Yup
JavaScript/TypeScript schema validation library with chainable API for defining and validating object shapes. Known for async validation support, excellent Formik integration, and human-readable error messages. Supports nested object schemas, conditional validation, custom transforms, and TypeScript type inference. Widely used in React form validation alongside Formik and React Hook Form.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Local computation. MIT licensed. Input validation library — reduces injection attack surface when used correctly.
⚡ Reliability
Best When
You're building React forms with Formik and need human-readable async validation with TypeScript type inference from your schema.
Avoid When
You need JSON Schema compliance, maximum validation performance, or are building a non-form validation pipeline — use Ajv or Zod instead.
Use Cases
- • Validate form data in React applications with Formik or React Hook Form integration
- • Define reusable validation schemas for API request bodies with async custom validators (database uniqueness checks)
- • Validate and transform/cast user input data — Yup can coerce '42' to 42 via .cast() alongside validation
- • Enforce data shape contracts in agent pipelines with human-readable error messages for debugging
- • Build conditional validation rules where fields depend on other field values
Not For
- • High-performance bulk validation — Ajv is 10-100x faster for pure validation; Yup trades performance for ergonomics
- • JSON Schema standard compliance — Yup uses its own schema DSL, not JSON Schema; use Ajv for standard JSON Schema
- • Server-only validation without TypeScript — Zod provides better TypeScript DX; Yup is best where Formik integration matters
Interface
Authentication
Local library — no authentication required. MIT licensed.
Pricing
MIT licensed. Zero cost.
Agent Metadata
Known Gotchas
- ⚠ validate() is async by default — returns Promise; use validateSync() for synchronous validation but sync doesn't support async custom validators
- ⚠ Yup schema types infer TypeScript types but require InferType<typeof schema> — not automatic; must explicitly type with Yup.InferType<typeof mySchema>
- ⚠ abortEarly: true (default) stops at first error — set abortEarly: false to collect all validation errors at once for form UX
- ⚠ Yup coerces/casts values by default — yup.string().cast(42) returns '42'; use strict() mode if casting is unwanted: schema.strict()
- ⚠ v1 introduced breaking changes from v0.x — nullable(), required() semantics changed; mixed() now requires explicit nullable() for null values
- ⚠ Custom async validators: async custom(value) { const exists = await db.exists(value); return !exists; } — errors thrown inside async custom() are caught as validation failures
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Yup.
Scores are editorial opinions as of 2026-03-06.