From e3af80d2acb7028f7dad1fc24233cc48528910ca Mon Sep 17 00:00:00 2001
From: tsushanth <78000697+tsushanth@users.noreply.github.com>
Date: Tue, 30 Jun 2026 09:12:21 -0700
Subject: [PATCH] fix(form-core): check StandardSchemaV1 before FieldValidateFn
 in UnwrapFieldValidateOrFn

Arktype's `Type` implements `Callable<fn>` which makes it structurally
extend `(data: unknown) => T`. TypeScript's contravariant parameter check
means `ArkType extends FieldValidateFn<any,any,any>` evaluates to `true`
(the param check reduces to `{value:any,...} extends unknown`), so the
`StandardSchemaV1` branch was never reached and `errors` entries resolved
to `ReturnType<ArkType>` instead of `StandardSchemaV1Issue[]`.

Fix: check `StandardSchemaV1` first on both the field and form-group
branches of `UnwrapFieldValidateOrFn`. Schemas that happen to be callable
(arktype, and future StandardSchema implementations) now take the correct
path; plain function validators still fall through to the `FieldValidateFn`
branch as before.

Adds a type-level test asserting that an arktype `type('string>0')` field
validator produces `StandardSchemaV1Issue[] | undefined` errors, not the
arktype output type.

Fixes #2221
---
 packages/form-core/src/types.ts               | 20 ++++++++---------
 .../tests/standardSchemaValidator.test-d.ts   | 22 +++++++++++++++++++
 2 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/packages/form-core/src/types.ts b/packages/form-core/src/types.ts
index 62513afc2..e02a4e2af 100644
--- a/packages/form-core/src/types.ts
+++ b/packages/form-core/src/types.ts
@@ -452,17 +452,17 @@ export type UnwrapFieldValidateOrFn<
             : undefined
           : undefined
       : never)
-  | ([TValidateOrFn] extends [FieldValidateFn<any, any, any>]
-      ? ReturnType<TValidateOrFn>
-      : [TValidateOrFn] extends [StandardSchemaV1<any, any>]
-        ? // TODO: Check if `disableErrorFlat` is enabled, if so, return StandardSchemaV1Issue[][]
-          StandardSchemaV1Issue[]
+  | ([TValidateOrFn] extends [StandardSchemaV1<any, any>]
+      ? // TODO: Check if `disableErrorFlat` is enabled, if so, return StandardSchemaV1Issue[][]
+        StandardSchemaV1Issue[]
+      : [TValidateOrFn] extends [FieldValidateFn<any, any, any>]
+        ? ReturnType<TValidateOrFn>
         : undefined)
-  | ([TValidateOrFn] extends [FormGroupValidateFn<any, any, any>]
-      ? ReturnType<TValidateOrFn>
-      : [TValidateOrFn] extends [StandardSchemaV1<any, any>]
-        ? // TODO: Check if `disableErrorFlat` is enabled, if so, return StandardSchemaV1Issue[][]
-          StandardSchemaV1Issue[]
+  | ([TValidateOrFn] extends [StandardSchemaV1<any, any>]
+      ? // TODO: Check if `disableErrorFlat` is enabled, if so, return StandardSchemaV1Issue[][]
+        StandardSchemaV1Issue[]
+      : [TValidateOrFn] extends [FormGroupValidateFn<any, any, any>]
+        ? ReturnType<TValidateOrFn>
         : undefined)
 
 /**
diff --git a/packages/form-core/tests/standardSchemaValidator.test-d.ts b/packages/form-core/tests/standardSchemaValidator.test-d.ts
index 2543ec48a..9b32cc105 100644
--- a/packages/form-core/tests/standardSchemaValidator.test-d.ts
+++ b/packages/form-core/tests/standardSchemaValidator.test-d.ts
@@ -1,5 +1,6 @@
 import { describe, expectTypeOf, it } from 'vitest'
 import { z } from 'zod'
+import { type } from 'arktype'
 import { FieldApi, FormApi, standardSchemaValidators } from '../src/index'
 import type { StandardSchemaV1Issue } from '../src/index'
 
@@ -80,6 +81,27 @@ describe('standard schema validator', () => {
     >()
   })
 
+  it('Should infer StandardSchemaV1Issue[] for arktype field validators (issue #2221)', () => {
+    const schema = type({ firstName: 'string>0' })
+    const form = new FormApi({
+      defaultValues: {
+        firstName: '',
+      },
+    })
+
+    const field = new FieldApi({
+      form,
+      name: 'firstName',
+      validators: {
+        onChange: type('string>0'),
+      },
+    })
+
+    expectTypeOf(field.getMeta().errorMap.onChange).toEqualTypeOf<
+      undefined | StandardSchemaV1Issue[]
+    >()
+  })
+
   it('Should return different Standard Schema Issue types from validateAsync based on scope', () => {
     const formSourceError = standardSchemaValidators.validateAsync(
       { value: '', validationSource: 'form' },