From cb8d079f49a2236f4b3407adaff873673e5c214e Mon Sep 17 00:00:00 2001
From: mtt-artis <mtt-artis@gmail.com>
Date: Wed, 28 Jan 2026 22:27:05 +0100
Subject: [PATCH 1/2] feat(createServerFn): allow FormData union type

---
 .../start-client-core/src/createServerFn.ts   |  2 +-
 .../src/tests/createServerFn.test-d.ts        | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/packages/start-client-core/src/createServerFn.ts b/packages/start-client-core/src/createServerFn.ts
index 39542d1e2a1..c3f444c28ff 100644
--- a/packages/start-client-core/src/createServerFn.ts
+++ b/packages/start-client-core/src/createServerFn.ts
@@ -468,7 +468,7 @@ export type ValidateValidatorInput<
   TMethod extends Method,
   TInputValidator,
 > = TMethod extends 'POST'
-  ? ResolveValidatorInput<TInputValidator> extends FormData
+  ? FormData extends ResolveValidatorInput<TInputValidator> 
     ? ResolveValidatorInput<TInputValidator>
     : ValidateSerializable<
         ResolveValidatorInput<TInputValidator>,
diff --git a/packages/start-client-core/src/tests/createServerFn.test-d.ts b/packages/start-client-core/src/tests/createServerFn.test-d.ts
index f0cb181a43d..2ba7f7a8fa1 100644
--- a/packages/start-client-core/src/tests/createServerFn.test-d.ts
+++ b/packages/start-client-core/src/tests/createServerFn.test-d.ts
@@ -879,3 +879,25 @@ test('createServerFn respects TsrSerializable', () => {
     Promise<{ nested: { custom: MyCustomTypeSerializable } }>
   >()
 })
+
+test('createServerFn POST with FormData union validator', () => {
+  const validator = createServerFn({ method: 'POST' }).inputValidator<
+    (input: FormData | { id: string }) => { id: 'string' }
+  >
+
+  expectTypeOf(validator)
+    .parameter(0)
+    .parameter(0)
+    .toEqualTypeOf<FormData | { id: string }>()
+})
+
+test('createServerFn GET cannot use FormData union validator', () => {
+  const validator = createServerFn({ method: 'GET' }).inputValidator<
+    (input: FormData | { id: string }) => { output: 'string' }
+  >
+
+  expectTypeOf(validator)
+    .parameter(0)
+    .parameter(0)
+    .not.toEqualTypeOf<FormData | { id: string }>()
+})

From ab4c68305176cf70f0d470891b03fd9a702d0967 Mon Sep 17 00:00:00 2001
From: mtt-artis <mtt-artis@gmail.com>
Date: Thu, 29 Jan 2026 00:07:23 +0100
Subject: [PATCH 2/2] fix(createServerFn): add regression test

---
 .../start-client-core/src/createServerFn.ts   | 13 ++++---
 .../src/tests/createServerFn.test-d.ts        | 35 ++++++++++++++++---
 2 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/packages/start-client-core/src/createServerFn.ts b/packages/start-client-core/src/createServerFn.ts
index c3f444c28ff..08e394eaf72 100644
--- a/packages/start-client-core/src/createServerFn.ts
+++ b/packages/start-client-core/src/createServerFn.ts
@@ -468,12 +468,17 @@ export type ValidateValidatorInput<
   TMethod extends Method,
   TInputValidator,
 > = TMethod extends 'POST'
-  ? FormData extends ResolveValidatorInput<TInputValidator> 
-    ? ResolveValidatorInput<TInputValidator>
-    : ValidateSerializable<
-        ResolveValidatorInput<TInputValidator>,
+  ? Extract<ResolveValidatorInput<TInputValidator>, FormData> extends never
+    ? ValidateSerializable<
+        Exclude<ResolveValidatorInput<TInputValidator>, FormData>,
         RegisteredSerializableInput<TRegister>
       >
+    :
+        | FormData
+        | ValidateSerializable<
+            Exclude<ResolveValidatorInput<TInputValidator>, FormData>,
+            RegisteredSerializableInput<TRegister>
+          >
   : ValidateSerializable<
       ResolveValidatorInput<TInputValidator>,
       RegisteredSerializableInput<TRegister>
diff --git a/packages/start-client-core/src/tests/createServerFn.test-d.ts b/packages/start-client-core/src/tests/createServerFn.test-d.ts
index 2ba7f7a8fa1..82dd3fd704d 100644
--- a/packages/start-client-core/src/tests/createServerFn.test-d.ts
+++ b/packages/start-client-core/src/tests/createServerFn.test-d.ts
@@ -896,8 +896,35 @@ test('createServerFn GET cannot use FormData union validator', () => {
     (input: FormData | { id: string }) => { output: 'string' }
   >
 
-  expectTypeOf(validator)
-    .parameter(0)
-    .parameter(0)
-    .not.toEqualTypeOf<FormData | { id: string }>()
+  expectTypeOf(validator).parameter(0).parameter(0).toEqualTypeOf<
+    | {
+        append: 'Function is not serializable'
+        delete: 'Function is not serializable'
+        get: 'Function is not serializable'
+        getAll: 'Function is not serializable'
+        has: 'Function is not serializable'
+        set: 'Function is not serializable'
+        forEach: 'Function is not serializable'
+        entries: 'Function is not serializable'
+        keys: 'Function is not serializable'
+        values: 'Function is not serializable'
+        [Symbol.iterator]: 'Function is not serializable'
+      }
+    | {
+        id: string
+      }
+  >()
+})
+
+test('createServerFn POST rejects FormData union with non-serializable', () => {
+  const validator = createServerFn({ method: 'POST' }).inputValidator<
+    (input: FormData | { func: () => void }) => { output: 'string' }
+  >
+
+  expectTypeOf(validator).parameter(0).parameter(0).toEqualTypeOf<
+    | FormData
+    | {
+        func: 'Function is not serializable'
+      }
+  >()
 })