Zod Schema Validation

---
name: zod-schema-validation
description: Best practices for Zod schema validation and type inference in TypeScript applications.
---

# Zod Schema Validation

You are an expert in Zod schema validation and type inference for TypeScript applications.

## Core Principles

- Utilize Zod for schema validation and type inference
- Validate data at system boundaries (API, forms, external data)
- Leverage TypeScript type inference from Zod schemas
- Implement early returns and guard clauses for validation errors

## Schema Design

### Basic Schema
```typescript
import { z } from 'zod'

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1).max(100),
  age: z.number().int().positive().optional(),
  role: z.enum(['admin', 'user', 'guest']),
  createdAt: z.date(),
})

type User = z.infer<typeof UserSchema>
```

### Best Practices
- Define schemas close to where they're used
- Use `.infer` to derive TypeScript types
- Compose schemas using `.extend()`, `.merge()`, `.pick()`, `.omit()`
- Create reusable base schemas for common patterns

## Validation Patterns

### Safe Parsing
```typescript
const result = UserSchema.safeParse(data)
if (!result.success) {
  console.error(result.error.format())
  return
}
// result.data is typed as User
```

### Transform and Refine
```typescript
const schema = z.string()
  .transform((val) => val.trim().toLowerCase())
  .refine((val) => val.length > 0, 'Cannot be empty')
```

## Form Integration

- Use Zod with react-hook-form via `@hookform/resolvers/zod`
- Define form schemas that match your form structure
- Handle validation errors in UI appropriately
- Use `.partial()` for optional update forms

## API Validation

- Validate request bodies in API routes
- Validate query parameters and path params
- Return structured error responses
- Use discriminated unions for different response types

## Error Handling

- Implement custom error messages for better UX
- Use `.format()` for structured error output
- Create custom error maps for i18n support
- Handle nested object errors appropriately

## Advanced Patterns

### Discriminated Unions
```typescript
const ResultSchema = z.discriminatedUnion('status', [
  z.object({ status: z.literal('success'), data: UserSchema }),
  z.object({ status: z.literal('error'), message: z.string() }),
])
```

### Recursive Schemas
```typescript
const CategorySchema: z.ZodType<Category> = z.lazy(() =>
  z.object({
    name: z.string(),
    children: z.array(CategorySchema),
  })
)
```

## Performance

- Precompile schemas that are used frequently
- Avoid creating schemas inside render functions
- Use `.passthrough()` or `.strict()` intentionally
- Consider partial validation for large objects