Rashi on Genesis
אני אל שדי I AM GOD, ALMIGHTY, for I have the power to bless because the blessings are Mine.
Sforno on Genesis
אני א-ל שדי, I swear by My own name when I swear an oath. One such example is found in Deut. 32,40 ואמרתי חי אנכי לעולם, “as I have said, I, the One Who lives forever.” G’d’s oaths to Yaakov were always by the attribute Shadday.
Radak on Genesis
ויאמר...אני א-ל שדי; strong and self-sufficient, able to guide all of history, in charge of all developments. We explained this on 17,1. The three matriarchs had all been genetically barren, and G’d overcame their genetic handicaps so that each bore children. This was a display of the attribute א-ל שדי at work.