained in a "Keli Rishon," meaning, the original utensil in which it is cooked. Once hot liquid has been transferred to a different utensil, such as if it is poured into a bowl, it is no longer considered Halachically capable of cooking food placed into it. And thus if one places pieces of bread into soup that is already poured into a bowl, and not while it is still in its original plot, he must recite "Ha’mosi" before eating the bread and Birkat Ha’mazon afterward. Since the bread did not undergo any Halachic change, as it was placed into a Keli Sheni – "second utensil," as opposed to the original utensil in which the soup was cooked – it retains its original status as bread. (Actually, if the bread was transferred with a ladle, then the bowl might even be considered a "Keli Shelishi" – a "third utensil.")
The more complicated question is whether the bread’s status changes if it is placed into soup that is still in the original utensil in which it was heated. This occurs if the bread pieces are poured directly into the original pot, either on the fire or even after it was taken off the fire but is still very hot, or if a restaurant warms up the customer’s bowl of soup in a microwave oven before serving it to him. In such case, the pieces of bread are halachically considered as having been cooked in the soup, and the question thus becomes whether their status vis-à-vis Berachot changes as a result of their having been cooked.
The Halacha in this case depends upon the size of the pieces of bread. If the pieces are smaller than a Ke’zayit, then they are "downgraded" to a status of "Mezonot." One would thus recite "Mezonot" before eating them, and, if he ate a Ke’zayit-worth of bread, he would recite "Al Ha’mihya" after eating. If, however, the pieces are the size of a Ke’zayit or larger, then they retain their original status of bread, and thus require "Ha’mosi" and Birkat Ha’mazon. This is the explicit ruling of the Shulhan Aruch (Orah Haim 168:10).
The Halachic authorities debate the question as to which Beracha one recites if the pieces of bread were the size of Ke’zayit when they were placed into the soup, but were then broken into smaller pieces before they were eaten. The Kaf Ha’haim (168:78) writes that since the pieces of bread were cooked when they were a Ke’zayit, they are considered bread and require "Ha’mosi." The Peri Megadim, however, leaves this question unresolved. Hacham David Yosef discusses this issue extensively in his Halacha Berura, and concludes that it is best to avoid this situation, and not break the pieces in the soup if they are the size of a Ke’zayit or larger. If one did break the pieces of bread, he should recite Ha’mosi over regular bread before eating the soup, in order to avoid this Halachic conundrum.
Summary: If pieces of bread were placed into soup while the soup was still in its original pot (or in a bowl that had been heated in a microwave oven), then one recites "Mezonot" and "Al Hamihya" over the pieces of bread, unless they were the size of a Ke’zayit or larger. If the pieces were placed in the soup after it had been transferred from the original pot, then they require "Ha’mosi" and Birkat Ha’mazon like ordinary bread, regardless of their size.