“Go up and prosper.” This is irony. The prophet Micaiah is mocking the false prophets who had been saying that (1 Kings 22:6, 12). What he really knew from God is in 1 Kings 22:17, which was a prophecy that the king would be killed in the battle (to understand that clearly, we have to know that culturally the king was referred to as the “shepherd” of the people, so for the people to have no shepherd meant the king had been killed). Micaiah’s prophecy came true, and Ahab was killed by an archer (1 Kings 22:34-35).
Verses like 1 Kings 22:15 require careful reading and an understanding of the context and the culture. The context reveals the irony to us because we cannot hear Micaiah’s tone of voice when he spoke. However, Ahab and Jehoshaphat did hear his voice and perhaps other gestures as well and immediately knew he was mocking the false prophets (1 Kings 22:16). Culturally, prophets did occasionally use irony to make their point emphatic (cp. Amos 4:4).