Yup schema validation library. Popular with Formik for form validation.
Use when building forms with React/Formik or needing schema-based validation.
USE WHEN: user mentions "Yup", "Formik validation", asks about "yup schema", "Formik setup", "form validation library"
DO NOT USE FOR: Zod projects (use zod skill), NestJS/class-validator (use class-validator skill), new projects (prefer Zod)