Why is 'misod chachomim unevonim' sometimes said and sometimes omitted in the repetition of the Amidah?

Question

In Askenazi communities, the piece beginning 'misod chachomim unevonim' is said at the repetition of the Amidah at Shacharis when there are piyuttim (except Purim) and on Musaf on Yomim Noraim as well as, in my Machzor, at Mincha and Neilah on Yom Kippur.

I thought the reason was to establish permission to “interrupt” the Amidah with these additional prayers.

But it is not said before the piyuttim in Musaf on (1) Shabbos Shekolim and (2) Shabbos Ha'Chodesh and at the beginning of the (3) Krovets on Purim morning.

Why not?

Answer 1

Possibly the difference has to do with the structure of the piyutim in each case.

The halachah is that you're allowed to add any kind of requests, even private ones, in the middle berachos of Shemoneh Esrei (Shulchan Aruch, Orach Chaim 119:1), and you can add public ones even in the first and last ones (ibid. 112:1). In either case, though, the addition has to be thematically related to the berachah.

Arguably, then, the relatively short paragraphs inserted into the berachos, in the instances mentioned in the question, are closely related enough to their parent berachos to not need any kind of special "request for permission" (which is basically what Misod is). In fact, I have a volume of ancient piyutim (found in the Cairo genizah) for individual Shabbasos throughout the year (מחזורי שבעתות לסדרים ולפרשות, ed. Shulamit Elitzur), from which it seems that there were times and places where they used these as replacements for the original bodies of the berachos (i.e., the chazzan would begin his repetition of Shemoneh Esrei by saying a four- or five-line piyut and then continue בא"י מגן אברהם, etc.) - so that there is precedent for considering these to be basically alternate versions of the berachos rather than interpolations.

Whereas when there are longer piyutim, even though the ones inserted in the berachos do (usually) end with a phrase containing the main theme of the berachah, it may be much harder to justify them on that basis.

Answer 2

I one time heard, although I do not recall the source, that we say it whenever there is an interruption between the Bracha of Mechayei Maisim and Kedusha, and we do not say it when there is no interruption between the Bracha of Mechayei Maisim and Kedusha. Thus the times you mentioned that it is not said, there is no interruption at that point and therefore it is not necessary to say.