npm install zod@^4.0.0

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

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

// 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() });

// 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

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

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

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

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

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

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),
]);

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

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

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

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

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

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

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();

// Find
z.record(valueSchema)

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

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

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

// 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);
  }
}

// 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

npx zod-v3-to-v4