Agni is each the sacred hearth and the deity of that fireplace who transmits oblations to another deities. As a result, this mantra is recited whilst invoking the deity Agni in the shape of fireplace as witness into the vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃstrikeā of your Śukla [White] Yajurveda https://directory-expert.com/listings13282207/everything-about-mantra