From 0ce7167fed7e794444aa48bc74b3460f80c831bd Mon Sep 17 00:00:00 2001
From: Caleb Cox <canac@users.noreply.github.com>
Date: Sun, 4 Jan 2026 08:36:47 -0600
Subject: [PATCH 1/3] fix(react-start): useServerFn returns undefined after
 redirect

---
 .../basic-auth/src/hooks/useMutation.ts            | 14 ++++++++------
 .../start-supabase-basic/src/hooks/useMutation.ts  | 14 ++++++++------
 packages/react-start/src/useServerFn.ts            |  2 +-
 3 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/e2e/react-start/basic-auth/src/hooks/useMutation.ts b/e2e/react-start/basic-auth/src/hooks/useMutation.ts
index 1ff7a4653bd..dbbf994c98d 100644
--- a/e2e/react-start/basic-auth/src/hooks/useMutation.ts
+++ b/e2e/react-start/basic-auth/src/hooks/useMutation.ts
@@ -1,7 +1,7 @@
 import * as React from 'react'
 
 export function useMutation<TVariables, TData, TError = Error>(opts: {
-  fn: (variables: TVariables) => Promise<TData>
+  fn: (variables: TVariables) => Promise<TData | void>
   onSuccess?: (ctx: { data: TData }) => void | Promise<void>
 }) {
   const [submittedAt, setSubmittedAt] = React.useState<number | undefined>()
@@ -20,11 +20,13 @@ export function useMutation<TVariables, TData, TError = Error>(opts: {
       //
       try {
         const data = await opts.fn(variables)
-        await opts.onSuccess?.({ data })
-        setStatus('success')
-        setError(undefined)
-        setData(data)
-        return data
+        if (data) {
+          await opts.onSuccess?.({ data })
+          setStatus('success')
+          setError(undefined)
+          setData(data)
+          return data
+        }
       } catch (err: any) {
         setStatus('error')
         setError(err)
diff --git a/examples/react/start-supabase-basic/src/hooks/useMutation.ts b/examples/react/start-supabase-basic/src/hooks/useMutation.ts
index afe59694d43..f5f94e35d02 100644
--- a/examples/react/start-supabase-basic/src/hooks/useMutation.ts
+++ b/examples/react/start-supabase-basic/src/hooks/useMutation.ts
@@ -1,7 +1,7 @@
 import * as React from 'react'
 
 export function useMutation<TVariables, TData, TError = Error>(opts: {
-  fn: (variables: TVariables) => Promise<TData>
+  fn: (variables: TVariables) => Promise<TData | void>
   onSuccess?: (ctx: { data: TData }) => void | Promise<void>
 }) {
   const [submittedAt, setSubmittedAt] = React.useState<number | undefined>()
@@ -20,11 +20,13 @@ export function useMutation<TVariables, TData, TError = Error>(opts: {
       //
       try {
         const data = await opts.fn(variables)
-        await opts.onSuccess?.({ data })
-        setStatus('success')
-        setError(undefined)
-        setData(data)
-        return data
+        if (data) {
+          await opts.onSuccess?.({ data })
+          setStatus('success')
+          setError(undefined)
+          setData(data)
+          return data
+        }
       } catch (err) {
         setStatus('error')
         setError(err as TError)
diff --git a/packages/react-start/src/useServerFn.ts b/packages/react-start/src/useServerFn.ts
index 67a09a4358c..45f8171b7f3 100644
--- a/packages/react-start/src/useServerFn.ts
+++ b/packages/react-start/src/useServerFn.ts
@@ -3,7 +3,7 @@ import { isRedirect, useRouter } from '@tanstack/react-router'
 
 export function useServerFn<T extends (...deps: Array<any>) => Promise<any>>(
   serverFn: T,
-): (...args: Parameters<T>) => ReturnType<T> {
+): (...args: Parameters<T>) => ReturnType<T> | Promise<void> {
   const router = useRouter()
 
   return React.useCallback(

From ddd6a34b12a19194052e25c4e7fdc8f8c8d2ff57 Mon Sep 17 00:00:00 2001
From: Caleb Cox <canac@users.noreply.github.com>
Date: Sun, 4 Jan 2026 12:26:30 -0600
Subject: [PATCH 2/3] Remove unneeded as any cast

---
 packages/react-start/src/useServerFn.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/react-start/src/useServerFn.ts b/packages/react-start/src/useServerFn.ts
index 45f8171b7f3..54202ea121f 100644
--- a/packages/react-start/src/useServerFn.ts
+++ b/packages/react-start/src/useServerFn.ts
@@ -26,5 +26,5 @@ export function useServerFn<T extends (...deps: Array<any>) => Promise<any>>(
       }
     },
     [router, serverFn],
-  ) as any
+  )
 }

From 31cadc5421dd1fe87bea0283975a5b94996c3265 Mon Sep 17 00:00:00 2001
From: Caleb Cox <canac@users.noreply.github.com>
Date: Sun, 4 Jan 2026 12:33:24 -0600
Subject: [PATCH 3/3] Use undefined instead of void

---
 packages/react-start/src/useServerFn.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/react-start/src/useServerFn.ts b/packages/react-start/src/useServerFn.ts
index 54202ea121f..fd4f9524dd5 100644
--- a/packages/react-start/src/useServerFn.ts
+++ b/packages/react-start/src/useServerFn.ts
@@ -3,7 +3,7 @@ import { isRedirect, useRouter } from '@tanstack/react-router'
 
 export function useServerFn<T extends (...deps: Array<any>) => Promise<any>>(
   serverFn: T,
-): (...args: Parameters<T>) => ReturnType<T> | Promise<void> {
+): (...args: Parameters<T>) => ReturnType<T> | Promise<undefined> {
   const router = useRouter()
 
   return React.useCallback(