ब्रुवो वचिः

Operational rule vidhi

वचि is the substitute of ब्रू 'to speak' when any आर्धधातुक affix is to be supplied.

Word by word (padaccheda)

Target ब्रुवः ṣaṣṭhī · ekavacanaSubstitute / term वचिः prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

ब्रुवः वचिः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

ब्रुवः → वचिः

Replace bruvaḥ with vaciḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

वचि is the substitute of ब्रू 'to speak' when any आर्धधातुक affix is to be supplied.
वक्ता 'he will speak,' वक्तुम् 'in order to speak.' The इ of वचि is for the sake of articulation only. The substitute being like the principal, वच् is Atmanepadi when the fruit of the action accrues to the agent, as ऊचे 'he spoke,' वक्ष्यते 'he will speak.'

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.