Zod vs io-ts vs Valibot: TypeScript validation library comparison for 2026

Three libraries. Same problem. Different tradeoffs. Here's which one to use based on what you're actually building.

The problem they solve

You have data from an external source (API response, form input, URL params, database query). TypeScript types disappear at runtime. You need to validate that the data matches what your code expects.

// TypeScript says this is User
const user: User = await fetch('/api/user').then(r => r.json());
// But at runtime? Could be anything. Could be { error: "not found" }.

All three libraries solve this: define a schema, validate data against it, get typed output.

Zod: the default choice

import { z } from 'zod';

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1).max(100),
  role: z.enum(['admin', 'user', 'guest']),
  metadata: z.record(z.string()).optional(),
});

type User = z.infer<typeof UserSchema>;

// Validate
const user = UserSchema.parse(unknownData); // throws on invalid
const result = UserSchema.safeParse(unknownData); // returns { success, data?, error? }

Pros:

• Largest ecosystem — used by tRPC, React Hook Form, AI SDK, Prisma, most Next.js apps
• Best TypeScript inference — z.infer<> works perfectly
• Readable, chainable API
• Good error messages out of the box
• Transforms (.transform(), .refine(), .preprocess())

Cons:

• 13KB minified — not the smallest
• No tree-shaking (you import the whole library)
• Some advanced type inference can be slow in large schemas

Use Zod when: You're building a web app with Next.js, tRPC, or any modern TypeScript stack. It's the ecosystem default and integrates with everything.

io-ts: the functional programming choice

import * as t from 'io-ts';
import { isRight } from 'fp-ts/Either';

const UserCodec = t.type({
  id: t.string,
  email: t.string,
  name: t.string,
  role: t.union([t.literal('admin'), t.literal('user'), t.literal('guest')]),
});

type User = t.TypeOf<typeof UserCodec>;

// Validate — returns Either<Errors, User>
const result = UserCodec.decode(unknownData);
if (isRight(result)) {
  const user = result.right; // typed as User
}

Pros:

• Functional programming model (Either, pipe, chain)
• Bidirectional codecs — can encode AND decode
• Composable with fp-ts ecosystem
• Battle-tested (oldest of the three)

Cons:

• Verbose syntax
• Requires fp-ts knowledge
• Smaller ecosystem of integrations
• Error messages require custom reporters
• Learning curve is steep if you don't know fp-ts

Use io-ts when: Your team uses fp-ts, your codebase is functional-first, or you need bidirectional encoding (serialize AND deserialize with the same schema).

Valibot: the bundle-size champion

import * as v from 'valibot';

const UserSchema = v.object({
  id: v.pipe(v.string(), v.uuid()),
  email: v.pipe(v.string(), v.email()),
  name: v.pipe(v.string(), v.minLength(1), v.maxLength(100)),
  role: v.picklist(['admin', 'user', 'guest']),
  metadata: v.optional(v.record(v.string(), v.string())),
});

type User = v.InferOutput<typeof UserSchema>;

// Validate
const result = v.safeParse(UserSchema, unknownData);
if (result.success) {
  const user = result.output; // typed as User
}

Pros:

• ~1KB minified (vs 13KB for Zod) — fully tree-shakeable
• Modular architecture — only import what you use
• API is intentionally similar to Zod (easy migration)
• Fast — benchmarks show 2-5x faster than Zod for complex schemas
• Growing ecosystem support

Cons:

• Newer — less ecosystem integration than Zod
• v.pipe() syntax is slightly more verbose than Zod's chaining
• Some tRPC/React Hook Form integrations require adapters
• Smaller community (fewer Stack Overflow answers)

Use Valibot when: Bundle size matters (edge functions, mobile web, Cloudflare Workers), you need maximum performance, or you want Zod-like DX with better tree-shaking.

The comparison table

Feature	Zod	io-ts	Valibot
Bundle size	13KB	8KB	~1KB
Tree-shakeable	No	Partial	Yes
TypeScript inference	Excellent	Good	Excellent
Error messages	Great	Requires reporter	Good
Ecosystem	Largest	fp-ts ecosystem	Growing
Learning curve	Low	High	Low
Performance	Good	Good	Best
Transforms	Built-in	Via pipes	Via pipes
tRPC integration	Native	Manual	Adapter
React Hook Form	Native	Manual	Adapter

Migration: Zod to Valibot

If you're considering the switch, it's mostly mechanical:

// Zod
const schema = z.object({
  name: z.string().min(1),
  age: z.number().int().positive(),
  email: z.string().email().optional(),
});

// Valibot equivalent
const schema = v.object({
  name: v.pipe(v.string(), v.minLength(1)),
  age: v.pipe(v.number(), v.integer(), v.minValue(1)),
  email: v.optional(v.pipe(v.string(), v.email())),
});

The API surface is similar enough that mechanical translation works for 90% of schemas. The remaining 10% (custom refinements, transforms, discriminated unions) requires more thought.

My recommendation

Default choice for 2026: Zod. The ecosystem integration is unmatched. tRPC, AI SDK, React Hook Form, Prisma — everything works with Zod natively. You'll spend less time on integration and more time on your product.

Switch to Valibot when: You're deploying to edge runtimes where 12KB matters, you have 50+ schemas and performance is measurable, or you're starting a new project and want to bet on the future.

Use io-ts only when: You're deep in the fp-ts ecosystem and functional programming is a team strength. Otherwise, the learning curve isn't worth it.

The validation schemas in the AI SaaS Starter Kit use Zod — it integrates with every part of the stack (tRPC, React Hook Form, AI SDK, Stripe webhook validation) without adapters.

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