Zod v4 best practices, patterns, and API guidance for schema validation, parsing, error handling, and type inference in TypeScript applications. Covers safeParse, object composition, refinements, transforms, codecs, branded types, v3→v4 migration, and testing schemas with Jest or Vitest. Baseline: z

Zod IMPORTANT: Your training data about zod may be outdated or incorrect — Zod v4 introduces breaking changes to string formats, enums, error handling, and recursive types. Always rely on this skill's rule files and the project's actual source code as the source of truth. Do not fall back on memorized v3 patterns when they conflict with the retrieved reference. When to Use Zod Zod is for runtime type validation — parsing untrusted data at system boundaries (API input, form data, env vars, external services). For compile time only types, plain TypeScript is sufficient. Need Recommended Tool API input/output validation Zod Form validation (React, Vue) Zod (with react hook form, formik, etc.) Env var parsing Zod (with t3 env or manual) Compile time types only Plain TypeScript Smaller bundle (~1kb) Valibot Maximum type inference ArkType Rule Categories by Priority Priority Category Impact Prefix 1 Parsing & Type Safety CRITICAL parse 2 Schema Design CRITICAL schema 3 Refinements & Transforms HIGH refine 4 Error Handling HIGH error 5 Performance & Composition MEDIUM perf 6 v4 Migration MEDIUM migrate 7 Advanced Patterns MEDIUM pattern 8 Architecture & Boundaries CRITICAL/HIGH arch 9 Observability HIGH/MEDIUM observe Quick Reference 1. Parsing & Type Safety (CRITICAL) parse use safeParse — Use safeParse() for user input instead of parse() which throws parse async required — Must use parseAsync() / safeParseAsync() when schema has async refinements parse infer types — Use z.infer<typeof Schema for output types; never manually duplicate types 2. Schema Design (CRITICAL) schema object unknowns — z.object() strips unknown keys; use strictObject or looseObject schema union discriminated — Use z.discriminatedUnion() for tagged unions, not z.union() schema coercion pitfalls — z.coerce.boolean() makes "false" → true ; use z.stringbool() schema recursive types — Use getter pattern for recursive schemas; z.lazy() is removed in v4 3. Refinements & Transforms (HIGH) refine never throw — Never throw inside .refine() or .transform() ; use ctx.addIssue() refine vs transform — .refine() for validation, .transform() for conversion, .pipe() for staging refine cross field — .superRefine() on parent object for cross field validation with path 4. Error Handling (HIGH) error custom messages — Use v4 error parameter; required error / invalid type error are removed error formatting — z.flattenError() for forms, z.treeifyError() for nested; formatError deprecated error input security — Never use reportInput: true in production; leaks sensitive data 5. Performance & Composition (MEDIUM) perf extend spread — Use { ...Schema.shape } spread over chained .extend() for large schemas perf reuse schemas — Define once, derive with .pick() , .omit() , .partial() perf zod mini — Use zod/v4/mini (1.88kb) for bundle critical client apps 6. v4 Migration (MEDIUM) migrate string formats — Use z.email() , z.uuid() , z.url() not z.string().email() migrate native enum — Use unified z.enum() for TS enums; z.nativeEnum() is deprecated migrate error api — Use error parameter everywhere; message , errorMap are removed 7. Advanced Patterns (MEDIUM) pattern branded types — .brand<"Name" () for nominal typing (USD vs EUR) pattern codecs — z.codec() for bidirectional transforms (parse + serialize) pattern pipe — .pipe() for staged parsing (string → number → validate range) 8. Architecture & Boundaries (CRITICAL/HIGH) arch boundary parsing — Parse at system boundaries (API handler, env, form, fetch); pass typed data to domain logic arch schema organization — Co locate schemas with their boundary layer; domain types use z.infer arch schema versioning — Additive changes only for non breaking evolution; new fields use .optional() 9. Observability (HIGH/MEDIUM) observe structured errors — Use z.flattenError() for compact structured logs with request correlation IDs observe error metrics — trackedSafeParse() wrapper to increment counters per schema and field on failure Schema Types Quick Reference Type Syntax String z.string() Number z.number() , z.int() , z.float() Boolean z.boolean() BigInt z.bigint() Date z.date() Undefined z.undefined() Null z.null() Void z.void() Any z.any() Unknown z.unknown() Never z.never() Literal z.literal("foo") , z.literal(42) Enum z.enum(["a", "b"]) , z.enum(TSEnum) Email z.email() URL z.url() UUID z.uuid() String→Bool z.stringbool() ISO DateTime z.iso.datetime() File z.file() JSON z.json() Array z.array(schema) Tuple z.tuple([a, b]) Object z.object({}) Strict Object z.strictObject({}) Loose Object z.looseObject({}) Record z.record(keySchema, valueSchema) Map z.map(keySchema, valueSchema) Set z.set(schema) Union z.union([a, b]) Disc. Union z.discriminatedUnion("key", [...]) Intersection z.intersection(a, b) How to Use Read individual rule files for detailed explanations and code examples: Each rule file contains: Brief explanation of why it matters Incorrect code example with explanation Correct code example with explanation Additional context and decision tables References Priority Reference When to read 1 references/schema types.md All primitives, string formats, numbers, enums, dates 2 references/parsing and inference.md parse, safeParse, z.infer, coercion 3 references/objects and composition.md Objects, arrays, unions, pick/omit/partial, recursive 4 references/refinements and transforms.md refine, superRefine, transform, pipe, defaults 5 references/error handling.md ZodError, flattenError, treeifyError, error customization 6 references/advanced features.md Codecs, branded types, JSON Schema, registries 7 references/anti patterns.md Common mistakes with BAD/GOOD examples 8 references/boundary architecture.md Where Zod fits: Express, tRPC, Next.js, React Hook Form, env, external APIs 9 references/linter and ci.md ESLint rules, CI schema snapshots, unused schema detection, circular deps 10 references/testing.md Testing schemas: safeParse in tests, boundary values, error assertions, mock data, property based testing 11 references/testing api.md Jest and Vitest API reference for schema tests 12 references/testing anti patterns.md Testing mistakes with BAD/GOOD examples Full Compiled Document For the complete guide with all rules expanded: AGENTS.md