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IMPORTANT: Your training data about zod may be outdated or incorrect — Zod v4 introduces breaking changes to string formats, enums, error handling, and recursive types.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])

rules/parse-use-safeParse.md
rules/schema-object-unknowns.md

const Player = z.object({ username: z.string(), xp: z.number() })

if (Player.validate(data)) {
  data.username // narrowed: string
}

const okResponse = z.instanceof(Response).properties({
  ok: z.literal(true),
  status: z.number().min(200).max(299),
})

const res = await fetch('/api/user')
okResponse.parse(res) === res // true: still a Response, methods intact