Zod 4 Expert Guide Skill

Zod 4 Expert Guide

Zod 4 is a major release with significant performance improvements, reduced TypeScript compilation times, and a cleaner API. This skill covers migration from v3 and idiomatic Zod 4 usage.

Quick Migration Checklist

Before diving deep, address these high-impact breaking changes:

| Change | Zod 3 | Zod 4 | |--------|-------|-------| | Record schemas | z.record(z.string()) | z.record(z.string(), z.string()) | | Strict objects | .strict() | z.strictObject({...}) | | Passthrough | .passthrough() | z.looseObject({...}) | | Error formatting | err.format() | z.treeifyError(err) | | Coerce input type | string | unknown |

Install Zod 4:

npm install zod@^4.0.0

For detailed breaking changes, see ./reference/breaking-changes.md.

Key Breaking Changes

1. z.record() Requires Two Arguments

// Zod 3 (BROKEN in v4)
z.record(z.string());

// Zod 4 (REQUIRED)
z.record(z.string(), z.string());

2. Strict/Loose Object Syntax

// Zod 3
z.object({ name: z.string() }).strict();
z.object({ name: z.string() }).passthrough();

// Zod 4
z.strictObject({ name: z.string() });
z.looseObject({ name: z.string() });

3. .default() Behavior Changed

In Zod 4, .default() short-circuits if input is undefined and returns the default directly (without parsing). Use .prefault() for the old behavior:

// Zod 4: default must match OUTPUT type
const schema = z.string()
  .transform(val => val.length)
  .default(0);  // Returns 0 directly, not parsed

// To parse the default (old behavior):
const schema = z.string()
  .transform(val => val.length)
  .prefault("tuna");  // "tuna" is parsed → 4

4. Error Handling Changes

// Zod 3
const formatted = err.format();
const flat = err.flatten();

// Zod 4
const tree = z.treeifyError(err);

// Adding issues
err.issues.push({ /* new issue */ });

5. z.coerce Input Type

const schema = z.coerce.string();
type Input = z.input<typeof schema>;
// Zod 3: string
// Zod 4: unknown

New Features

z.file() - File Validation

const fileSchema = z.file()
  .min(10_000)        // minimum bytes
  .max(1_000_000)     // maximum bytes
  .mime(["image/png", "image/jpeg"]);

z.templateLiteral() - Template Literal Types

const css = z.templateLiteral([z.number(), z.enum(["px", "em", "rem"])]);
// `${number}px` | `${number}em` | `${number}rem`

const email = z.templateLiteral([
  z.string().min(1),
  "@",
  z.string().max(64),
]);

.meta() - Schema Metadata

z.string().meta({
  id: "email_address",
  title: "Email address",
  description: "User's email",
  examples: ["user@example.com"]
});

z.globalRegistry - Global Schema Registry

z.globalRegistry.add(mySchema, {
  id: "user_schema",
  title: "User",
  description: "User data structure"
});

z.locales - Internationalization

import { z } from "zod";
import { en } from "zod/locales/en";

z.config(z.locales.en());  // Configure error messages

z.strictObject() / z.looseObject()

// Rejects unknown keys
z.strictObject({ name: z.string() });

// Allows unknown keys (passthrough)
z.looseObject({ name: z.string() });

For complete new features guide, see ./reference/new-features.md.

Zod Mini

Zod Mini (zod/mini) provides a smaller bundle with tree-shakable, functional API:

import * as z from "zod/mini";

// Functional checks instead of methods
const schema = z.pipe(
  z.string(),
  z.minLength(1),
  z.maxLength(100),
  z.regex(/^[a-z]+$/)
);

// Available functions
z.lt(value);
z.gt(value);
z.positive();
z.negative();
z.minLength(value);
z.maxLength(value);
z.regex(pattern);
z.trim();
z.toLowerCase();
z.toUpperCase();

Migration Patterns

Pattern 1: Update z.record() Calls

Search and replace:

// Find
z.record(valueSchema)

// Replace with
z.record(z.string(), valueSchema)

Pattern 2: Update Strict Objects

// Find
z.object({...}).strict()

// Replace with
z.strictObject({...})

Pattern 3: Update Error Handling

// Find
try {
  schema.parse(data);
} catch (err) {
  if (err instanceof z.ZodError) {
    const formatted = err.format();
  }
}

// Replace with
try {
  schema.parse(data);
} catch (err) {
  if (err instanceof z.ZodError) {
    const tree = z.treeifyError(err);
  }
}

Pattern 4: Fix Default Values

If using .default() with transforms, check if default matches output type:

// If this breaks:
z.string().transform(s => s.length).default("hello")

// Change to:
z.string().transform(s => s.length).prefault("hello")
// OR
z.string().transform(s => s.length).default(5)  // Match output type

For complete migration checklist, see ./reference/migration-checklist.md.

Common Issues

| Error | Cause | Fix | |-------|-------|-----| | Expected 2 arguments, got 1 | z.record() single arg | Add key schema: z.record(z.string(), ...) | | Property 'strict' does not exist | .strict() removed | Use z.strictObject() | | Property 'format' does not exist | .format() removed | Use z.treeifyError(err) | | Type mismatch on .default() | Default must match output | Use .prefault() or fix default type |

Codemod

A community-maintained codemod is available:

npx zod-v3-to-v4

This automates many of the breaking change fixes.

Agent Skills: Zod 4 Expert Guide

Install this agent skill to your local

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