Yup is a schema builder and validation library for JavaScript and TypeScript, designed for runtime value parsing and validation. It provides a fluent, chainable API to define schemas that can cast input values to the correct types and assert constraints. The library is built around a modular class hierarchy: a base Schema class provides common methods (validate, cast, isValid, etc.), and concrete types (StringSchema, NumberSchema, BooleanSchema, DateSchema, ArraySchema, ObjectSchema, TupleSchema) extend it with type-specific transforms and tests. MixedSchema allows arbitrary types with custom type guards. The library also supports lazy evaluation via LazySchema, conditional logic via when(), and references via Ref for cross-field validation. Yup is fully asynchronous by default but provides synchronous variants for use cases without async tests. It integrates with the Standard Schema specification for interoperability. External dependencies include property-expr for path expressions, tiny-case for string case transformations, toposort for dependency ordering, and type-fest for TypeScript utilities. The codebase is written in TypeScript with comprehensive type inference, allowing static types to be inferred from schema definitions. The architecture emphasizes immutability (each method returns a new schema instance) and composability (schemas can be concatenated, nested, and reused). Key entry points are the factory functions (string(), number(), boolean(), date(), object(), array(), tuple(), mixed(), lazy(), ref()) and the methods on schema instances (validate, cast, isValid, describe, etc.). The validation flow involves resolving conditions and references, running transforms (coercion pipeline), then executing tests (assertions). Errors are collected into ValidationError objects with detailed context.