As the mistake was done by the god Indra, he grants Hanuman a wish that his body could well be as potent as Indra's Vajra, and that his Vajra could also not damage him. In conjunction with Indra other gods have also granted him needs: the God Agni granted Hanuman https://erickpsqpl.thelateblog.com/37492368/the-smart-trick-of-hanuman-that-no-one-is-discussing