Schema validation with Zod

Description

Build, advise on, and review runtime validation with Zod — schemas at trust boundaries (API inputs, Server Actions, route handlers, env vars, forms, webhooks, LLM output), parse vs safeParse, type inference (z.infer/z.input/z.output), coercion and transforms, refinements, discriminated unions, error formatting (z.treeifyError/z.flattenError/z.prettifyError), Zod 4 API changes from v3, zod/mini, and JSON Schema export. Use whenever zod is in package.json, code imports from "zod", the user validates input or env vars, types an API response, or hits Zod 3→4 migration errors — even if they just say "validate this form" or "make sure this payload is safe".