Agni is both the sacred fire as well as the deity of that hearth who transmits oblations to the opposite deities. Thus, this mantra is recited though invoking the deity Agni in the shape of hearth as witness for the vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃhitā in the Śukla [White] https://thedirectoryblog.com/listings13295059/the-basic-principles-of-baglamukhi