At most one validation library

Two validation libraries produce two schema syntaxes, two inferred-type systems, two error-object shapes, and two ways to compose refinements. A backend route validated with Zod cannot share its schema with a frontend form validated by Yup, so every endpoint has its contract defined twice and they drift the moment someone adds a field to only one side. Error rendering components branch on library-specific shapes, and TypeScript type narrowing works on one side but not the other.

Standardize on one library — Zod if you want TypeScript-first inference, Yup if you have deep Formik-era code. Colocate every schema in src/lib/schemas/ and import from both client and server code.

// src/lib/schemas/login.ts
import { z } from 'zod'
export const LoginSchema = z.object({
  email: z.string().email(),
  password: z.string().min(8),
})
export type LoginInput = z.infer<typeof LoginSchema>

Remove the loser with npm uninstall yup @hookform/resolvers.