zod-validation-utilities

SKILL.md

Zod Validation Utilities

Overview

Production-ready Zod v4 patterns for reusable, type-safe validation with minimal boilerplate. Focuses on modern APIs, predictable error handling, and form integration.

When to Use

Defining request/response validation schemas in TypeScript services

Parsing untrusted input from APIs, forms, env vars, or external systems

Standardizing coercion, transforms, and cross-field validation

Building reusable schema utilities across teams

Integrating React Hook Form with Zod using zodResolver

Instructions

Start with strict object schemas and explicit field constraints

Prefer modern Zod v4 APIs and the error option for error messages

Use coercion at boundaries (z.coerce.*) when input types are uncertain

Keep business invariants in refine/superRefine close to schema definitions

Export both schema and inferred types (z.input/z.output) for consistency

Reuse utility schemas (email, id, dates, pagination) to reduce duplication

Validation Workflow

When integrating validation into an API handler or service:

Define the schema at the boundary (handler, queue, config loader)

Parse with safeParse to handle errors gracefully

Check result.success to branch on failure/success

Use result.data with full type inference in success path

Return formatted errors or proceed with validated data

See example 7 (safeParse workflow) for the complete pattern.

Examples

1) Modern Zod 4 primitives and object errors

import { z } from "zod";

export const UserIdSchema = z.uuid({ error: "Invalid user id" });
export const EmailSchema = z.email({ error: "Invalid email" });
export const WebsiteSchema = z.url({ error: "Invalid URL" });

export const UserProfileSchema = z.object(
  {
    id: UserIdSchema,
    email: EmailSchema,
    website: WebsiteSchema.optional(),
  },
  { error: "Invalid user profile payload" }
);

2) Coercion, preprocess, and transform

import { z } from "zod";

export const PaginationQuerySchema = z.object({
  page: z.coerce.number().int().min(1).default(1),
  pageSize: z.coerce.number().int().min(1).max(100).default(20),
  includeArchived: z.coerce.boolean().default(false),
});

export const DateFromUnknownSchema = z.preprocess(
  (value) => (typeof value === "string" || value instanceof Date ? value : undefined),
  z.coerce.date({ error: "Invalid date" })
);

export const NormalizedEmailSchema = z
  .string()
  .trim()
  .toLowerCase()
  .email({ error: "Invalid email" })
  .transform((value) => value as Lowercase<string>);

3) Complex schema structures

import { z } from "zod";

const TagSchema = z.string().trim().min(1).max(40);

export const ProductSchema = z.object({
  sku: z.string().min(3).max(24),
  tags: z.array(TagSchema).max(15),
  attributes: z.record(z.string(), z.union([z.string(), z.number(), z.boolean()])),
  dimensions: z.tuple([z.number().positive(), z.number().positive(), z.number().positive()]),
});

export const PaymentMethodSchema = z.discriminatedUnion("type", [
  z.object({ type: z.literal("card"), last4: z.string().regex(/^\d{4}$/) }),
  z.object({ type: z.literal("paypal"), email: z.email() }),
  z.object({ type: z.literal("wire"), iban: z.string().min(10) }),
]);

4) refine and superRefine

import { z } from "zod";

export const PasswordSchema = z
  .string()
  .min(12)
  .refine((v) => /[A-Z]/.test(v), { error: "Must include an uppercase letter" })
  .refine((v) => /\d/.test(v), { error: "Must include a number" });

export const RegisterSchema = z
  .object({
    email: z.email(),
    password: PasswordSchema,
    confirmPassword: z.string(),
  })
  .superRefine((data, ctx) => {
    if (data.password !== data.confirmPassword) {
      ctx.addIssue({
        code: "custom",
        path: ["confirmPassword"],
        message: "Passwords do not match",
      });
    }
  });

5) Optional, nullable, nullish, and default

import { z } from "zod";

export const UserPreferencesSchema = z.object({
  nickname: z.string().min(2).optional(),      // undefined allowed
  bio: z.string().max(280).nullable(),         // null allowed
  avatarUrl: z.url().nullish(),                // null or undefined allowed
  locale: z.string().default("en"),           // fallback when missing
});

6) React Hook Form integration (zodResolver)

import { useForm } from "react-hook-form";
import { zodResolver } from "@hookform/resolvers/zod";
import { z } from "zod";

const ProfileFormSchema = z.object({
  name: z.string().min(2, { error: "Name too short" }),
  email: z.email({ error: "Invalid email" }),
  age: z.coerce.number().int().min(18),
});

type ProfileFormInput = z.input<typeof ProfileFormSchema>;
type ProfileFormOutput = z.output<typeof ProfileFormSchema>;

const form = useForm<ProfileFormInput, unknown, ProfileFormOutput>({
  resolver: zodResolver(ProfileFormSchema),
  criteriaMode: "all",
});

7) Error handling workflow with safeParse

import { z } from "zod";
import type { ZodError } from "zod";

const ResultSchema = z.object({ id: z.string(), name: z.string() });

function parseAndHandle(input: unknown) {
  const result = ResultSchema.safeParse(input);

  if (!result.success) {
    const error = result.error as ZodError;
    console.error("Validation failed:", error.errors);
    return { success: false as const, error: error.format() };
  }

  return { success: true as const, data: result.data };
}

Tip: For advanced discriminated union patterns and complex React Hook Form workflows, see references/advanced-patterns.md.

Best Practices

Keep schemas near boundaries (HTTP handlers, queues, config loaders)

Prefer safeParse for recoverable flows; parse for fail-fast execution

Share small schema utilities (id, email, slug) to enforce consistency

Use z.input and z.output when transforms/coercions change runtime shape

Avoid overusing preprocess; prefer explicit z.coerce.* where possible

Treat external payloads as untrusted and always validate before use

Constraints and Warnings

Ensure examples match your installed zod major version (v4 APIs shown)

error is the preferred option for custom errors in Zod v4 patterns

Discriminated unions require a stable discriminator key across variants

Coercion can hide bad upstream data; add bounds and refinements defensively