As the mistake was done because of the god Indra, he grants Hanuman a desire that his overall body could well be as solid 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 https://hanumanshabharmantra10345.blogkoo.com/the-smart-trick-of-panchmukhi-hanuman-that-nobody-is-discussing-52274696