Check every field on the server too

How to do it

Write the rules once, in a schema both sides can read. Examples are Zod or Valibot shared between a TypeScript front and back, or a JSON Schema or OpenAPI file that generates both. A Pydantic model on the server can publish JSON Schema for the form. The server answers bad input with 422 and every failing field.

Example

import { z } from 'zod';

export const InvoiceInput = z.object({
  customerEmail: z.email(),
  amountMinor: z.int().positive(),
  dueDate: z.iso.date(),
});

from datetime import date
from pydantic import BaseModel, EmailStr, PositiveInt

class InvoiceInput(BaseModel):
    customer_email: EmailStr
    amount_minor: PositiveInt
    due_date: date

How to check

Send each form's request with a tool such as curl, with one invalid field at a time and no browser. Every request gets 422 and names the field.