diff --git a/packages/react-start-client/src/tests/createServerFn.test-d.tsx b/packages/react-start-client/src/tests/createServerFn.test-d.tsx
index 77f39cbedae..e7d8c7be08e 100644
--- a/packages/react-start-client/src/tests/createServerFn.test-d.tsx
+++ b/packages/react-start-client/src/tests/createServerFn.test-d.tsx
@@ -1,6 +1,8 @@
 import { expectTypeOf, test } from 'vitest'
 import { createServerFn } from '@tanstack/start-client-core'
 
+/*
+// disabled until we really support RSC
 test.skip('createServerFn returns RSC', () => {
   const fn = createServerFn().handler(() => ({
     rscs: [
@@ -14,4 +16,40 @@ test.skip('createServerFn returns RSC', () => {
       rscs: readonly [ReadableStream, ReadableStream]
     }>
   >()
+})*/
+
+test('createServerFn returns async array', () => {
+  const result: Array<{ a: number }> = [{ a: 1 }]
+  const serverFn = createServerFn({ method: 'GET' }).handler(async () => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Array<{ a: number }>>>()
+})
+
+test('createServerFn returns sync array', () => {
+  const result: Array<{ a: number }> = [{ a: 1 }]
+  const serverFn = createServerFn({ method: 'GET' }).handler(() => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Array<{ a: number }>>>()
+})
+
+test('createServerFn returns async union', () => {
+  const result = '1' as string | number
+  const serverFn = createServerFn({ method: 'GET' }).handler(async () => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<string | number>>()
+})
+
+test('createServerFn returns sync union', () => {
+  const result = '1' as string | number
+  const serverFn = createServerFn({ method: 'GET' }).handler(() => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<string | number>>()
 })
diff --git a/packages/start-client-core/src/client-rpc/serverFnFetcher.ts b/packages/start-client-core/src/client-rpc/serverFnFetcher.ts
index 1611a4cba6e..25b38c10357 100644
--- a/packages/start-client-core/src/client-rpc/serverFnFetcher.ts
+++ b/packages/start-client-core/src/client-rpc/serverFnFetcher.ts
@@ -7,7 +7,11 @@ import {
 import { fromCrossJSON, toJSONAsync } from 'seroval'
 import invariant from 'tiny-invariant'
 import { getDefaultSerovalPlugins } from '../getDefaultSerovalPlugins'
-import { TSS_FORMDATA_CONTEXT, X_TSS_SERIALIZED } from '../constants'
+import {
+  TSS_FORMDATA_CONTEXT,
+  X_TSS_RAW_RESPONSE,
+  X_TSS_SERIALIZED,
+} from '../constants'
 import type { FunctionMiddlewareClientFnOptions } from '../createMiddleware'
 import type { Plugin as SerovalPlugin } from 'seroval'
 
@@ -156,6 +160,9 @@ async function getResponse(fn: () => Promise<Response>) {
     }
   })()
 
+  if (response.headers.get(X_TSS_RAW_RESPONSE) === 'true') {
+    return response
+  }
   const contentType = response.headers.get('content-type')
   invariant(contentType, 'expected content-type header to be set')
   const serializedByStart = !!response.headers.get(X_TSS_SERIALIZED)
diff --git a/packages/start-client-core/src/constants.ts b/packages/start-client-core/src/constants.ts
index 0a39ffaeed3..1e541af3231 100644
--- a/packages/start-client-core/src/constants.ts
+++ b/packages/start-client-core/src/constants.ts
@@ -5,4 +5,5 @@ export const TSS_SERVER_FUNCTION_FACTORY = Symbol.for(
 )
 
 export const X_TSS_SERIALIZED = 'x-tss-serialized'
+export const X_TSS_RAW_RESPONSE = 'x-tss-raw'
 export {}
diff --git a/packages/start-client-core/src/createServerFn.ts b/packages/start-client-core/src/createServerFn.ts
index 3ae9a3682cc..e4941c58715 100644
--- a/packages/start-client-core/src/createServerFn.ts
+++ b/packages/start-client-core/src/createServerFn.ts
@@ -14,7 +14,6 @@ import type {
   ResolveValidatorInput,
   ValidateSerializable,
   ValidateSerializableInput,
-  ValidateSerializableInputResult,
   Validator,
 } from '@tanstack/router-core'
 import type { JsonResponse } from '@tanstack/router-core/ssr/client'
@@ -174,7 +173,7 @@ export async function executeMiddleware(
   env: 'client' | 'server',
   opts: ServerFnMiddlewareOptions,
 ): Promise<ServerFnMiddlewareResult> {
-  const globalMiddlewares = getStartOptions().functionMiddleware || []
+  const globalMiddlewares = getStartOptions()?.functionMiddleware || []
   const flattenedMiddlewares = flattenMiddlewares([
     ...globalMiddlewares,
     ...middlewares,
@@ -243,10 +242,10 @@ export type CompiledFetcherFnOptions = {
   context?: any
 }
 
-export type Fetcher<TRegister, TMiddlewares, TInputValidator, TResponse> =
+export type Fetcher<TMiddlewares, TInputValidator, TResponse> =
   undefined extends IntersectAllValidatorInputs<TMiddlewares, TInputValidator>
-    ? OptionalFetcher<TRegister, TMiddlewares, TInputValidator, TResponse>
-    : RequiredFetcher<TRegister, TMiddlewares, TInputValidator, TResponse>
+    ? OptionalFetcher<TMiddlewares, TInputValidator, TResponse>
+    : RequiredFetcher<TMiddlewares, TInputValidator, TResponse>
 
 export interface FetcherBase {
   [TSS_SERVER_FUNCTION]: true
@@ -260,26 +259,18 @@ export interface FetcherBase {
   }) => Promise<unknown>
 }
 
-export interface OptionalFetcher<
-  TRegister,
-  TMiddlewares,
-  TInputValidator,
-  TResponse,
-> extends FetcherBase {
+export interface OptionalFetcher<TMiddlewares, TInputValidator, TResponse>
+  extends FetcherBase {
   (
     options?: OptionalFetcherDataOptions<TMiddlewares, TInputValidator>,
-  ): Promise<FetcherData<TRegister, TResponse>>
+  ): Promise<FetcherData<TResponse>>
 }
 
-export interface RequiredFetcher<
-  TRegister,
-  TMiddlewares,
-  TInputValidator,
-  TResponse,
-> extends FetcherBase {
+export interface RequiredFetcher<TMiddlewares, TInputValidator, TResponse>
+  extends FetcherBase {
   (
     opts: RequiredFetcherDataOptions<TMiddlewares, TInputValidator>,
-  ): Promise<FetcherData<TRegister, TResponse>>
+  ): Promise<FetcherData<TResponse>>
 }
 
 export type FetcherBaseOptions = {
@@ -297,22 +288,23 @@ export interface RequiredFetcherDataOptions<TMiddlewares, TInputValidator>
   data: Expand<IntersectAllValidatorInputs<TMiddlewares, TInputValidator>>
 }
 
-export type FetcherData<TRegister, TResponse> = TResponse extends Response
-  ? Response
-  : TResponse extends JsonResponse<any>
-    ? ValidateSerializableInputResult<TRegister, ReturnType<TResponse['json']>>
-    : ValidateSerializableInputResult<TRegister, TResponse>
-
 export type RscStream<T> = {
   __cacheState: T
 }
 
 export type Method = 'GET' | 'POST'
 
+export type FetcherData<TResponse> =
+  Awaited<TResponse> extends Response
+    ? Awaited<TResponse>
+    : Awaited<TResponse> extends JsonResponse<any>
+      ? ReturnType<Awaited<TResponse>['json']>
+      : Awaited<TResponse>
+
 export type ServerFnReturnType<TRegister, TResponse> =
-  | Response
-  | Promise<ValidateSerializableInput<TRegister, TResponse>>
-  | ValidateSerializableInput<TRegister, TResponse>
+  Awaited<TResponse> extends Response
+    ? TResponse
+    : ValidateSerializableInput<TRegister, TResponse>
 
 export type ServerFn<
   TRegister,
@@ -521,7 +513,7 @@ export interface ServerFnHandler<
       TInputValidator,
       TNewResponse
     >,
-  ) => Fetcher<TRegister, TMiddlewares, TInputValidator, TNewResponse>
+  ) => Fetcher<TMiddlewares, TInputValidator, TNewResponse>
 }
 
 export interface ServerFnBuilder<TRegister, TMethod extends Method = 'GET'>
diff --git a/packages/start-client-core/src/getDefaultSerovalPlugins.ts b/packages/start-client-core/src/getDefaultSerovalPlugins.ts
index 3c4fb6c3224..61e48b9d896 100644
--- a/packages/start-client-core/src/getDefaultSerovalPlugins.ts
+++ b/packages/start-client-core/src/getDefaultSerovalPlugins.ts
@@ -7,7 +7,7 @@ import type { AnySerializationAdapter } from '@tanstack/router-core'
 
 export function getDefaultSerovalPlugins() {
   const start = getStartOptions()
-  const adapters = start.serializationAdapters as
+  const adapters = start?.serializationAdapters as
     | Array<AnySerializationAdapter>
     | undefined
   return [
diff --git a/packages/start-client-core/src/getStartOptions.ts b/packages/start-client-core/src/getStartOptions.ts
index d4ba38cf4e1..7c08e1f49a6 100644
--- a/packages/start-client-core/src/getStartOptions.ts
+++ b/packages/start-client-core/src/getStartOptions.ts
@@ -1,6 +1,8 @@
 import { getStartContext } from '@tanstack/start-storage-context'
 import { createIsomorphicFn } from './createIsomorphicFn'
+import type { AnyStartInstanceOptions } from './createStart'
 
-export const getStartOptions = createIsomorphicFn()
-  .client(() => window.__TSS_START_OPTIONS__!)
-  .server(() => getStartContext().startOptions)
+export const getStartOptions: () => AnyStartInstanceOptions | undefined =
+  createIsomorphicFn()
+    .client(() => window.__TSS_START_OPTIONS__)
+    .server(() => getStartContext().startOptions)
diff --git a/packages/start-client-core/src/index.tsx b/packages/start-client-core/src/index.tsx
index 2552f31a156..abb082d02b7 100644
--- a/packages/start-client-core/src/index.tsx
+++ b/packages/start-client-core/src/index.tsx
@@ -85,6 +85,7 @@ export {
   TSS_FORMDATA_CONTEXT,
   TSS_SERVER_FUNCTION,
   X_TSS_SERIALIZED,
+  X_TSS_RAW_RESPONSE,
 } from './constants'
 
 export type * from './serverRoute'
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 177c679774c..47718d74f2b 100644
--- a/packages/start-client-core/src/tests/createServerFn.test-d.ts
+++ b/packages/start-client-core/src/tests/createServerFn.test-d.ts
@@ -317,11 +317,7 @@ test('createServerFn returns undefined', () => {
 test('createServerFn cannot return function', () => {
   expectTypeOf(createServerFn().handler<{ func: () => 'func' }>)
     .parameter(0)
-    .returns.toEqualTypeOf<
-      | Response
-      | { func: 'Function is not serializable' }
-      | Promise<{ func: 'Function is not serializable' }>
-    >()
+    .returns.toEqualTypeOf<{ func: 'Function is not serializable' }>()
 })
 
 test('createServerFn cannot validate function', () => {
@@ -598,3 +594,47 @@ test('createServerFn fetcher itself is serializable', () => {
   const fn1 = createServerFn().handler(() => ({}))
   const fn2 = createServerFn().handler(() => fn1)
 })
+
+test('createServerFn returns async Response', () => {
+  const serverFn = createServerFn().handler(async () => {
+    return new Response(new Blob([JSON.stringify({ a: 1 })]), {
+      status: 200,
+      headers: {
+        'Content-Type': 'application/json',
+      },
+    })
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Response>>()
+})
+
+test('createServerFn returns sync Response', () => {
+  const serverFn = createServerFn().handler(() => {
+    return new Response(new Blob([JSON.stringify({ a: 1 })]), {
+      status: 200,
+      headers: {
+        'Content-Type': 'application/json',
+      },
+    })
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Response>>()
+})
+
+test('createServerFn returns async array', () => {
+  const result: Array<{ a: number }> = [{ a: 1 }]
+  const serverFn = createServerFn({ method: 'GET' }).handler(async () => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Array<{ a: number }>>>()
+})
+
+test('createServerFn returns sync array', () => {
+  const result: Array<{ a: number }> = [{ a: 1 }]
+  const serverFn = createServerFn({ method: 'GET' }).handler(() => {
+    return result
+  })
+
+  expectTypeOf(serverFn()).toEqualTypeOf<Promise<Array<{ a: number }>>>()
+})
diff --git a/packages/start-server-core/src/server-functions-handler.ts b/packages/start-server-core/src/server-functions-handler.ts
index 47cf996f75c..d6beb47ac98 100644
--- a/packages/start-server-core/src/server-functions-handler.ts
+++ b/packages/start-server-core/src/server-functions-handler.ts
@@ -2,6 +2,7 @@ import { isNotFound } from '@tanstack/router-core'
 import invariant from 'tiny-invariant'
 import {
   TSS_FORMDATA_CONTEXT,
+  X_TSS_RAW_RESPONSE,
   X_TSS_SERIALIZED,
   getDefaultSerovalPlugins,
 } from '@tanstack/start-client-core'
@@ -145,6 +146,7 @@ export const handleServerAction = async ({
       // Any time we get a Response back, we should just
       // return it immediately.
       if (result.result instanceof Response) {
+        result.result.headers.set(X_TSS_RAW_RESPONSE, 'true')
         return result.result
       }