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: Server functions are API endpoints. They are reachable independently of whichever route renders the calling UI. Auth must be enforced inside the handler (or via middleware) for any server function that touches private data. Route beforeLoad is UX, not the data boundary. See start-core/auth-server-primitives for the session/middleware pattern. 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!' }
})