What will be the formal of “Je suis d’accord avec toi.

Bonjour Harmeet,

To supplement Lovepreet's answer, here are two links:

- distinction between 'Tu' (informal) and 'Vous' (formal or informal)

- stress pronoun 'you informal' -> 'toi' /  stress pronoun 'you formal - informal' -> 'vous'

I hope this is helpful.

Bonne journée !