Valibot

Valibot is a TypeScript schema validation library built around many small, independently importable functions rather than a handful of large chainable methods. Schemas are composed by piping together primitive validators and transformation actions (v.pipe(v.string(), v.email())), which lets bundlers tree-shake away everything not actually used — Valibot advertises bundle sizes starting under 1KB versus tens of kilobytes for comparable libraries like Zod.

Beyond runtime validation and parsing (parse, safeParse, is), the project is a monorepo that also ships companion packages for JSON Schema conversion (@valibot/to-json-schema) and internationalized error messages, plus a codemod for migrating existing Zod schemas. Full static type inference means a validated schema’s TypeScript type is derived automatically via InferOutput.

What You Get

• Composable schema builders (v.object, v.string, v.array, v.union, etc.) combined via v.pipe() with validation/transformation actions
• parse() (throws on failure), safeParse() (returns a result object), and is() (type guard) APIs for validating unknown data
• Full static type inference via v.InferInput/v.InferOutput, so a schema’s TypeScript type never has to be hand-written
• Companion @valibot/to-json-schema package for converting Valibot schemas to standard JSON Schema
• An official Zod-to-Valibot codemod for migrating existing schema definitions
• Internationalized error message packages for validation failures in multiple languages

Common Use Cases

• Validating incoming API request bodies or query parameters on a server with automatic TypeScript types
• Validating and parsing form input on the client with detailed, per-field error messages
• Validating environment variables and configuration files at startup
• Converting existing schemas to JSON Schema for OpenAPI docs or LLM tool-call argument validation
• Migrating an existing Zod-based codebase to a smaller-bundle alternative via the provided codemod

Under The Hood

Architecture: The monorepo’s library/ package holds the actual valibot npm package; library/src/schemas/ defines type-specific schema constructors (string, number, object, array, union, and dozens more), library/src/actions/ defines composable validation/transformation actions (email, minLength, trim, brand, etc.), and library/src/methods/ implements the parse/safeParse/is entry points that run a schema’s pipeline against input data. packages/to-json-schema and packages/i18n are separate, independently versioned workspace packages consuming the core library’s schema objects.

Tech Stack: Pure TypeScript with zero runtime dependencies, managed as a pnpm workspace (pnpm-workspace.yaml) with per-package package.json/build config, and Vitest for testing (library/src/vitest).

Code Quality: The library/src tree contains 272+ .test.ts files, essentially one per schema type and action, giving near-total coverage of the composable primitives — consistent with the README’s claim of 100% test coverage. Naming is highly consistent (each action/schema is one small file matching its exported name), and the codebase carries zero runtime dependencies, minimizing supply-chain and version-conflict risk for consumers.

API Design: The core design decision — many small single-purpose functions composed via pipe() instead of a large chainable builder object — is explicitly built to let bundlers eliminate unused code, and it keeps the public API surface predictable: every validator/action is a plain function with the same shape, so learning one teaches you the pattern for all of them.