Fixed type inference with Infer for Zod 4

Author: ciscoheat

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Possibly fixed the SuperDebug broken import on Svelte 5 in enforced runes mode [#599](https://github.com/ciscoheat/sveltekit-superforms/issues/599)
 - Zod 4 error messages should now take the current locale into account as default. [#618](https://github.com/ciscoheat/sveltekit-superforms/issues/618), [#639](https://github.com/ciscoheat/sveltekit-superforms/issues/639)
 - Zod 3 fix for URL parsing - A default boolean value of `true` returned `false` when parsing a URL with `superValidate`. [#633](https://github.com/ciscoheat/sveltekit-superforms/issues/633)
+- Fixed type inference with `Infer` for Zod 4
 
 ## [2.27.4] - 2025-10-14
 

src/lib/adapters/typeSchema.ts

@@ -15,7 +15,7 @@ import type {
 } from 'valibot';
 import type { Schema as Schema$2, InferType } from 'yup';
 import type { ZodTypeAny, input, output } from 'zod/v3';
-import type { $ZodType, input as zod4Input, output as zod4Output } from 'zod/v4/core';
+import type { $ZodType as $Zod4Type, input as zod4Input, output as zod4Output } from 'zod/v4/core';
 import type { SchemaTypes, Infer as VineInfer } from '@vinejs/vine/types';
 import type { FromSchema, JSONSchema } from 'json-schema-to-ts';
 import type { Struct, Infer as Infer$2 } from 'superstruct';
@@ -93,14 +93,20 @@ type CustomSchema<T = any> = (data: unknown) => Promise<T> | T;
 interface CustomResolver extends Resolver {
 	base: CustomSchema;
 	input: this['schema'] extends CustomSchema
-		? keyof this['schema'] extends never
-			? Awaited<ReturnType<this['schema']>>
-			: never
+		? // Exclude schemas that are Zod types (they have _def property)
+			this['schema'] extends { _def: unknown }
+			? never
+			: keyof this['schema'] extends never
+				? Awaited<ReturnType<this['schema']>>
+				: never
 		: never;
 	output: this['schema'] extends CustomSchema
-		? keyof this['schema'] extends never
-			? Awaited<ReturnType<this['schema']>>
-			: never
+		? // Exclude schemas that are Zod types (they have _def property)
+			this['schema'] extends { _def: unknown }
+			? never
+			: keyof this['schema'] extends never
+				? Awaited<ReturnType<this['schema']>>
+				: never
 		: never;
 }
 
@@ -135,11 +141,10 @@ interface ZodResolver extends Resolver {
 }
 
 interface Zod4Resolver extends Resolver {
-	base: $ZodType;
-	input: this['schema'] extends $ZodType ? zod4Input<this['schema']> : never;
-	output: this['schema'] extends $ZodType ? zod4Output<this['schema']> : never;
+	base: $Zod4Type;
+	input: this['schema'] extends $Zod4Type ? zod4Input<this['schema']> : never;
+	output: this['schema'] extends $Zod4Type ? zod4Output<this['schema']> : never;
 }
-
 interface VineResolver extends Resolver {
 	base: SchemaTypes;
 	input: this['schema'] extends SchemaTypes
@@ -229,14 +234,20 @@ export type Registry = {
 };
 
 type Infer<TSchema extends Schema, Keys extends keyof Registry = keyof Registry> = UnknownIfNever<
-	{
-		[K in Keys]: IfDefined<InferOutput<Registry[K], TSchema>>;
-	}[Keys]
+	// If schema has _zod property, it's Zod v4 - use only zod4 resolver
+	TSchema extends { _zod: unknown }
+		? IfDefined<InferOutput<Registry['zod4'], TSchema>>
+		: {
+				[K in Keys]: IfDefined<InferOutput<Registry[K], TSchema>>;
+			}[Keys]
 >;
 type InferIn<TSchema extends Schema, Keys extends keyof Registry = keyof Registry> = UnknownIfNever<
-	{
-		[K in Keys]: IfDefined<InferInput<Registry[K], TSchema>>;
-	}[Keys]
+	// If schema has _zod property, it's Zod v4 - use only zod4 resolver
+	TSchema extends { _zod: unknown }
+		? IfDefined<InferInput<Registry['zod4'], TSchema>>
+		: {
+				[K in Keys]: IfDefined<InferInput<Registry[K], TSchema>>;
+			}[Keys]
 >;
 
 /*