Start Server Core (@tanstack/start-server-core)

Server-side runtime for TanStack Start. Provides the request handler, request/response utilities, cookie management, and session management. All utilities are available anywhere in the call stack during a request via AsyncLocalStorage.

CRITICAL: These utilities are SERVER-ONLY. Import them from @tanstack/<framework>-start/server, not from the main entry point. They throw if called outside a server request context.
CRITICAL: Types are FULLY INFERRED. Never cast, never annotate inferred values.

createStartHandler

Creates the main request handler that processes all incoming requests through three phases: server functions, server routes, then app SSR.

// src/server.ts
// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createStartHandler } from '@tanstack/react-start/server'
import { defaultStreamHandler } from '@tanstack/react-start/server'

export default createStartHandler({
  handler: defaultStreamHandler,
})

With asset URL transforms (CDN):

export default createStartHandler({
  handler: defaultStreamHandler,
  transformAssetUrls: 'https://cdn.example.com',
})

Request Utilities

All imported from @tanstack/<framework>-start/server. Available anywhere during request handling — no parameter passing needed.

Reading Request Data

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from '@tanstack/react-start'
import {
  getRequest,
  getRequestHeaders,
  getRequestHeader,
  getRequestIP,
  getRequestHost,
  getRequestUrl,
  getRequestProtocol,
} from '@tanstack/react-start/server'

const serverFn = createServerFn({ method: 'GET' }).handler(async () => {
  const request = getRequest()
  const headers = getRequestHeaders()
  const auth = getRequestHeader('authorization')
  const ip = getRequestIP({ xForwardedFor: true })
  const host = getRequestHost()
  const url = getRequestUrl()
  const protocol = getRequestProtocol()

  return { ip, host }
})

Setting Response Data

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from '@tanstack/react-start'
import {
  setResponseHeader,
  setResponseHeaders,
  setResponseStatus,
  getResponseHeaders,
  getResponseHeader,
  getResponseStatus,
  removeResponseHeader,
  clearResponseHeaders,
} from '@tanstack/react-start/server'

const serverFn = createServerFn({ method: 'POST' }).handler(async () => {
  setResponseStatus(201)
  setResponseHeader('x-custom', 'value')
  setResponseHeaders({ 'cache-control': 'no-store' })

  return { created: true }
})

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from '@tanstack/react-start'
import {
  getCookies,
  getCookie,
  setCookie,
  deleteCookie,
} from '@tanstack/react-start/server'

const serverFn = createServerFn({ method: 'POST' }).handler(async () => {
  const allCookies = getCookies()
  const token = getCookie('session-token')

  setCookie('preference', 'dark', {
    httpOnly: true,
    secure: true,
    maxAge: 60 * 60 * 24 * 30, // 30 days
    path: '/',
  })

  deleteCookie('old-cookie')
})

Session Management

Encrypted sessions stored in cookies. Requires a password for encryption.

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from '@tanstack/react-start'
import {
  useSession,
  getSession,
  updateSession,
  clearSession,
} from '@tanstack/react-start/server'

const sessionConfig = {
  password: process.env.SESSION_SECRET!,
  name: 'my-app-session',
  maxAge: 60 * 60 * 24 * 7, // 7 days
}

// Full session manager
const getUser = createServerFn({ method: 'GET' }).handler(async () => {
  const session = await useSession<{ userId: string }>(sessionConfig)
  return session.data
})

// Update session
const login = createServerFn({ method: 'POST' })
  .inputValidator((data: { userId: string }) => data)
  .handler(async ({ data }) => {
    await updateSession(sessionConfig, { userId: data.userId })
    return { success: true }
  })

// Clear session
const logout = createServerFn({ method: 'POST' }).handler(async () => {
  await clearSession(sessionConfig)
  return { success: true }
})

Session Config

Option	Type	Default	Description
password	string	required	Encryption key
name	string	'start'	Cookie name
maxAge	number	undefined	Expiry in seconds
cookie	false \| CookieOptions	undefined	Cookie settings

Session Manager Methods

const session = await useSession<{ userId: string }>(config)

session.id // Session ID (string | undefined)
session.data // Session data (typed)
await session.update({ userId: '123' }) // Persist session data
await session.clear() // Clear session data

Query Validation

Validate query string parameters using a Standard Schema:

// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { getValidatedQuery } from '@tanstack/react-start/server'
import { z } from 'zod'

const serverFn = createServerFn({ method: 'GET' }).handler(async () => {
  const query = await getValidatedQuery(
    z.object({
      page: z.coerce.number().default(1),
      limit: z.coerce.number().default(20),
    }),
  )

  return { page: query.page }
})

Note: getValidatedQuery accepts a Standard Schema validator, not a callback function.

How Request Handling Works

createStartHandler processes requests in three phases:

Server Function Dispatch — If URL matches the server function prefix (/_serverFn), deserializes the payload, runs global request middleware, executes the server function, and returns the serialized result.
Server Route Handler — For non-server-function requests, matches the URL against routes with server.handlers. Runs route middleware, then the matched HTTP method handler. Handlers can return a Response or call next() to fall through to SSR.
App Router SSR — Loads all route loaders, dehydrates state for client hydration, and calls the handler callback (e.g., defaultStreamHandler) to render HTML.

Common Mistakes

1. CRITICAL: Importing server utilities in client code

Server utilities use AsyncLocalStorage and only work during server request handling. Importing them in client code causes build errors or runtime crashes.

// WRONG — importing in a component file that runs on client
import { getCookie } from '@tanstack/react-start/server'

function MyComponent() {
  const token = getCookie('auth') // crashes on client
}

// CORRECT — use inside server functions only
// Use @tanstack/<framework>-start for your framework (react, solid, vue)
import { createServerFn } from '@tanstack/react-start'
import { getCookie } from '@tanstack/react-start/server'

const getAuth = createServerFn({ method: 'GET' }).handler(async () => {
  return getCookie('auth')
})

start-core/server-functions — creating server functions that use these utilities
start-core/middleware — request middleware
start-core/server-routes — server route handlers

@tanstack/start-server-core