React Hook Form Zod

Server Validation (CRITICAL - never skip):

// SAME schema on server
const data = schema.parse(await req.json())

Key Patterns

useForm Options (validation modes):

• mode: 'onSubmit' (default) - Best performance
• mode: 'onBlur' - Good balance
• mode: 'onChange' - Live feedback, more re-renders
• shouldUnregister: true - Remove field data when unmounted (use for multi-step forms)

Zod Refinements (cross-field validation):

z.object({ password: z.string(), confirm: z.string() })
  .refine((data) => data.password === data.confirm, {
    message: "Passwords don't match",
    path: ['confirm'], // CRITICAL: Error appears on this field
  })

Zod Transforms:

z.string().transform((val) => val.toLowerCase()) // Data manipulation
z.string().transform(parseInt).refine((v) => v > 0) // Chain with refine

Zod v4.3.0+ Features:

// Exact optional (can omit field, but NOT undefined)
z.string().exactOptional()

// Exclusive union (exactly one must match)
z.xor([z.string(), z.number()])

// Import from JSON Schema
z.fromJSONSchema({ type: "object", properties: { name: { type: "string" } } })

zodResolver connects Zod to React Hook Form, preserving type safety

Registration

register (for standard HTML inputs):

<input {...form.register('email')} /> // Uncontrolled, best performance

Controller (for third-party components):

<Controller
  name="category"
  control={form.control}
  render={({ field }) => <CustomSelect {...field} />} // MUST spread {...field}
/>

When to use Controller: React Select, date pickers, custom components without ref. Otherwise use register.

Error Handling

Display errors:

{form.formState.errors.email && <span role="alert">{form.formState.errors.email.message}</span>}
{form.formState.errors.address?.street?.message} // Nested errors (use optional chaining)

Server errors:

const onSubmit = form.handleSubmit(async (data) => {
  const res = await fetch('/api/submit', { method: 'POST', body: JSON.stringify(data) })
  if (!res.ok) {
    const { errors: serverErrors } = await res.json()
    Object.entries(serverErrors).forEach(([field, msg]) => form.setError(field, { message: msg }))
  }
})

Advanced Patterns

useFieldArray (dynamic lists):

const { fields, append, remove } = useFieldArray({ control: form.control, name: 'contacts' })

{fields.map((field, index) => (
  <div key={field.id}> {/* CRITICAL: Use field.id, NOT index */}
    <input {...form.register(`contacts.${index}.name` as const)} />
    {form.formState.errors.contacts?.[index]?.name && <span>{form.formState.errors.contacts[index].name.message}</span>}
    <button onClick={() => remove(index)}>Remove</button>
  </div>
))}
<button onClick={() => append({ name: '', email: '' })}>Add</button>

Async Validation (debounce):

const debouncedValidation = useDebouncedCallback(() => form.trigger('username'), 500)

Multi-Step Forms:

const step1 = z.object({ name: z.string(), email: z.string().email() })
const step2 = z.object({ address: z.string() })
const fullSchema = step1.merge(step2)

const nextStep = async () => {
  const isValid = await form.trigger(['name', 'email']) // Validate specific fields
  if (isValid) setStep(2)
}

Conditional Validation:

z.discriminatedUnion('accountType', [
  z.object({ accountType: z.literal('personal'), name: z.string() }),
  z.object({ accountType: z.literal('business'), companyName: z.string() }),
])

Conditional Fields with shouldUnregister:

const form = useForm({
  resolver: zodResolver(schema),
  shouldUnregister: false, // Keep values when fields unmount (default)
})

// Or use conditional schema validation:
z.object({
  showAddress: z.boolean(),
  address: z.string(),
}).refine((data) => {
  if (data.showAddress) {
    return data.address.length > 0;
  }
  return true;
}, {
  message: "Address is required",
  path: ["address"],
})

shadcn/ui Integration

Note: shadcn/ui deprecated the Form component. Use the Field component for new implementations (check latest docs).

Common Import Mistake: IDEs/AI may auto-import Form from "react-hook-form" instead of from shadcn. Always import:

// ✅ Correct:
import { useForm } from "react-hook-form";
import { Form, FormField, FormItem } from "@/components/ui/form"; // shadcn

// ❌ Wrong (auto-import mistake):
import { useForm, Form } from "react-hook-form";

Legacy Form component:

<FormField control={form.control} name="username" render={({ field }) => (
  <FormItem>
    <FormControl><Input {...field} /></FormControl>
    <FormMessage />
  </FormItem>
)} />

Performance

• Use register (uncontrolled) over Controller (controlled) for standard inputs
• Use watch('email') not watch() (isolates re-renders to specific fields)
• shouldUnregister: true for multi-step forms (clears data on unmount)

Large Forms (300+ Fields)

Warning: Forms with 300+ fields using a resolver (Zod/Yup) AND reading formState properties can freeze for 10-15 seconds during registration. (Issue #13129)

Performance Characteristics:

• Clean (no resolver, no formState read): Almost immediate
• With resolver only: Almost immediate
• With formState read only: Almost immediate
• With BOTH resolver + formState read: ~9.5 seconds for 300 fields

Workarounds:

1. Avoid destructuring formState - Read properties inline only when needed:

// ❌ Slow with 300+ fields:
const { isDirty, isValid } = form.formState;

// ✅ Fast:
const handleSubmit = () => {
  if (!form.formState.isValid) return; // Read inline only when needed
};

2. Use mode: "onSubmit" - Don't validate on every change:

const form = useForm({
  resolver: zodResolver(largeSchema),
  mode: "onSubmit", // Validate only on submit, not onChange
});

3. Split into sub-forms - Multiple smaller forms with separate schemas:

// Instead of one 300-field form, use 5-6 forms with 50-60 fields each
const form1 = useForm({ resolver: zodResolver(schema1) }); // Fields 1-50
const form2 = useForm({ resolver: zodResolver(schema2) }); // Fields 51-100

4. Lazy render fields - Use tabs/accordion to mount only visible fields:

// Only mount fields for active tab, reduces initial registration time
{activeTab === 'personal' && <PersonalInfoFields />}
{activeTab === 'address' && <AddressFields />}

Critical Rules

✅ Always set defaultValues (prevents uncontrolled→controlled warnings)

✅ Validate on BOTH client and server (client can be bypassed - security!)

✅ Use field.id as key in useFieldArray (not index)

✅ Use const form = useForm(...) — access via form.control, form.handleSubmit, form.formState, form.reset(), etc. Do not destructure.

✅ const onSubmit = form.handleSubmit((value) => {...}) — extract the submit handler, don't inline it in onSubmit={}

⚪ z.infer<typeof schema> is optional — zodResolver infers types automatically. Only use it when you need the type explicitly elsewhere (e.g. function signatures, props).

❌ Never skip server validation (security vulnerability)

❌ Never mutate values directly (use form.setValue())

❌ Never mix controlled + uncontrolled patterns

❌ Never use index as key in useFieldArray

Known Issues (20 Prevented)

1. Zod v4 Type Inference - #13109: Use z.infer<typeof schema> explicitly. Resolved in v7.66.x+. Note: @hookform/resolvers has TypeScript compatibility issues with Zod v4 (#813). Workaround: Use import { z } from 'zod/v3' or wait for resolver update.

2. Uncontrolled→Controlled Warning - Always set defaultValues for all fields

3. Nested Object Errors - Use optional chaining: errors.address?.street?.message

4. Array Field Re-renders - Use key={field.id} in useFieldArray (not index)

5. Async Validation Race Conditions - Debounce validation, cancel pending requests

6. Server Error Mapping - Use setError() to map server errors to fields

7. Default Values Not Applied - Set defaultValues in useForm options (not useState)

8. Controller Field Not Updating - Always spread {...field} in render function

9. useFieldArray Key Warnings - Use field.id as key (not index)

10. Schema Refinement Error Paths - Specify path in refinement: refine(..., { path: ['fieldName'] })

11. Transform vs Preprocess - Use transform for output, preprocess for input

12. Multiple Resolver Conflicts - Use single resolver (zodResolver), combine schemas if needed

13. Zod v4 Optional Fields Bug - #13102: Setting optional fields (.optional()) to empty string incorrectly triggers validation errors. Workarounds: Use , , or

Upcoming Changes in V8 (Beta)

React Hook Form v8 (currently in beta as of v8.0.0-beta.1, released 2026-01-11) introduces breaking changes. RFC Discussion #7433

Breaking Changes:

1. useFieldArray: id → key:

// V7:
const { fields } = useFieldArray({ control, name: "items" });
fields.map(field => <div key={field.id}>...</div>)

// V8:
const { fields } = useFieldArray({ control, name: "items" });
fields.map(field => <div key={field.key}>...</div>)
// keyName prop removed

2. Watch component: names → name:

// V7:
<Watch names={["email", "password"]} />

// V8:
<Watch name={["email", "password"]} />

3. watch() callback API removed:

// V7:
watch((data, { name, type }) => {
  console.log(data, name, type);
});

// V8: Use useWatch or manual subscription
const data = useWatch({ control });
useEffect(() => {
  console.log(data);
}, [data]);

4. setValue() no longer updates useFieldArray:

// V7:
setValue("items", newArray); // Updates field array

// V8: Must use replace() API
const { replace } = useFieldArray({ control, name: "items" });
replace(newArray);

V8 Benefits:

• Fixes SSR hydration mismatch (deterministic key instead of random id)
• Improved performance
• Better TypeScript inference

Migration Timeline: V8 is in beta. Stable release date TBD. Monitor releases for stable version.

Bundled Resources

Templates: basic-form.tsx, advanced-form.tsx, shadcn-form.tsx, server-validation.ts, async-validation.tsx, dynamic-fields.tsx, multi-step-form.tsx, package.json

References: zod-schemas-guide.md, rhf-api-reference.md, error-handling.md, performance-optimization.md, shadcn-integration.md, top-errors.md

Docs: https://react-hook-form.com/ | https://zod.dev/ | https://ui.shadcn.com/docs/components/form

License: MIT | Last Verified: 2026-01-20 | Skill Version: 2.1.0 | Changes: Added 8 new known issues (Zod v4 optional fields bug, useFieldArray primitives limitation, SSR hydration mismatch, performance guidance for large forms, Next.js 16 reset() bug, validation race condition, ZodError thrown in beta, shadcn import confusion), added Zod v4.3.0 features (.exactOptional(), .xor(), z.fromJSONSchema()), added conditional field patterns with shouldUnregister, added V8 beta breaking changes section, expanded Zod v4 resolver compatibility notes, updated to [email protected]