Form validation without Formik or React Hook Form: treat your rules as domain logic

We've all been here. A new form shows up, you install React Hook Form, add Zod or Yup, and in ten minutes you have something that "works." The problem doesn't surface that day. It surfaces three months later, when the same VIN you validate in the create car form also has to be validated in edit , in import from Excel , and it turns out the rule —"17 characters, the last 5 numeric"— is written three times, each one slightly different, and none of them lives in a place you can point to and say "here is what a valid VIN is." A typical form with a library looks roughly like this: const schema = z . object ({ vin : z . string (). length ( 17 , " The VIN must be 17 characters " ), miles : z . number (). min ( 0 , " Miles cannot be negative " ), // ...and 8 more fields }); const { register , handleSubmit , watch , formState : { errors }, } = useForm ({ resolver : zodResolver ( schema ), }); It works. But if you stop to look at it, you're paying three costs that almost never get named: 1. Clean code dissolves. The business rule ends up scattered across the schema , the resolver , the register calls, the Controller s, and the JSX. The knowledge — what makes a car valid — has no home. It's wired into the UI. And what's wired into the UI doesn't get reused: it gets copied. 2. Performance and coupling are paid silently. These libraries live on subscriptions: watch , re-renders on every keystroke, internal state to keep in sync. For a contact form, who cares. For a screen with 15 fields, sub-forms, and cross-field validation, your component is tied to the library's lifecycle —not yours— and you start fighting it instead of using it. 3. Developer convenience is a trap. It's wonderfully convenient at first . But that same rule: how do you test it without mounting a component? How do you move it to the backend? How do you translate it into two languages without polluting the schema? Everything the library gave you for free, it charges you for the day you need to step outside its mo