Start Core/server Functions

Server Functions

Server functions are type-safe RPCs created with createServerFn. They run exclusively on the server but can be called from anywhere — loaders, components, hooks, event handlers, or other server functions.

CRITICAL: Loaders are ISOMORPHIC — they run on BOTH client and server. Database queries, file system access, and secret API keys MUST go inside createServerFn, NOT in loaders directly. CRITICAL: Do not use "use server" directives, getServerSideProps, or any Next.js/Remix server patterns. TanStack Start uses createServerFn exclusively.

Basic Usage

import { createServerFn } from "@tanstack/react-start"

// GET (default)
const getData = createServerFn().handler(async () => {
  return { message: "Hello from server!" }
})

// POST
const saveData = createServerFn({ method: "POST" }).handler(async () => {
  return { success: true }
})

Calling from Loaders

import { createFileRoute } from "@tanstack/react-router"
import { createServerFn } from "@tanstack/react-start"

const getPosts = createServerFn({ method: "GET" }).handler(async () => {
  const posts = await db.query("SELECT * FROM posts")
  return { posts }
})

export const Route = createFileRoute("/posts")({
  loader: () => getPosts(),
  component: PostList,
})

function PostList() {
  const { posts } = Route.useLoaderData()
  return (
    <ul>
      {posts.map((p) => (
        <li key={p.id}>{p.title}</li>
      ))}
    </ul>
  )
}

Calling from Components

Use the useServerFn hook to call server functions from event handlers:

import { useServerFn } from "@tanstack/react-start"

const deletePost = createServerFn({ method: "POST" })
  .inputValidator((data: { id: string }) => data)
  .handler(async ({ data }) => {
    await db.delete("posts").where({ id: data.id })
    return { success: true }
  })

function DeleteButton({ postId }: { postId: string }) {
  const deletePostFn = useServerFn(deletePost)

  return (
    <button => deletePostFn({ data: { id: postId } })}>
      Delete
    </button>
  )
}

Input Validation

Basic Validator

const greetUser = createServerFn({ method: "GET" })
  .inputValidator((data: { name: string }) => data)
  .handler(async ({ data }) => {
    return `Hello, ${data.name}!`
  })

await greetUser({ data: { name: "John" } })

Zod Validator

import { z } from "zod"

const createUser = createServerFn({ method: "POST" })
  .inputValidator(
    z.object({
      name: z.string().min(1),
      age: z.number().min(0),
    }),
  )
  .handler(async ({ data }) => {
    return `Created user: ${data.name}, age ${data.age}`
  })

FormData

const submitForm = createServerFn({ method: "POST" })
  .inputValidator((data) => {
    if (!(data instanceof FormData)) {
      throw new Error("Expected FormData")
    }
    return {
      name: data.get("name")?.toString() || "",
      email: data.get("email")?.toString() || "",
    }
  })
  .handler(async ({ data }) => {
    return { success: true }
  })

Error Handling

Errors

const riskyFunction = createServerFn().handler(async () => {
  throw new Error("Something went wrong!")
})

try {
  await riskyFunction()
} catch (error) {
  console.log(error.message) // "Something went wrong!"
}

Redirects

import { redirect } from "@tanstack/react-router"

const requireAuth = createServerFn().handler(async () => {
  const user = await getCurrentUser()
  if (!user) {
    throw redirect({ to: "/login" })
  }
  return user
})

Not Found

import { notFound } from "@tanstack/react-router"

const getPost = createServerFn()
  .inputValidator((data: { id: string }) => data)
  .handler(async ({ data }) => {
    const post = await db.findPost(data.id)
    if (!post) {
      throw notFound()
    }
    return post
  })

Server Context Utilities

Access request/response details inside server function handlers:

import { createServerFn } from "@tanstack/react-start"
import {
  getRequest,
  getRequestHeader,
  setResponseHeaders,
  setResponseStatus,
} from "@tanstack/react-start/server"

const getCachedData = createServerFn({ method: "GET" }).handler(async () => {
  const request = getRequest()
  const authHeader = getRequestHeader("Authorization")

  setResponseHeaders({
    "Cache-Control": "public, max-age=300",
  })
  setResponseStatus(200)

  return fetchData()
})

Available utilities:

• getRequest() — full Request object
• getRequestHeader(name) — single request header
• setResponseHeader(name, value) — single response header
• setResponseHeaders(headers) — multiple response headers
• setResponseStatus(code) — HTTP status code

File Organization

src/utils/
├── users.functions.ts   # createServerFn wrappers (safe to import anywhere)
├── users.server.ts      # Server-only helpers (DB queries, internal logic)
└── schemas.ts           # Shared validation schemas (client-safe)

// users.server.ts — server-only helpers
import { db } from "~/db"

export async function findUserById(id: string) {
  return db.query.users.findFirst({ where: eq(users.id, id) })
}

// users.functions.ts — server functions
import { createServerFn } from "@tanstack/react-start"
import { findUserById } from "./users.server"

export const getUser = createServerFn({ method: "GET" })
  .inputValidator((data: { id: string }) => data)
  .handler(async ({ data }) => {
    return findUserById(data.id)
  })

Static imports of server functions are safe — the build replaces implementations with RPC stubs in client bundles.

Common Mistakes

1. CRITICAL: Putting server-only code in loaders

// WRONG — loader is ISOMORPHIC, runs on BOTH client and server
export const Route = createFileRoute("/posts")({
  loader: async () => {
    const posts = await db.query("SELECT * FROM posts")
    return { posts }
  },
})

// CORRECT — use createServerFn for server-only logic
const getPosts = createServerFn({ method: "GET" }).handler(async () => {
  const posts = await db.query("SELECT * FROM posts")
  return { posts }
})

export const Route = createFileRoute("/posts")({
  loader: () => getPosts(),
})

2. CRITICAL: Using Next.js/Remix server patterns

// WRONG — "use server" is a React directive, not used in TanStack Start
'use server'
export async function getUser() { ... }

// WRONG — getServerSideProps is Next.js
export async function getServerSideProps() { ... }

// CORRECT — TanStack Start uses createServerFn
const getUser = createServerFn({ method: 'GET' })
  .handler(async () => { ... })

3. HIGH: Dynamic imports for server functions

// WRONG — can cause bundler issues
const { getUser } = await import("~/utils/users.functions")

// CORRECT — static imports are safe, build handles environment shaking
import { getUser } from "~/utils/users.functions"

4. HIGH: Awaiting server function without calling it

createServerFn returns a function — it must be invoked with ():

// WRONG — getItems is a function, not a Promise
const data = await getItems

// CORRECT — call the function
const data = await getItems()

// With validated input
const data = await getItems({ data: { id: "1" } })

5. MEDIUM: Not using useServerFn for component calls

When calling server functions from event handlers in components, use useServerFn to get proper React integration:

// WRONG — direct call doesn't integrate with React lifecycle
<button => deletePost({ data: { id } })}>Delete</button>

// CORRECT — useServerFn integrates with React
const deletePostFn = useServerFn(deletePost)
<button => deletePostFn({ data: { id } })}>Delete</button>

Cross-References

• start-core/execution-model — understanding where code runs
• start-core/middleware — composing server functions with middleware