Start server functions

TanStack Start Server Functions

Server functions are same-origin, type-safe RPC endpoints created with createServerFn. They run only on the server but are callable from loaders, components, hooks, and other server functions. This skill targets @tanstack/react-start v1.166.2.

Agent Workflow (MANDATORY)

Before implementing, verify current APIs against Context7 (/websites/tanstack_start_framework_react) + Exa, then explore the target codebase. After changes, run fuse-ai-pilot:sniper.

Critical Rules (read first)

1. A server function is an independent HTTP endpoint. A route beforeLoad redirect protects the route UI, NOT the RPC — an attacker can hit the endpoint directly. Put auth inside the .handler() or in middleware for every function touching private data.
2. Never use Next.js/Remix patterns. No "use server", getServerSideProps, Remix loader/action, or react-router-dom. Use createServerFn exclusively (from @tanstack/react-start).
3. Loaders are isomorphic (run on client AND server). DB queries, secrets, and file-system access MUST live in a server function, never in a loader.
4. useServerFn is MANDATORY when the function does throw redirect() or throw notFound() — the hook wires the throw into the router. Optional for plain-data functions (a direct call or useMutation/useQuery is fine).
5. Strict serialization is on by default. Validator input and handler return types must be serializable (FormData allowed as POST input, Response allowed as output). strict: false disables only the TS check, not runtime.

Overview

When to Use	Do NOT Use
Internal type-safe RPC from your own app	Public/cross-origin API (use start-server-routes)
Data fetching in loaders	Raw HTTP method routing on a URL path
Mutations from event handlers	Composable auth/logging chains (use start-middleware)

Architecture

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

The build replaces server function bodies with RPC stubs in the client bundle, so static imports of .functions.ts from client components are safe.

→ See crud-server-functions.md

Reference Guide

Concepts

Topic	Reference	Load when
Creating functions	creating.md	Defining createServerFn, validators, serialization
Calling functions	calling.md	Invoking from loaders/components, useServerFn, redirect/notFound
Security	security.md	Enforcing auth, CSRF, caching auth'd responses

Templates

Template	When to Use
crud-server-functions.md	Building a full CRUD module
form-with-validation.md	Handling FormData submissions with Zod

Quick Reference

Basic function with validation

import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'

export const createUser = createServerFn({ method: 'POST' })
  .validator(z.object({ name: z.string().min(1) }))
  .handler(async ({ data }) => db.users.create(data))

await createUser({ data: { name: 'John' } })

Redirect (requires useServerFn in components)

import { useServerFn } from '@tanstack/react-start'
import { redirect } from '@tanstack/react-router'

const signup = createServerFn({ method: 'POST' })
  .handler(async () => { throw redirect({ to: '/dashboard' }) })

// In a component:
const signupFn = useServerFn(signup)

→ See calling.md for the full redirect/notFound rules

Best Practices

DO

• Enforce auth inside every handler that reads/writes private data
• Split server-only helpers into .server.ts, wrappers into .functions.ts
• Validate every input crossing the network boundary with Zod
• Wrap with useServerFn when in doubt (no-op for plain-data functions)

DON'T

• Rely on route guards to protect a server function
• Use "use server", getServerSideProps, or Remix action
• Put DB queries or secrets in a route loader
• Dynamically import() server functions (breaks bundler shaking)