zod vs class-validator

validation

zod

class-validator

Zod and class-validator represent two fundamentally different approaches to runtime validation in TypeScript applications. Zod is a schema-first validation library with a functional, chainable API that emphasizes type inference and framework-agnostic design. Class-validator uses a decorator-based approach that integrates tightly with class definitions and the NestJS ecosystem, making validation feel like a natural extension of object-oriented TypeScript.

This comparison matters because choosing between them often determines your application architecture and developer workflow. Zod appeals to developers who value composable schemas, functional programming patterns, and maximum TypeScript type safety across any framework. Class-validator targets NestJS developers and teams who prefer decorator-driven development with automatic validation pipelines built into their request handling.

Head-to-Head

API Design & Schema Definition→ zod wins

zod

Functional, chainable API with schema-first approach. Validation logic is separate from data structures: `z.object({ email: z.string().email() })`. Highly composable with schema transformations and refinements.

class-validator

Decorator-based approach where validation rules live directly on class properties: `@IsEmail()`. Validation is coupled to class definitions, which feels natural in OOP but less composable for functional patterns.

TypeScript Type Inference→ zod wins

zod

Exceptional type inference. Schema definitions automatically generate TypeScript types with `z.infer<typeof schema>`. Single source of truth for both runtime validation and compile-time types, eliminating duplication.

class-validator

Requires manual TypeScript type definitions on classes. Decorators provide runtime validation but no automatic type generation. You maintain validation rules and types separately, risking drift between them.

NestJS Integration→ class-validator wins

zod

Requires custom integration with NestJS. You need to manually wire Zod schemas into ValidationPipe or create custom pipes. More boilerplate but offers flexibility in how validation errors are handled.

class-validator

First-class NestJS integration through ValidationPipe. Automatic request body validation, transformation, and error handling with zero configuration. The standard choice for NestJS applications with comprehensive documentation.

Runtime Performancetie

zod

Fast for typical use cases but operates as a runtime interpreter. Each validation walks the schema tree dynamically. In high-throughput scenarios (thousands of validations per second), this interpretive overhead becomes measurable.

class-validator

Adequate performance for most applications. Uses reflection and decorators which add overhead, but cached metadata improves repeated validations. Not optimized for extreme performance but sufficient for API validation workloads.

Bundle Size & Dependencies→ zod wins

zod

Lightweight with minimal dependencies. Tree-shakeable design allows bundlers to eliminate unused validators. Approximately 12-15KB minified+gzipped for common use cases, making it suitable for browser and edge runtimes.

class-validator

Heavier footprint due to dependencies on class-transformer and reflect-metadata. Approximately 30-40KB minified+gzipped. The decorator metadata system adds unavoidable overhead, less ideal for bundle-sensitive environments.

Framework Portability→ zod wins

zod

Completely framework-agnostic. Use the same schemas across Express, Fastify, Next.js API routes, tRPC, React Hook Form, and more. Validation logic is portable across frontend and backend without modification.

class-validator

Primarily designed for NestJS and class-based backends. While technically usable elsewhere, the decorator approach feels out of place in functional frameworks. Limited value outside the NestJS ecosystem.

Verdict

For most new TypeScript projects, Zod is the stronger choice unless you're deeply committed to the NestJS ecosystem. Its superior type inference eliminates the redundancy of maintaining separate validation rules and TypeScript types, which is a massive productivity gain that compounds over time. The functional, composable API fits modern TypeScript patterns better than decorators, and its framework-agnostic nature means your validation logic remains portable as your architecture evolves. The bundle size advantage also makes Zod viable for fullstack applications where validation runs in the browser.

However, if you're building within NestJS and want to follow framework conventions, class-validator is the pragmatic choice. Its zero-config integration with ValidationPipe, automatic error formatting, and tight coupling with NestJS's request lifecycle provide immediate productivity for NestJS teams. The decorator syntax aligns with other NestJS patterns (controllers, guards, interceptors), creating a consistent codebase aesthetic. Choose class-validator when staying within NestJS best practices matters more than portability or when your team strongly prefers OOP patterns over functional composition.

When to Use Each

If you are building a Next.js application with API routes and client-side form validation → use Zod because you can share the exact same validation schemas between frontend form libraries (React Hook Form, Formik) and backend API routes, maintaining a single source of truth across your fullstack application.

If you are building a traditional NestJS REST API following framework conventions → use class-validator because it integrates seamlessly with ValidationPipe, automatically validates DTOs, and provides consistent error responses with zero custom configuration, letting you follow documented NestJS patterns.

If you are building a tRPC API with end-to-end type safety → use Zod because tRPC is built around Zod schemas for input validation, and the type inference creates automatic TypeScript types on both client and server without code generation.

If you are building microservices where bundle size matters for cold starts (AWS Lambda, Cloudflare Workers) → use Zod because its smaller footprint (12-15KB vs 30-40KB) and minimal dependencies reduce initialization time and memory consumption in serverless environments where every millisecond counts.

Learn More

zod →class-validator →

Head-to-Head

API Design & Schema Definition→ zod wins

zod

class-validator

TypeScript Type Inference→ zod wins

zod

class-validator

NestJS Integration→ class-validator wins

zod

class-validator

Runtime Performancetie

zod

class-validator

Bundle Size & Dependencies→ zod wins

zod

class-validator

Framework Portability→ zod wins

zod

class-validator

Verdict

When to Use Each