Zod: Runtime Schema Validation That Infers TypeScript Types

**Zod** is a TypeScript-first schema library (`zod.dev`) that validates data at runtime and infers static types via `z.infer<typeof schema>`. It fits API handlers, form parsers, and config loaders.

Patterns: `schema.safeParse(input)` returns discriminated unions for error handling; `.parse` throws on failure. Compose objects, unions, discriminated unions, and refinements for business rules.

Pair Zod with OpenAPI generators or tRPC routers to avoid duplicating shapes. For large payloads, validate only required fields early, then stream or lazy-parse nested sections if needed.

Alternatives include Valibot and ArkType with different bundle size tradeoffs. Pin major versions and read migration guides when upgrading.