diff --git a/packages/react-router/src/index.tsx b/packages/react-router/src/index.tsx
index 79987033b83..6b2d9bec3e0 100644
--- a/packages/react-router/src/index.tsx
+++ b/packages/react-router/src/index.tsx
@@ -290,7 +290,13 @@ export {
 export type { SearchSerializer, SearchParser } from './searchParams'
 
 export { defaultTransformer } from './transformer'
-export type { RouterTransformer } from './transformer'
+export type {
+  RouterTransformer,
+  TransformerParse,
+  TransformerStringify,
+  DefaultTransformerParse,
+  DefaultTransformerStringify,
+} from './transformer'
 
 export { useBlocker, Block } from './useBlocker'
 
diff --git a/packages/react-router/src/transformer.ts b/packages/react-router/src/transformer.ts
index 2277a3dbb8d..15a6af55e77 100644
--- a/packages/react-router/src/transformer.ts
+++ b/packages/react-router/src/transformer.ts
@@ -47,3 +47,22 @@ const transformers = [
     parse: () => undefined,
   },
 ] as const
+
+export type TransformerStringify<T, TSerializable> = T extends TSerializable
+  ? T
+  : T extends (...args: Array<any>) => any
+    ? 'Function is not serializable'
+    : { [K in keyof T]: TransformerStringify<T[K], TSerializable> }
+
+export type TransformerParse<T, TSerializable> = T extends TSerializable
+  ? T
+  : T extends JSX.Element
+    ? ReadableStream
+    : { [K in keyof T]: TransformerParse<T[K], TSerializable> }
+
+export type DefaultTransformerStringify<T> = TransformerStringify<
+  T,
+  Date | undefined
+>
+
+export type DefaultTransformerParse<T> = TransformerParse<T, Date | undefined>
diff --git a/packages/start/src/client/createMiddleware.ts b/packages/start/src/client/createMiddleware.ts
index 61af24f88fa..52e352282a3 100644
--- a/packages/start/src/client/createMiddleware.ts
+++ b/packages/start/src/client/createMiddleware.ts
@@ -1,7 +1,7 @@
-import type { Method } from './createServerFn'
+import type { ConstrainValidator, Method } from './createServerFn'
 import type {
-  AnyValidator,
   Constrain,
+  DefaultTransformerStringify,
   Expand,
   MergeAll,
   ResolveValidatorInput,
@@ -129,7 +129,7 @@ export interface MiddlewareOptions<
 > {
   validateClient?: boolean
   middleware?: TMiddlewares
-  validator?: Constrain<TValidator, AnyValidator>
+  validator?: ConstrainValidator<TValidator>
   client?: MiddlewareClientFn<
     TMiddlewares,
     TValidator,
@@ -166,7 +166,7 @@ export type MiddlewareServerFn<
     TNewClientAfterContext = undefined,
   >(ctx?: {
     context?: TNewServerContext
-    sendContext?: TNewClientAfterContext
+    sendContext?: DefaultTransformerStringify<TNewClientAfterContext>
   }) => Promise<
     ServerResultWithContext<TNewServerContext, TNewClientAfterContext>
   >
@@ -186,7 +186,7 @@ export type MiddlewareClientFn<
   method: Method
   next: <TNewServerContext = undefined, TNewClientContext = undefined>(ctx?: {
     context?: TNewClientContext
-    sendContext?: TNewServerContext
+    sendContext?: DefaultTransformerStringify<TNewServerContext>
     headers?: HeadersInit
   }) => Promise<ClientResultWithContext<TNewServerContext, TNewClientContext>>
 }) =>
@@ -272,7 +272,7 @@ export interface MiddlewareValidator<
   TClientAfterContext,
 > {
   validator: <TNewValidator>(
-    input: TNewValidator,
+    input: ConstrainValidator<TNewValidator>,
   ) => MiddlewareAfterMiddleware<
     TId,
     TMiddlewares,
diff --git a/packages/start/src/client/createServerFn.ts b/packages/start/src/client/createServerFn.ts
index d450abcfb80..1bc91498126 100644
--- a/packages/start/src/client/createServerFn.ts
+++ b/packages/start/src/client/createServerFn.ts
@@ -1,7 +1,14 @@
 import invariant from 'tiny-invariant'
 import { defaultTransformer } from '@tanstack/react-router'
 import { mergeHeaders } from './headers'
-import type { AnyValidator, Constrain } from '@tanstack/react-router'
+import type {
+  AnyValidator,
+  Constrain,
+  DefaultTransformerParse,
+  DefaultTransformerStringify,
+  ResolveValidatorInput,
+  Validator,
+} from '@tanstack/react-router'
 import type {
   AnyMiddleware,
   MergeAllServerContext,
@@ -61,20 +68,10 @@ export interface OptionalFetcherDataOptions<TInput> extends FetcherBaseOptions {
   data?: TInput
 }
 
-export type FetcherData<TResponse> = WrapRSCs<
+export type FetcherData<TResponse> = DefaultTransformerParse<
   TResponse extends JsonResponse<infer TData> ? TData : TResponse
 >
 
-export type WrapRSCs<T> = T extends JSX.Element
-  ? ReadableStream
-  : T extends Record<string, any>
-    ? {
-        [K in keyof T]: WrapRSCs<T[K]>
-      }
-    : T extends Array<infer U>
-      ? Array<WrapRSCs<U>>
-      : T
-
 export type RscStream<T> = {
   __cacheState: T
 }
@@ -83,7 +80,9 @@ export type Method = 'GET' | 'POST'
 
 export type ServerFn<TMethod, TMiddlewares, TValidator, TResponse> = (
   ctx: ServerFnCtx<TMethod, TMiddlewares, TValidator>,
-) => Promise<TResponse> | TResponse
+) =>
+  | Promise<DefaultTransformerStringify<TResponse>>
+  | DefaultTransformerStringify<TResponse>
 
 export type ServerFnCtx<TMethod, TMiddlewares, TValidator> = {
   method: TMethod
@@ -105,13 +104,23 @@ type ServerFnBaseOptions<
   method: TMethod
   validateClient?: boolean
   middleware?: Constrain<TMiddlewares, ReadonlyArray<AnyMiddleware>>
-  validator?: Constrain<TInput, AnyValidator>
+  validator?: ConstrainValidator<TInput>
   extractedFn?: CompiledFetcherFn<TResponse>
   serverFn?: ServerFn<TMethod, TMiddlewares, TInput, TResponse>
   filename: string
   functionId: string
 }
 
+export type ConstrainValidator<TValidator> = unknown extends TValidator
+  ? TValidator
+  : Constrain<
+      TValidator,
+      Validator<
+        DefaultTransformerStringify<ResolveValidatorInput<TValidator>>,
+        any
+      >
+    >
+
 type ServerFnBase<
   TMethod extends Method = 'GET',
   TResponse = unknown,
@@ -126,7 +135,7 @@ type ServerFnBase<
     'validator' | 'handler'
   >
   validator: <TValidator>(
-    validator: Constrain<TValidator, AnyValidator>,
+    validator: ConstrainValidator<TValidator>,
   ) => Pick<
     ServerFnBase<TMethod, TResponse, TMiddlewares, TValidator>,
     'handler' | 'middleware'
diff --git a/packages/start/src/client/index.tsx b/packages/start/src/client/index.tsx
index bdedb47f085..5a2361e8e66 100644
--- a/packages/start/src/client/index.tsx
+++ b/packages/start/src/client/index.tsx
@@ -9,7 +9,6 @@ export {
   type CompiledFetcherFn,
   type Fetcher,
   type RscStream,
-  type WrapRSCs,
   type FetcherImpl,
   type FetcherData,
   type FetcherBaseOptions,
diff --git a/packages/start/src/client/tests/createServerFn.test-d.ts b/packages/start/src/client/tests/createServerFn.test-d.tsx
similarity index 75%
rename from packages/start/src/client/tests/createServerFn.test-d.ts
rename to packages/start/src/client/tests/createServerFn.test-d.tsx
index 88853e7d901..f2907a809c6 100644
--- a/packages/start/src/client/tests/createServerFn.test-d.ts
+++ b/packages/start/src/client/tests/createServerFn.test-d.tsx
@@ -1,6 +1,7 @@
 import { expectTypeOf, test } from 'vitest'
 import { createServerFn } from '../createServerFn'
 import { createMiddleware } from '../createMiddleware'
+import type { Constrain, Validator } from '@tanstack/react-router'
 
 test('createServerFn without middleware', () => {
   createServerFn({ method: 'GET' }).handler((options) => {
@@ -178,3 +179,56 @@ test('createServerFn where data is optional if there is no validator', () => {
     | undefined
   >()
 })
+
+test('createServerFn returns Date', () => {
+  const fn = createServerFn().handler(() => ({
+    dates: [new Date(), new Date()] as const,
+  }))
+
+  expectTypeOf(fn()).toEqualTypeOf<Promise<{ dates: readonly [Date, Date] }>>()
+})
+
+test('createServerFn returns RSC', () => {
+  const fn = createServerFn().handler(() => ({
+    rscs: [
+      <div key="0">I'm an RSC</div>,
+      <div key="1">I'm an RSC</div>,
+    ] as const,
+  }))
+
+  expectTypeOf(fn()).toEqualTypeOf<
+    Promise<{ rscs: readonly [ReadableStream, ReadableStream] }>
+  >()
+})
+
+test('createServerFn returns undefined', () => {
+  const fn = createServerFn().handler(() => ({
+    nothing: undefined,
+  }))
+
+  expectTypeOf(fn()).toEqualTypeOf<Promise<{ nothing: undefined }>>()
+})
+
+test('createServerFn cannot return function', () => {
+  expectTypeOf(createServerFn().handler<{ func: () => 'func' }>)
+    .parameter(0)
+    .returns.toEqualTypeOf<
+      | { func: 'Function is not serializable' }
+      | Promise<{ func: 'Function is not serializable' }>
+    >()
+})
+
+test('createServerFn cannot validate function', () => {
+  const validator = createServerFn().validator<
+    (input: { func: () => 'string' }) => { output: 'string' }
+  >
+
+  expectTypeOf(validator)
+    .parameter(0)
+    .toEqualTypeOf<
+      Constrain<
+        (input: { func: () => 'string' }) => { output: 'string' },
+        Validator<{ func: 'Function is not serializable' }, any>
+      >
+    >()
+})
diff --git a/packages/start/src/client/tests/createServerMiddleware.test-d.ts b/packages/start/src/client/tests/createServerMiddleware.test-d.ts
index d3f464e9957..c714a91c29d 100644
--- a/packages/start/src/client/tests/createServerMiddleware.test-d.ts
+++ b/packages/start/src/client/tests/createServerMiddleware.test-d.ts
@@ -1,5 +1,6 @@
 import { expectTypeOf, test } from 'vitest'
 import { createMiddleware } from '../createMiddleware'
+import type { Constrain, Validator } from '@tanstack/react-router'
 
 test('createServeMiddleware removes middleware after middleware,', () => {
   const middleware = createMiddleware()
@@ -228,7 +229,7 @@ test('createMiddleware merges server context and client context, sends server co
         fromServer1: string
         fromServer2: string
         fromServer3: string
-        toServer1: string
+        toServer1: 'toServer1'
       }>()
       return next({
         context: { fromServer4: 'fromServer4' },
@@ -244,8 +245,45 @@ test('createMiddleware merges server context and client context, sends server co
         fromClient3: string
         clientAfter3: string
         fromClient4: string
-        toClient1: string
+        toClient1: 'toClient1'
       }>
       return next({ context: { clientAfter4: 'clientAfter4' } })
     })
 })
+
+test('createMiddleware sendContext cannot send a function', () => {
+  createMiddleware()
+    .client(({ next }) => {
+      expectTypeOf(next<{ func: () => 'func' }>)
+        .parameter(0)
+        .exclude<undefined>()
+        .toHaveProperty('sendContext')
+        .toEqualTypeOf<{ func: 'Function is not serializable' } | undefined>()
+
+      return next()
+    })
+    .server(({ next }) => {
+      expectTypeOf(next<undefined, { func: () => 'func' }>)
+        .parameter(0)
+        .exclude<undefined>()
+        .toHaveProperty('sendContext')
+        .toEqualTypeOf<{ func: 'Function is not serializable' } | undefined>()
+
+      return next()
+    })
+})
+
+test('createMiddleware cannot validate function', () => {
+  const validator = createMiddleware().validator<
+    (input: { func: () => 'string' }) => { output: 'string' }
+  >
+
+  expectTypeOf(validator)
+    .parameter(0)
+    .toEqualTypeOf<
+      Constrain<
+        (input: { func: () => 'string' }) => { output: 'string' },
+        Validator<{ func: 'Function is not serializable' }, any>
+      >
+    >()
+})