Or HaChaim on Genesis 18:17/אור החיים על בראשית י״ח:י״ז

1 והשם אמר המכסה אני מאברהם את אשר אני עשה G'd had said: "Should I conceal from Abraham what I am about to do?" G'd chose the word המכסה deliberately. Abraham noted that the third angel had a mission which had so far not been revealed to him, in contrast with the missions of the other two angels which had become manifest to him already. One had come to heal him and the other to tell Sarah about her impending motherhood. Moreover, when the angels departed from him, Abraham noticed that two of them walked in the direction of Sodom. Abraham accompanied those two angels to see them off. He realised that these angels still had a mission to perform on earth, something which had been concealed from him. Abraham would become aware of those angels' mission after the event. G'd was concerned lest he would feel badly in retrospect if He would not now take him into His confidence. Our sages (Tanchuma item 5 on our portion) state that the reason G'd decided to take Abraham into His confidence was because Abraham entertained doubts about the justice of G'd having brought on the deluge. He found it impossible to believe that there had not been at least ten or twenty righteous people at that time. In his opinion G'd should have suspended judgment on their account. The words אשר אני עושה then mean: "as I am used to do," i.e. I do not destroy a nation unless there are no righteous people left.

2G'd had another reason to reveal to Abraham what He had in mind for Sodom, seeing it had been Abraham who had saved the people of those cities at the risk of his own life when he rescued Lot. Inasmuch as Abraham thought that these people were safe, G'd thought He owed it to Abraham to inform him about His plans for them. He did not have to spell out what it was that He would conceal from Abraham as this is self-evident.