Runtime Type Safety with Zod: Validating at Every Boundary

TypeScript gives you confidence inside your codebase. The moment data crosses a boundary — an API response, a form submission, an environment variable, a message queue payload — that confidence evaporates. You're casting to unknown, asserting with as, and hoping the shape matches the type you declared. Zod fixes this by making the schema the source of truth for both the runtime validator and the TypeScript type.

This isn't just about validation. It's about moving the type definition to the only place where it can be verified: the boundary itself.