Ssr

SSR (Server-Side Rendering)

WARNING: SSR APIs are experimental. They share internal implementations with TanStack Start and may change. TanStack Start is the recommended way to do SSR in production — use manual SSR setup only when integrating with an existing server.

CRITICAL: TanStack Router is CLIENT-FIRST. Loaders run on the client by default. With SSR enabled, loaders run on BOTH client AND server. They are NOT server-only like Remix/Next.js loaders. See router-core/data-loading.

CRITICAL: Do not generate Next.js patterns (getServerSideProps, App Router, server components) or Remix patterns (server-only loader exports). TanStack Router has its own SSR API.

Concepts

There are two SSR flavors:

• Non-streaming: Full page rendered on server, sent as one HTML response, then hydrated on client.
• Streaming: Critical first paint sent immediately; remaining content streamed incrementally as it resolves.

Key behaviors:

• Memory history is used automatically on the server (no window).
• Loader data is automatically dehydrated on the server and hydrated on the client.
• Data serialization supports Date, Error, FormData, and undefined out of the box.

Setup: Shared Router Factory

The router must be created identically on server and client. Export a factory function from a shared file:

// src/router.tsx
import { createRouter as createTanstackRouter } from '@tanstack/react-router'
import { routeTree } from './routeTree.gen'

export function createRouter() {
  return createTanstackRouter({ routeTree })
}

declare module '@tanstack/react-router' {
  interface Register {
    router: ReturnType<typeof createRouter>
  }
}

Non-Streaming SSR

Server Entry (using defaultRenderHandler)

// src/entry-server.tsx
import {
  createRequestHandler,
  defaultRenderHandler,
} from '@tanstack/react-router/ssr/server'
import { createRouter } from './router'

export async function render({ request }: { request: Request }) {
  const handler = createRequestHandler({ request, createRouter })
  return await handler(defaultRenderHandler)
}

Server Entry (using renderRouterToString for custom wrappers)

// src/entry-server.tsx
import {
  createRequestHandler,
  renderRouterToString,
  RouterServer,
} from '@tanstack/react-router/ssr/server'
import { createRouter } from './router'

export function render({ request }: { request: Request }) {
  const handler = createRequestHandler({ request, createRouter })

  return handler(({ responseHeaders, router }) =>
    renderRouterToString({
      responseHeaders,
      router,
      children: <RouterServer router={router} />,
    }),
  )
}

Client Entry

// src/entry-client.tsx
import { hydrateRoot } from 'react-dom/client'
import { RouterClient } from '@tanstack/react-router/ssr/client'
import { createRouter } from './router'

const router = createRouter()

hydrateRoot(document, <RouterClient router={router} />)

Streaming SSR

Server Entry (using defaultStreamHandler)

// src/entry-server.tsx
import {
  createRequestHandler,
  defaultStreamHandler,
} from '@tanstack/react-router/ssr/server'
import { createRouter } from './router'

export async function render({ request }: { request: Request }) {
  const handler = createRequestHandler({ request, createRouter })
  return await handler(defaultStreamHandler)
}

Server Entry (using renderRouterToStream for custom wrappers)

// src/entry-server.tsx
import {
  createRequestHandler,
  renderRouterToStream,
  RouterServer,
} from '@tanstack/react-router/ssr/server'
import { createRouter } from './router'

export function render({ request }: { request: Request }) {
  const handler = createRequestHandler({ request, createRouter })

  return handler(({ request, responseHeaders, router }) =>
    renderRouterToStream({
      request,
      responseHeaders,
      router,
      children: <RouterServer router={router} />,
    }),
  )
}

Streaming is automatic — deferred data (unawaited promises from loaders) and streamed markup just work when using defaultStreamHandler or renderRouterToStream.

Document Head Management

Use the head route option to manage <title>, <meta>, <link>, and <style> tags. Render <HeadContent /> in <head> and <Scripts /> in <body>.

Root Route with Head

// src/routes/__root.tsx
import {
  createRootRoute,
  HeadContent,
  Outlet,
  Scripts,
} from '@tanstack/react-router'

export const Route = createRootRoute({
  head: () => ({
    meta: [
      { charSet: 'UTF-8' },
      { name: 'viewport', content: 'width=device-width, initial-scale=1.0' },
      { title: 'My App' },
    ],
    links: [{ rel: 'icon', href: '/favicon.ico' }],
  }),
  component: RootComponent,
})

function RootComponent() {
  return (
    <html lang="en">
      <head>
        <HeadContent />
      </head>
      <body>
        <Outlet />
        <Scripts />
      </body>
    </html>
  )
}

Per-Route Head (Nested Deduplication)

Child route title and meta tags override parent tags with the same name/property:

// src/routes/posts/$postId.tsx
import { createFileRoute } from '@tanstack/react-router'

export const Route = createFileRoute('/posts/$postId')({
  loader: async ({ params }) => {
    const post = await fetchPost(params.postId)
    return { post }
  },
  head: ({ loaderData }) => ({
    meta: [
      { title: loaderData.post.title },
      { name: 'description', content: loaderData.post.excerpt },
    ],
  }),
  component: PostPage,
})

function PostPage() {
  const { post } = Route.useLoaderData()
  return <article>{post.content}</article>
}

SPA Head (No Full HTML Control)

For SPAs without server-rendered HTML, render <HeadContent /> at the top of the component tree:

import { createRootRoute, HeadContent, Outlet } from '@tanstack/react-router'

const rootRoute = createRootRoute({
  head: () => ({
    meta: [{ title: 'My SPA' }],
  }),
  component: () => (
    <>
      <HeadContent />
      <Outlet />
    </>
  ),
})

Body Scripts

Use scripts (separate from head.scripts) to inject scripts into <body> before the app entry point:

export const Route = createRootRoute({
  scripts: () => [{ children: 'console.log("runs before hydration")' }],
})

The <Scripts /> component renders these. Place it at the end of <body>.

ScriptOnce for Pre-Hydration Scripts

ScriptOnce renders a <script> during SSR that executes immediately and self-removes. On client navigation, it does nothing (no duplicate execution).

import { ScriptOnce } from '@tanstack/react-router'

const themeScript = `(function() {
  try {
    const theme = localStorage.getItem('theme') || 'auto';
    const resolved = theme === 'auto'
      ? (matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light')
      : theme;
    document.documentElement.classList.add(resolved);
  } catch (e) {}
})();`

function ThemeProvider({ children }: { children: React.ReactNode }) {
  return (
    <>
      <ScriptOnce children={themeScript} />
      {children}
    </>
  )
}

If the script modifies the DOM (e.g., adds a class to <html>), use suppressHydrationWarning on the element:

<html lang="en" suppressHydrationWarning>

Express Integration Example

createRequestHandler expects a Web API Request and returns a Web API Response. For Express, convert between formats:

// src/entry-server.tsx
import { pipeline } from 'node:stream/promises'
import {
  RouterServer,
  createRequestHandler,
  renderRouterToString,
} from '@tanstack/react-router/ssr/server'
import { createRouter } from './router'
import type express from 'express'

export async function render({
  req,
  res,
}: {
  req: express.Request
  res: express.Response
}) {
  const protocol = req.get('x-forwarded-proto') ?? req.protocol
  const host = req.get('x-forwarded-host') ?? req.get('host')
  const url = new URL(req.originalUrl || req.url, `${protocol}://${host}`).href

  const request = new Request(url, {
    method: req.method,
    headers: (() => {
      const headers = new Headers()
      for (const [key, value] of Object.entries(req.headers)) {
        headers.set(key, value as any)
      }
      return headers
    })(),
  })

  const handler = createRequestHandler({ request, createRouter })

  const response = await handler(({ responseHeaders, router }) =>
    renderRouterToString({
      responseHeaders,
      router,
      children: <RouterServer router={router} />,
    }),
  )

  res.status(response.status)
  response.headers.forEach((value, name) => {
    res.setHeader(name, value)
  })

  return pipeline(response.body as any, res)
}

Common Mistakes

1. HIGH: Using browser APIs in loaders without environment check

Loaders run on BOTH client and server with SSR. Browser-only APIs (window, document, localStorage) throw on the server.

// WRONG — crashes on server
loader: async () => {
  const token = localStorage.getItem('token')
  return fetchData(token)
}

// CORRECT — guard with environment check
loader: async () => {
  const token =
    typeof window !== 'undefined' ? localStorage.getItem('token') : null
  return fetchData(token)
}

2. MEDIUM: Using hash fragments for server-rendered content

Hash fragments (#section) are never sent to the server. Conditional rendering based on hash causes hydration mismatches.

// WRONG — server has no hash, client does → mismatch
component: () => {
  const hash = window.location.hash
  return hash === '#admin' ? <AdminPanel /> : <UserPanel />
}

// CORRECT — use search params for server-visible state
validateSearch: z.object({ view: fallback(z.enum(['admin', 'user']), 'user') }),
component: () => {
  const { view } = Route.useSearch()
  return view === 'admin' ? <AdminPanel /> : <UserPanel />
}

3. CRITICAL: Generating Next.js, Remix, or React Router DOM patterns

TanStack Router does NOT use getServerSideProps, getStaticProps, App Router page.tsx, Remix-style server-only loader exports, or anything from react-router-dom.

Wrong file structures

WRONG (Next.js Pages Router):
  src/pages/index.tsx
  src/pages/_app.tsx
  src/pages/posts/[id].tsx

WRONG (Next.js App Router):
  app/layout.tsx
  app/page.tsx
  app/posts/[id]/page.tsx

WRONG (Next.js custom App):
  _app/index.tsx
  pages/_app.tsx, pages/_document.tsx

CORRECT (TanStack Router file-based routing):
  src/routes/__root.tsx
  src/routes/index.tsx
  src/routes/posts/$postId.tsx

Wrong imports

// WRONG — react-router-dom is a different library
import {
  Link,
  useNavigate,
  BrowserRouter,
  Route,
  Routes,
} from 'react-router-dom'

// WRONG — Next.js Link/router
import Link from 'next/link'
import { useRouter } from 'next/router' // Pages Router
import { useRouter } from 'next/navigation' // App Router

// CORRECT — everything routing-related lives in @tanstack/react-router
import {
  Link,
  useNavigate,
  useRouter,
  useLocation,
  redirect,
} from '@tanstack/react-router'

Wrong loader/data-fetching patterns

// WRONG — Next.js Pages Router
export async function getServerSideProps() {
  return { props: { data: await fetchData() } }
}

// WRONG — Remix
export async function loader({ request }: LoaderFunctionArgs) {
  return json({ data: await fetchData() })
}

// CORRECT — TanStack Router
export const Route = createFileRoute('/data')({
  loader: async () => {
    const data = await fetchData()
    return { data }
  },
  component: DataPage,
})

function DataPage() {
  const { data } = Route.useLoaderData()
  return <div>{data}</div>
}

If you see src/pages/, app/layout.tsx, react-router-dom, or any of the above in agent output, the agent is generating for the wrong framework. The build will either fail or produce duplicate / routes that conflict at runtime.

Tension: Client-First Loaders vs SSR

TanStack Router loaders are client-first by design. When SSR is enabled, they run in both environments. This means:

• Browser APIs work by default (client-only) but break under SSR
• Database access does NOT belong in loaders (unlike Remix/Next) — use API routes
• For server-only data logic with SSR, use TanStack Start's server functions

See router-core/data-loading for loader fundamentals.

Cross-References

• router-core/data-loading — SSR changes where loaders execute
• compositions/router-query — SSR dehydration/hydration with TanStack Query