Valibot

What is it?

TypeScript schema validation library designed for minimal bundle size. Built from many small, independent functions enabling aggressive tree-shaking. Zero dependencies, runs in any JavaScript environment.

What is it for?

• Schema validation for server-side data (API endpoints, database inputs)
• Client-side form validation (SvelteKit, React, Vue)
• Configuration file validation
• Runtime type guarantees for unknown data structures

Why was it chosen?

Bundle impact: Login form example: Valibot 1.37 KB vs Zod 13.5 KB (90% reduction)

Key advantages:

• Simplest schemas start at <300 bytes
• Functional composition with pipes (pipe(string(), email(), minLength(5)))
• Modular imports (only bundles used validators)
• Full TypeScript inference (no codegen)
• Composable schemas (build complex from simple)
• Custom error messages (i18n-friendly)

Superforms integration:

• Native Valibot adapter (sveltekit-superforms/adapters)
• Automatic FormData coercion
• Schema must be defined at module top-level for caching

Known limitations

Ecosystem maturity:

• ~1 year old (newer than Zod/Yup)
• Smaller community; support primarily via GitHub discussions
• Growing library support (Superforms, NestJS, Drizzle) but not as comprehensive as Zod

Documentation:

• Identified as "biggest blocker" for adoption
• API reference finalized with 600+ pages (recent improvement)
• Still considered risky for large production projects

API differences from Zod:

• No method chaining (functional composition instead)
• v.parse(schema, data) instead of schema.parse(data)
• Single string error format vs Zod's differentiated messages
• No coerce object (requires explicit transform)
• Migration codemod available but in beta

JSON Schema:

• @valibot/to-json-schema available
• Does not support transform actions (limits practical use)

Related

• superforms.md - Form handling
• ../core/sveltekit.md - Server validation