Jamieson, Fausset and Brown on Haggai 2:5

5. According to the word that--literally, "(I am with you) the word (or thing) which I covenanted"; that is, I am with you as I covenanted with you when ye came out of Egypt (Ex 19:5, 6; 34:10, 11). The covenant promise of God to the elect people at Sinai is an additional motive for their persevering. The Hebrew for to "covenant" is literally "to cut," alluding to the sacrificial victims cut in ratification of a covenant.

so--or, "and."

my Spirit remaineth among you--to strengthen you for the work (Hag 1:14; Zec 4:6). The inspiration of Haggai and Zechariah at this time was a specimen of the presence of God's Spirit remaining still with His people, as He had been with Moses and Israel of old (Ezr 5:1; Is 63:11).