What Changed in Zod 4, and How I Migrated Production Schemas

Headline: Zod 4 is a rewrite of the TypeScript-first schema validation library, released as the stable major in 2025. Four changes hit my code directly: string formats moved to top-level functions (z.email() instead of z.string().email()), the four error options collapsed into one error parameter, error formatting moved to standalone helpers (z.flattenError, z.treeifyError, z.prettifyError), and .strict()/.passthrough() became z.strictObject()/z.looseObject(). The deprecated Zod 3 APIs still work with warnings, so I migrated incrementally.

Key takeaways

Zod 4 is the stable major of the TypeScript-first schema validator, released in 2025; it requires TypeScript 5.5 or newer.

String formats are now top-level tree-shakeable functions — z.email(), z.uuid(), z.url() — and z.string().email() is deprecated but still works.

A single error parameter replaces Zod 3's message, invalid_type_error, required_error, and errorMap.