TanStack · hokkyss · Jan 25, 2026 · Jan 25, 2026 · Jan 25, 2026 · Jan 25, 2026
diff --git a/e2e/react-start/serialization-adapters/src/data.tsx b/e2e/react-start/serialization-adapters/src/data.tsx
@@ -46,11 +46,95 @@ export const carAdapter = createSerializationAdapter({
     makeCar(value),
 })
 
-export function makeData() {
-  function makeFoo(suffix: string = '') {
-    return new Foo(typeof window === 'undefined' ? 'server' : 'client' + suffix)
+export class AsyncFoo {
+  private readonly internalValue: string
+  constructor(value: string) {
+    this.internalValue = value
+  }
+
+  get valueAsync() {
+    return new Promise<[string]>((resolve) => {
+      setTimeout(() => {
+        resolve([this.internalValue])
+      }, 1000)
+    })
+  }
+
+  get value() {
+    return this.internalValue
+  }
+
+  public echo() {
+    return { message: `echo`, value: this.internalValue }
   }
+}
+
+export const asyncFooAdapter = createSerializationAdapter({
+  key: 'asyncFoo',
+  test: (value) => value instanceof AsyncFoo,
+  toSerializable: (foo) => foo.value,
+  toSerializableAsync: (foo) => foo.valueAsync,
+  fromSerializable: (value) =>
+    new AsyncFoo(typeof value === 'string' ? value : value[0]),
+})
+
+export interface AsyncCar {
+  __type: 'asynccar'
+  make: string
+  model: string
+  year: number
+  honk: () => { message: string; make: string; model: string; year: number }
+}
+
+export function makeAsyncCar(opts: {
+  make: string
+  model: string
+  year: number
+}): AsyncCar {
   return {
+    ...opts,
+    __type: 'asynccar',
+    honk: () => {
+      return { message: `Asynchronous Car Honk!`, ...opts }
+    },
+  }
+}
+
+export const asyncCarAdapter = createSerializationAdapter({
+  key: 'asyncCar',
+  test: (value: any): value is AsyncCar =>
+    '__type' in (value as AsyncCar) && value.__type === 'asynccar',
+  toSerializable: (car) => ({
+    make: car.make,
+    model: car.model,
+    year: car.year,
+  }),
+  fromSerializable: (value: { make: string; model: string; year: number }) =>
+    makeAsyncCar(value),
+})
+
+export function makeAsyncFoo(suffix: string = '') {
+  return new AsyncFoo(
+    (typeof window === 'undefined' ? 'server' : 'client') + '-async' + suffix,
+  )
+}
+
+export function makeFoo(suffix: string = '') {
+  return new Foo(typeof window === 'undefined' ? 'server' : 'client' + suffix)
+}
+
+export function makeData() {
+  return {
+    asyncFoo: {
+      singleInstance: makeAsyncFoo(),
+    },
+    asyncCar: {
+      singleInstance: makeAsyncCar({
+        make: 'Toyota',
+        model: 'Camry',
+        year: 2020,
+      }),
+    },
     foo: {
       singleInstance: makeFoo(),
       array: [makeFoo('0'), makeFoo('1'), makeFoo('2')],
@@ -148,12 +232,35 @@ export function RenderData({
           year: data.car.singleInstance.year,
         })}
       </div>
+      <h3>Async Car</h3>
+      <h4>expected</h4>
+      <div data-testid={`${id}-async-car-expected`}>
+        {JSON.stringify({
+          make: localData.asyncCar.singleInstance.make,
+          model: localData.asyncCar.singleInstance.model,
+          year: localData.asyncCar.singleInstance.year,
+        })}
+      </div>
+      <h4>actual</h4>
+      <div data-testid={`${id}-async-car-actual`}>
+        {JSON.stringify({
+          make: data.asyncCar.singleInstance.make,
+          model: data.asyncCar.singleInstance.model,
+          year: data.asyncCar.singleInstance.year,
+        })}
+      </div>
       <b>Foo</b>
       <div data-testid={`${id}-foo`}>
         {JSON.stringify({
           value: data.foo.singleInstance.value,
         })}
       </div>
+      <b>Async Foo</b>
+      <div data-testid={`${id}-async-foo`}>
+        {JSON.stringify({
+          value: data.asyncFoo.singleInstance.value,
+        })}
+      </div>
     </div>
   )
 }

diff --git a/e2e/react-start/serialization-adapters/src/routeTree.gen.ts b/e2e/react-start/serialization-adapters/src/routeTree.gen.ts
@@ -15,6 +15,7 @@ import { Route as SsrNestedRouteImport } from './routes/ssr/nested'
 import { Route as SsrDataOnlyRouteImport } from './routes/ssr/data-only'
 import { Route as ServerFunctionNestedRouteImport } from './routes/server-function/nested'
 import { Route as ServerFunctionCustomErrorRouteImport } from './routes/server-function/custom-error'
+import { Route as ServerFunctionClassInstanceRouteImport } from './routes/server-function/class-instance'
 
 const IndexRoute = IndexRouteImport.update({
   id: '/',
@@ -47,9 +48,16 @@ const ServerFunctionCustomErrorRoute =
     path: '/server-function/custom-error',
     getParentRoute: () => rootRouteImport,
   } as any)
+const ServerFunctionClassInstanceRoute =
+  ServerFunctionClassInstanceRouteImport.update({
+    id: '/server-function/class-instance',
+    path: '/server-function/class-instance',
+    getParentRoute: () => rootRouteImport,
+  } as any)
 
 export interface FileRoutesByFullPath {
   '/': typeof IndexRoute
+  '/server-function/class-instance': typeof ServerFunctionClassInstanceRoute
   '/server-function/custom-error': typeof ServerFunctionCustomErrorRoute
   '/server-function/nested': typeof ServerFunctionNestedRoute
   '/ssr/data-only': typeof SsrDataOnlyRoute
@@ -58,6 +66,7 @@ export interface FileRoutesByFullPath {
 }
 export interface FileRoutesByTo {
   '/': typeof IndexRoute
+  '/server-function/class-instance': typeof ServerFunctionClassInstanceRoute
   '/server-function/custom-error': typeof ServerFunctionCustomErrorRoute
   '/server-function/nested': typeof ServerFunctionNestedRoute
   '/ssr/data-only': typeof SsrDataOnlyRoute
@@ -67,6 +76,7 @@ export interface FileRoutesByTo {
 export interface FileRoutesById {
   __root__: typeof rootRouteImport
   '/': typeof IndexRoute
+  '/server-function/class-instance': typeof ServerFunctionClassInstanceRoute
   '/server-function/custom-error': typeof ServerFunctionCustomErrorRoute
   '/server-function/nested': typeof ServerFunctionNestedRoute
   '/ssr/data-only': typeof SsrDataOnlyRoute
@@ -77,6 +87,7 @@ export interface FileRouteTypes {
   fileRoutesByFullPath: FileRoutesByFullPath
   fullPaths:
     | '/'
+    | '/server-function/class-instance'
     | '/server-function/custom-error'
     | '/server-function/nested'
     | '/ssr/data-only'
@@ -85,6 +96,7 @@ export interface FileRouteTypes {
   fileRoutesByTo: FileRoutesByTo
   to:
     | '/'
+    | '/server-function/class-instance'
     | '/server-function/custom-error'
     | '/server-function/nested'
     | '/ssr/data-only'
@@ -93,6 +105,7 @@ export interface FileRouteTypes {
   id:
     | '__root__'
     | '/'
+    | '/server-function/class-instance'
     | '/server-function/custom-error'
     | '/server-function/nested'
     | '/ssr/data-only'
@@ -102,6 +115,7 @@ export interface FileRouteTypes {
 }
 export interface RootRouteChildren {
   IndexRoute: typeof IndexRoute
+  ServerFunctionClassInstanceRoute: typeof ServerFunctionClassInstanceRoute
   ServerFunctionCustomErrorRoute: typeof ServerFunctionCustomErrorRoute
   ServerFunctionNestedRoute: typeof ServerFunctionNestedRoute
   SsrDataOnlyRoute: typeof SsrDataOnlyRoute
@@ -153,11 +167,19 @@ declare module '@tanstack/react-router' {
       preLoaderRoute: typeof ServerFunctionCustomErrorRouteImport
       parentRoute: typeof rootRouteImport
     }
+    '/server-function/class-instance': {
+      id: '/server-function/class-instance'
+      path: '/server-function/class-instance'
+      fullPath: '/server-function/class-instance'
+      preLoaderRoute: typeof ServerFunctionClassInstanceRouteImport
+      parentRoute: typeof rootRouteImport
+    }
   }
 }
 
 const rootRouteChildren: RootRouteChildren = {
   IndexRoute: IndexRoute,
+  ServerFunctionClassInstanceRoute: ServerFunctionClassInstanceRoute,
   ServerFunctionCustomErrorRoute: ServerFunctionCustomErrorRoute,
   ServerFunctionNestedRoute: ServerFunctionNestedRoute,
   SsrDataOnlyRoute: SsrDataOnlyRoute,

diff --git a/e2e/react-start/serialization-adapters/src/routes/server-function/class-instance.tsx b/e2e/react-start/serialization-adapters/src/routes/server-function/class-instance.tsx
@@ -0,0 +1,38 @@
+import { createFileRoute } from '@tanstack/react-router'
+import { createServerFn } from '@tanstack/react-start'
+import { useState } from 'react'
+import { makeAsyncFoo, makeFoo } from '~/data'
+
+const serverFn = createServerFn().handler(() => {
+  return {
+    asyncFoo: makeAsyncFoo('-serverFn'),
+    foo: makeFoo('-serverFn'),
+  }
+})
+
+export const Route = createFileRoute('/server-function/class-instance')({
+  component: RouteComponent,
+})
+
+function RouteComponent() {
+  const [resp, setResp] = useState<null | Awaited<ReturnType<typeof serverFn>>>(
+    null,
+  )
+
+  return (
+    <div>
+      <button
+        data-testid="server-function-btn"
+        onClick={() => serverFn().then(setResp)}
+      >
+        trigger serverFn
+      </button>
+      <div data-testid="server-function-foo-response">
+        {JSON.stringify(resp?.foo?.value)}
+      </div>
+      <div data-testid="server-function-async-foo-response">
+        {JSON.stringify(resp?.asyncFoo?.echo())}
+      </div>
+    </div>
+  )
+}
diff --git a/e2e/react-start/serialization-adapters/src/routes/ssr/data-only.tsx b/e2e/react-start/serialization-adapters/src/routes/ssr/data-only.tsx
@@ -18,15 +18,20 @@ export const Route = createFileRoute('/ssr/data-only')({
 
     const honkState = loaderData.car.singleInstance.honk()
 
+    const expectedAsyncHonkState = localData.asyncCar.singleInstance.honk()
+    const asyncHonkState = loaderData.asyncCar.singleInstance.honk()
+
     return (
       <div data-testid="data-only-container">
         <h2 data-testid="data-only-heading">data-only</h2>
         <div>
           context: <RenderData id="context" data={context} />
         </div>
+        <hr />
         <div>
           loader: <RenderData id="loader" data={loaderData} />
         </div>
+        <hr />
         <div data-testid="honk-container">
           <h3>honk</h3>
           <div>
@@ -43,6 +48,21 @@ export const Route = createFileRoute('/ssr/data-only')({
           </div>
         </div>
         <hr />
+        <div data-testid="async-honk-container">
+          <h3>async car honk</h3>
+          <div>
+            expected:{' '}
+            <div data-testid="async-honk-expected-state">
+              {JSON.stringify(expectedAsyncHonkState)}
+            </div>
+          </div>
+          <div>
+            actual:{' '}
+            <div data-testid="async-honk-actual-state">
+              {JSON.stringify(asyncHonkState)}
+            </div>
+          </div>
+        </div>
         <Outlet />
       </div>
     )

diff --git a/e2e/react-start/serialization-adapters/src/start.tsx b/e2e/react-start/serialization-adapters/src/start.tsx
@@ -1,5 +1,11 @@
 import { createStart } from '@tanstack/react-start'
-import { carAdapter, fooAdapter, nestedOuterAdapter } from './data'
+import {
+  asyncCarAdapter,
+  asyncFooAdapter,
+  carAdapter,
+  fooAdapter,
+  nestedOuterAdapter,
+} from './data'
 import { customErrorAdapter } from './CustomError'
 
 export const startInstance = createStart(() => {
@@ -11,6 +17,8 @@ export const startInstance = createStart(() => {
       customErrorAdapter,
       // only register nestedOuterAdapter here, nestedInnerAdapter is registered as an "extends" of nestedOuterAdapter
       nestedOuterAdapter,
+      asyncCarAdapter,
+      asyncFooAdapter,
     ],
   }
 })