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IMPORTANT: Your training data about zod may be outdated or incorrect — Zod v4 introduces breaking changes to string formats, enums, error handling, and recursive types.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

Rule Categories by Priority

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

How to Use

Read individual rule files for detailed explanations and code examples:

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

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

Zod 4.6 Highlights

New in Zod 4.6 (latest: 4.6.7). Full details in the official release notes.

.validate() — cheap boolean validation

.validate() answers "is this input valid?" without building a result or a ZodError. It short-circuits on the first issue and returns a type guard on the schema's input type. Up to 35x faster than .safeParse().success on invalid input when the schema is compiled with z.compile() (5.9x uncompiled).

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

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

Use .safeParse() when you need the parsed value or the issues; use .validate() when you only need a yes/no. validateAsync() covers async refinements.

z.properties() — validate properties in place

Unlike z.object(), .properties() validates specific properties of an object in place, preserving class instances and prototypes instead of returning a plain object.

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

ZodInstanceOf.properties() is the method form; Zod Mini has z.properties() as a check.

z.withParser() — install a parser built elsewhere

z.compile() builds parsers with new Function, which a strict Content Security Policy blocks. z.withParser() installs a parser generated at build time (or by a native compiler) under the same contract, for CSP-restricted environments.

z.iban() and expanded fromJSONSchema()

z.iban() validates an electronic-format IBAN including the ISO 7064 MOD 97-10 checksum. fromJSONSchema() now also supports minProperties/maxProperties, uniqueItems, and contains/minContains/maxContains.

Two behavioral gotchas

• Error maps run lazily. Global, locale, and per-schema error maps now run on the first read of result.error, not at parse time. Code that swaps z.config() between the parse and the read gets the newer configuration. Throwing parses (.parse()) are unaffected.
• z.emoji() requires a pictograph. Component-only strings (a lone zero-width joiner, variation selector, skin tone modifier, digits) no longer pass.

Also in 4.6: @zod/mini ships as a standalone package versioned in lockstep with zod, CommonJS require() of z.validate() is ~3x faster, and a memory-retention regression in recursive schemas is fixed.

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md