fix(start-server-core): properly merge middleware context objects

e2e/react-start/server-routes/.gitignore

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+node_modules
+package-lock.json
+yarn.lock
+
+.DS_Store
+.cache
+.env
+.vercel
+.output
+/build/
+/api/
+/server/build
+/public/build# Sentry Config File
+.env.sentry-build-plugin
+/test-results/
+/playwright-report/
+/blob-report/
+/playwright/.cache/

e2e/react-start/server-routes/.prettierignore

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+**/build
+**/public
+pnpm-lock.yaml
+routeTree.gen.ts

e2e/react-start/server-routes/package.json

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+{
+  "name": "tanstack-react-start-e2e-server-routes",
+  "private": true,
+  "sideEffects": false,
+  "type": "module",
+  "scripts": {
+    "dev": "vite dev --port 3000",
+    "dev:e2e": "vite dev",
+    "build": "vite build && tsc --noEmit",
+    "start": "node .output/server/index.mjs",
+    "test:e2e": "playwright test --project=chromium"
+  },
+  "dependencies": {
+    "@tanstack/react-router": "workspace:^",
+    "@tanstack/react-router-devtools": "workspace:^",
+    "@tanstack/react-start": "workspace:^",
+    "js-cookie": "^3.0.5",
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
+    "redaxios": "^0.5.1",
+    "tailwind-merge": "^2.6.0",
+    "vite": "^6.3.5",
+    "zod": "^3.24.2"
+  },
+  "devDependencies": {
+    "@playwright/test": "^1.50.1",
+    "@tanstack/router-e2e-utils": "workspace:^",
+    "@types/js-cookie": "^3.0.6",
+    "@types/node": "^22.10.2",
+    "@types/react": "^19.0.8",
+    "@types/react-dom": "^19.0.3",
+    "@vitejs/plugin-react": "^4.3.4",
+    "autoprefixer": "^10.4.20",
+    "combinate": "^1.1.11",
+    "postcss": "^8.5.1",
+    "tailwindcss": "^3.4.17",
+    "typescript": "^5.7.2",
+    "vite-tsconfig-paths": "^5.1.4"
+  }
+}

e2e/react-start/server-routes/playwright.config.ts

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+import { defineConfig, devices } from '@playwright/test'
+import { derivePort } from '@tanstack/router-e2e-utils'
+import packageJson from './package.json' with { type: 'json' }
+
+export const PORT = await derivePort(packageJson.name)
+const baseURL = `http://localhost:${PORT}`
+
+/**
+ * See https://playwright.dev/docs/test-configuration.
+ */
+export default defineConfig({
+  testDir: './tests',
+  workers: 1,
+
+  reporter: [['line']],
+
+  use: {
+    /* Base URL to use in actions like `await page.goto('/')`. */
+    baseURL,
+  },
+
+  webServer: {
+    command: `VITE_SERVER_PORT=${PORT} pnpm build && PORT=${PORT} VITE_SERVER_PORT=${PORT} pnpm start`,
+    url: baseURL,
+    reuseExistingServer: !process.env.CI,
+    stdout: 'pipe',
+  },
+
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+  ],
+})

e2e/react-start/server-routes/postcss.config.mjs

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+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}

e2e/react-start/server-routes/src/components/DefaultCatchBoundary.tsx

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+import {
+  ErrorComponent,
+  Link,
+  rootRouteId,
+  useMatch,
+  useRouter,
+} from '@tanstack/react-router'
+import type { ErrorComponentProps } from '@tanstack/react-router'
+
+export function DefaultCatchBoundary({ error }: ErrorComponentProps) {
+  const router = useRouter()
+  const isRoot = useMatch({
+    strict: false,
+    select: (state) => state.id === rootRouteId,
+  })
+
+  console.error(error)
+
+  return (
+    <div className="min-w-0 flex-1 p-4 flex flex-col items-center justify-center gap-6">
+      <ErrorComponent error={error} />
+      <div className="flex gap-2 items-center flex-wrap">
+        <button
+          onClick={() => {
+            router.invalidate()
+          }}
+          className={`px-2 py-1 bg-gray-600 dark:bg-gray-700 rounded text-white uppercase font-extrabold`}
+        >
+          Try Again
+        </button>
+        {isRoot ? (
+          <Link
+            to="/"
+            className={`px-2 py-1 bg-gray-600 dark:bg-gray-700 rounded text-white uppercase font-extrabold`}
+          >
+            Home
+          </Link>
+        ) : (
+          <Link
+            to="/"
+            className={`px-2 py-1 bg-gray-600 dark:bg-gray-700 rounded text-white uppercase font-extrabold`}
+            onClick={(e) => {
+              e.preventDefault()
+              window.history.back()
+            }}
+          >
+            Go Back
+          </Link>
+        )}
+      </div>
+    </div>
+  )
+}

e2e/react-start/server-routes/src/components/NotFound.tsx

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+import { Link } from '@tanstack/react-router'
+
+export function NotFound({ children }: { children?: any }) {
+  return (
+    <div className="space-y-2 p-2" data-testid="default-not-found-component">
+      <div className="text-gray-600 dark:text-gray-400">
+        {children || <p>The page you are looking for does not exist.</p>}
+      </div>
+      <p className="flex items-center gap-2 flex-wrap">
+        <button
+          onClick={() => window.history.back()}
+          className="bg-emerald-500 text-white px-2 py-1 rounded uppercase font-black text-sm"
+        >
+          Go back
+        </button>
+        <Link
+          to="/"
+          className="bg-cyan-600 text-white px-2 py-1 rounded uppercase font-black text-sm"
+        >
+          Start Over
+        </Link>
+      </p>
+    </div>
+  )
+}

e2e/react-start/server-routes/src/routeTree.gen.ts

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+/* eslint-disable */
+
+// @ts-nocheck
+
+// noinspection JSUnusedGlobalSymbols
+
+// This file was automatically generated by TanStack Router.
+// You should NOT make any changes in this file as it will be overwritten.
+// Additionally, you should also exclude this file from your linter and/or formatter to prevent it from being checked or modified.
+
+import { createServerRootRoute } from '@tanstack/react-start/server'
+
+import { Route as rootRouteImport } from './routes/__root'
+import { Route as MergeServerFnMiddlewareContextRouteImport } from './routes/merge-server-fn-middleware-context'
+import { Route as IndexRouteImport } from './routes/index'
+import { ServerRoute as ApiMiddlewareContextServerRouteImport } from './routes/api/middleware-context'
+
+const rootServerRouteImport = createServerRootRoute()
+
+const MergeServerFnMiddlewareContextRoute =
+  MergeServerFnMiddlewareContextRouteImport.update({
+    id: '/merge-server-fn-middleware-context',
+    path: '/merge-server-fn-middleware-context',
+    getParentRoute: () => rootRouteImport,
+  } as any)
+const IndexRoute = IndexRouteImport.update({
+  id: '/',
+  path: '/',
+  getParentRoute: () => rootRouteImport,
+} as any)
+const ApiMiddlewareContextServerRoute =
+  ApiMiddlewareContextServerRouteImport.update({
+    id: '/api/middleware-context',
+    path: '/api/middleware-context',
+    getParentRoute: () => rootServerRouteImport,
+  } as any)
+
+export interface FileRoutesByFullPath {
+  '/': typeof IndexRoute
+  '/merge-server-fn-middleware-context': typeof MergeServerFnMiddlewareContextRoute
+}
+export interface FileRoutesByTo {
+  '/': typeof IndexRoute
+  '/merge-server-fn-middleware-context': typeof MergeServerFnMiddlewareContextRoute
+}
+export interface FileRoutesById {
+  __root__: typeof rootRouteImport
+  '/': typeof IndexRoute
+  '/merge-server-fn-middleware-context': typeof MergeServerFnMiddlewareContextRoute
+}
+export interface FileRouteTypes {
+  fileRoutesByFullPath: FileRoutesByFullPath
+  fullPaths: '/' | '/merge-server-fn-middleware-context'
+  fileRoutesByTo: FileRoutesByTo
+  to: '/' | '/merge-server-fn-middleware-context'
+  id: '__root__' | '/' | '/merge-server-fn-middleware-context'
+  fileRoutesById: FileRoutesById
+}
+export interface RootRouteChildren {
+  IndexRoute: typeof IndexRoute
+  MergeServerFnMiddlewareContextRoute: typeof MergeServerFnMiddlewareContextRoute
+}
+export interface FileServerRoutesByFullPath {
+  '/api/middleware-context': typeof ApiMiddlewareContextServerRoute
+}
+export interface FileServerRoutesByTo {
+  '/api/middleware-context': typeof ApiMiddlewareContextServerRoute
+}
+export interface FileServerRoutesById {
+  __root__: typeof rootServerRouteImport
+  '/api/middleware-context': typeof ApiMiddlewareContextServerRoute
+}
+export interface FileServerRouteTypes {
+  fileServerRoutesByFullPath: FileServerRoutesByFullPath
+  fullPaths: '/api/middleware-context'
+  fileServerRoutesByTo: FileServerRoutesByTo
+  to: '/api/middleware-context'
+  id: '__root__' | '/api/middleware-context'
+  fileServerRoutesById: FileServerRoutesById
+}
+export interface RootServerRouteChildren {
+  ApiMiddlewareContextServerRoute: typeof ApiMiddlewareContextServerRoute
+}
+
+declare module '@tanstack/react-router' {
+  interface FileRoutesByPath {
+    '/merge-server-fn-middleware-context': {
+      id: '/merge-server-fn-middleware-context'
+      path: '/merge-server-fn-middleware-context'
+      fullPath: '/merge-server-fn-middleware-context'
+      preLoaderRoute: typeof MergeServerFnMiddlewareContextRouteImport
+      parentRoute: typeof rootRouteImport
+    }
+    '/': {
+      id: '/'
+      path: '/'
+      fullPath: '/'
+      preLoaderRoute: typeof IndexRouteImport
+      parentRoute: typeof rootRouteImport
+    }
+  }
+}
+declare module '@tanstack/react-start/server' {
+  interface ServerFileRoutesByPath {
+    '/api/middleware-context': {
+      id: '/api/middleware-context'
+      path: '/api/middleware-context'
+      fullPath: '/api/middleware-context'
+      preLoaderRoute: typeof ApiMiddlewareContextServerRouteImport
+      parentRoute: typeof rootServerRouteImport
+    }
+  }
+}
+
+const rootRouteChildren: RootRouteChildren = {
+  IndexRoute: IndexRoute,
+  MergeServerFnMiddlewareContextRoute: MergeServerFnMiddlewareContextRoute,
+}
+export const routeTree = rootRouteImport
+  ._addFileChildren(rootRouteChildren)
+  ._addFileTypes<FileRouteTypes>()
+const rootServerRouteChildren: RootServerRouteChildren = {
+  ApiMiddlewareContextServerRoute: ApiMiddlewareContextServerRoute,
+}
+export const serverRouteTree = rootServerRouteImport
+  ._addFileChildren(rootServerRouteChildren)
+  ._addFileTypes<FileServerRouteTypes>()

e2e/react-start/server-routes/src/router.tsx

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+import { createRouter as createTanStackRouter } from '@tanstack/react-router'
+import { routeTree } from './routeTree.gen'
+import { DefaultCatchBoundary } from './components/DefaultCatchBoundary'
+import { NotFound } from './components/NotFound'
+
+export function createRouter() {
+  const router = createTanStackRouter({
+    routeTree,
+    defaultPreload: 'intent',
+    defaultErrorComponent: DefaultCatchBoundary,
+    defaultNotFoundComponent: () => <NotFound />,
+    scrollRestoration: true,
+  })
+
+  return router
+}
+
+declare module '@tanstack/react-router' {
+  interface Register {
+    router: ReturnType<typeof createRouter>
+  }
+}