פירוש על דברי הימים ב 18:29

disguise myself and go The meaning is: “I must make myself like a stranger and disguise myself in other clothes, so that they do not recognize that I am the king of Israel,” for he was wary of Micaiah’s words, saying, “If you return in peace, etc.” And he said to Jehoshaphat, “I wish to disguise myself and go to war, for I fear that the Arameans heard that Micaiah said, ‘If you return in peace,’ and they will attack only me.”

but you may wear your garb The royal costume and the coat of mail and the royal weapons, for there is no need for anyone to be wary of the prophet’s words except the king of Israel, for his [the king of Aram’s] spies told [him] the words of Micaiah.