Zod 4 adapter now handles top-level discriminated unions.

Author: ciscoheat

CHANGELOG.md

@@ -5,6 +5,12 @@ Headlines: Added, Changed, Deprecated, Removed, Fixed, Security
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Zod 4 adapter now handles top-level discriminated unions.
+
 ## [2.26.0] - 2025-06-03
 
 ### Added

src/lib/adapters/index.ts

@@ -15,7 +15,7 @@ export {
 	type ZodObjectTypes,
 	type ZodObjectType
 } from './zod.js';
-export { zod as zod4, zodClient as zod4Client } from './zod4.js';
+export { zod as zod4, zodClient as zod4Client, type ZodValidationSchema } from './zod4.js';
 export { vine, vineClient } from './vine.js';
 export { schemasafe, schemasafeClient } from './schemasafe.js';
 export { superstruct, superstructClient } from './superstruct.js';

src/lib/adapters/zod4.ts

@@ -1,4 +1,10 @@
-import { type $ZodObject, type $ZodErrorMap, safeParseAsync, toJSONSchema } from 'zod/v4/core';
+import {
+	type $ZodObject,
+	type $ZodErrorMap,
+	safeParseAsync,
+	toJSONSchema,
+	$ZodDiscriminatedUnion
+} from 'zod/v4/core';
 import type { JSONSchema7 } from 'json-schema';
 import {
 	type AdapterOptions,
@@ -13,6 +19,8 @@ import { memoize } from '$lib/memoize.js';
 
 type Options = NonNullable<Parameters<typeof toJSONSchema>[1]>;
 
+export type ZodValidationSchema = $ZodObject | $ZodDiscriminatedUnion<$ZodObject[]>;
+
 const defaultJSONSchemaOptions = {
 	unrepresentable: 'any',
 	override: (ctx) => {
@@ -25,11 +33,11 @@ const defaultJSONSchemaOptions = {
 } satisfies Options;
 
 /* @__NO_SIDE_EFFECTS__ */
-export const zodToJSONSchema = <S extends $ZodObject>(schema: S, options?: Options) => {
+export const zodToJSONSchema = <S extends ZodValidationSchema>(schema: S, options?: Options) => {
 	return toJSONSchema(schema, { ...defaultJSONSchemaOptions, ...options }) as JSONSchema7;
 };
 
-async function validate<T extends $ZodObject>(
+async function validate<T extends ZodValidationSchema>(
 	schema: T,
 	data: unknown,
 	error: $ZodErrorMap | undefined
@@ -48,7 +56,7 @@ async function validate<T extends $ZodObject>(
 	};
 }
 
-function _zod4<T extends $ZodObject>(
+function _zod4<T extends ZodValidationSchema>(
 	schema: T,
 	options?: AdapterOptions<Infer<T, 'zod4'>> & { error?: $ZodErrorMap; config?: Options }
 ): ValidationAdapter<Infer<T, 'zod4'>, InferIn<T, 'zod4'>> {
@@ -62,7 +70,7 @@ function _zod4<T extends $ZodObject>(
 	});
 }
 
-function _zod4Client<T extends $ZodObject>(
+function _zod4Client<T extends ZodValidationSchema>(
 	schema: T,
 	options?: { error?: $ZodErrorMap }
 ): ClientValidationAdapter<Infer<T, 'zod4'>, InferIn<T, 'zod4'>> {

src/routes/(v2)/v2/zod4-discriminated/+page.svelte

@@ -0,0 +1,77 @@
+<script lang="ts">
+	import { z } from 'zod/v4';
+	import { superForm, defaults } from '$lib/index.js';
+	import { zod as zod4 } from '$lib/adapters/zod4.js';
+
+	// ==================== VALIDATORS ====================
+
+	/**
+	 * Base validator for common UI schema properties.
+	 */
+	const BaseUiSchemaItemValidator = z.object({
+		defaultValueCompute: z.string().nullable(),
+		dynamicValueCompute: z
+			.object({
+				dependencies: z.array(z.string()),
+				fn: z.string()
+			})
+			.nullable()
+	});
+
+	/**
+	 * Text UI Schema variant
+	 */
+	const TextUiSchemaValidator = BaseUiSchemaItemValidator.extend({
+		dataType: z.literal('text'),
+		selectedWidget: z.enum(['text_input', 'textarea']).default('text_input')
+	});
+
+	/**
+	 * Boolean UI Schema variant
+	 */
+	const BoolUiSchemaValidator = BaseUiSchemaItemValidator.extend({
+		dataType: z.literal('bool'),
+		selectedWidget: z.enum(['checkbox', 'toggle']).default('checkbox')
+	});
+
+	/**
+	 * DISCRIMINATED UNION - This is what causes the error
+	 * TypeError: Cannot read properties of undefined (reading '_zod')
+	 */
+	const UiSchemaItemValidator = z.discriminatedUnion('dataType', [
+		TextUiSchemaValidator,
+		BoolUiSchemaValidator
+	]);
+
+	// ==================== SUPERFORM SETUP ====================
+
+	// THIS IS WHERE THE ERROR OCCURS
+	const data = defaults(zod4(UiSchemaItemValidator));
+	const { form, enhance } = superForm(data, {
+		SPA: true,
+		resetForm: false,
+		dataType: 'json',
+		validators: zod4(UiSchemaItemValidator),
+		id: crypto.randomUUID(),
+		validationMethod: 'submit-only',
+		onUpdate: async (e) => {
+			console.log(e.form.data);
+		}
+	});
+</script>
+
+<!-- ==================== TEMPLATE ==================== -->
+
+<form method="POST" use:enhance>
+	{#if $form.dataType === 'text'}
+		<input type="text" bind:value={$form.selectedWidget} name="selectedWidget" />
+	{:else if $form.dataType === 'bool'}
+		<select bind:value={$form.selectedWidget} name="selectedWidget">
+			<option value="checkbox">Checkbox</option>
+			<option value="toggle">Toggle</option>
+		</select>
+	{/if}
+	<div>
+		<button>Submit</button>
+	</div>
+</form>