TanStack Start

Full-stack React framework built on TanStack Router + Vite. Client-first with opt-in server capabilities. Type-safe from routes to server functions.

When to Use

Triggers: tanstack start, tanstack router, server function, createServerFn, createFileRoute, gremlin-cms, tanstack app, or any work in a TanStack Start project.

Core Concepts

Execution Model — Critical

Route loaders are ISOMORPHIC — they run on BOTH server and client. This is the #1 gotcha.

// ❌ WRONG — loader runs on client too, exposes secrets
export const Route = createFileRoute('/users')({
  loader: () => {
    const secret = process.env.SECRET // Exposed to client!
    return fetch(`/api/users?key=${secret}`)
  },
})

// ✅ CORRECT — server function wraps server-only logic
const getUsers = createServerFn().handler(() => {
  const secret = process.env.SECRET // Server-only
  return fetch(`/api/users?key=${secret}`)
})

export const Route = createFileRoute('/users')({
  loader: () => getUsers(), // Isomorphic call to server function
})

Server Functions

Type-safe RPC that replaces REST/tRPC/GraphQL for internal data access. Build process replaces server implementations with RPC stubs in client bundles.

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

// GET (default)
export const getPosts = createServerFn().handler(async () => {
  return db.posts.findMany()
})

// POST with input validation
export const createPost = createServerFn({ method: 'POST' })
  .inputValidator(z.object({ title: z.string().min(1), body: z.string() }))
  .handler(async ({ data }) => {
    return db.posts.create(data)
  })

The schema validates runtime input; a TypeScript annotation with an identity function does not. Match the validator API to the project’s pinned TanStack version.

Where to call server functions:

• Route loaders — data fetching
• Components — via useServerFn() hook
• Other server functions — compose server logic
• Event handlers — form submissions, clicks

Server-Only Functions

For utilities that must NEVER reach the client bundle:

import { createServerOnlyFn } from '@tanstack/react-start'

const getDbUrl = createServerOnlyFn(() => process.env.DATABASE_URL)
// Calling from client THROWS — crashes intentionally

File-Based Routing

app/
├── routes/
│   ├── __root.tsx          # Root layout
│   ├── index.tsx           # /
│   ├── about.tsx           # /about
│   ├── posts/
│   │   ├── index.tsx       # /posts
│   │   └── $postId.tsx     # /posts/:postId
│   └── _authed/
│       └── dashboard.tsx   # /dashboard (with auth layout)
├── client.tsx              # Client entry
├── router.tsx              # Router config
└── ssr.tsx                 # SSR entry

• $param = dynamic segment
• _prefix = pathless layout route (groups routes without adding URL segments)
• __root.tsx = root layout (wraps everything)

Route Definition

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

export const Route = createFileRoute('/posts/$postId')({
  // Loader runs before render (isomorphic!)
  loader: ({ params }) => getPost({ data: params.postId }),

  // Component receives loader data
  component: PostPage,

  // Error boundary
  errorComponent: ({ error }) => <div>Error: {error.message}</div>,

  // Pending component (while loader runs)
  pendingComponent: () => <div>Loading...</div>,
})

function PostPage() {
  const post = Route.useLoaderData()
  return <h1>{post.title}</h1>
}

Middleware

Compose reusable server function middleware:

import { createMiddleware } from '@tanstack/react-start'

const authMiddleware = createMiddleware({ type: 'function' }).server(
  async ({ next }) => {
    const session = await getSessionFn()
    if (!session?.user) throw new Error('Unauthorized')
    return next({ context: { session } })
  }
)

// Use in server functions
export const listPosts = createServerFn({ method: 'GET' })
  .middleware([authMiddleware])
  .handler(async ({ context }) => {
    return db.posts.where({ userId: context.session.user.id })
  })

Server Routes (API endpoints)

export const Route = createFileRoute('/api/health')({
  server: {
    handlers: ({ createHandlers }) => createHandlers({
      GET: async ({ request }) => {
        return new Response(JSON.stringify({ ok: true }), {
          headers: { 'Content-Type': 'application/json' },
        })
      },
    }),
  },
})

Using with TanStack Query

import { useServerFn } from '@tanstack/react-start'
import { useQuery, useMutation } from '@tanstack/react-query'

function PostList() {
  const getPostsFn = useServerFn(getPosts)
  const createPostFn = useServerFn(createPost)

  const { data } = useQuery({
    queryKey: ['posts'],
    queryFn: () => getPostsFn(),
  })

  const mutation = useMutation({
    mutationFn: (data) => createPostFn({ data }),
    onSuccess: () => queryClient.invalidateQueries({ queryKey: ['posts'] }),
  })
}

File Organization (Large Apps)

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)

• .functions.ts — server function wrappers, safe to import anywhere
• .server.ts — server-only helpers, NEVER import from client code

App Config

// app.config.ts
import { defineConfig } from '@tanstack/react-start/config'
import tsConfigPaths from 'vite-tsconfig-paths'

export default defineConfig({
  vite: {
    plugins: [tsConfigPaths({ projects: ['./tsconfig.json'] })],
  },
})

BetterAuth Integration

// Server function for session
import { createServerFn } from '@tanstack/react-start'
import { getRequestHeaders } from '@tanstack/react-start/server'
import { auth } from '@/lib/auth'

export const getSessionFn = createServerFn({ method: 'GET' }).handler(
  async () => {
    const headers = getRequestHeaders()
    return auth.api.getSession({ headers })
  }
)

// Protected layout route
export const Route = createFileRoute('/_authed')({
  beforeLoad: async () => {
    const session = await getSessionFn()
    if (!session?.user) throw redirect({ to: '/sign-in' })
  },
  component: () => <Outlet />,
})

Deployment

TanStack Start deploys to any Node/Bun target, Vercel, Cloudflare Workers, Netlify.

Vercel (Critical)

You MUST add the nitro() Vite plugin — without it, Vercel builds succeed but serve 404s.

// vite.config.ts
import { nitro } from 'nitro/vite'

export default defineConfig({
  plugins: [
    tanstackStart(),
    nitro(),        // ← REQUIRED for Vercel
    viteReact(),
  ],
})

Install: pnpm add nitro

Monorepo (pnpm + Turborepo) quirks:

• Set rootDirectory in Vercel project settings to the app dir (e.g., apps/gremlin-cms)
• Framework preset should be TanStack Start or auto-detect — never Next.js
• Do NOT manually set outputDirectory — Nitro generates .vercel/output automatically
• Build command from repo root: turbo run build --filter=<app-name> or let Vercel auto-detect
• If you get 404 after successful build, check: (1) nitro plugin present, (2) framework preset correct, (3) no stale .vercel config

No CLI deploys — push to git, let Vercel auto-deploy. Only use vercel --prod for emergency hotfixes.

Rules

1. Never access process.env in loaders directly — use createServerFn or createServerOnlyFn
2. Loaders are isomorphic — they run on both server AND client during navigation
3. Server functions are the boundary — anything that touches DB, env vars, or secrets goes through createServerFn
4. Prefer server functions over API routes for internal data access — type-safe, no manual fetch
5. Use useServerFn() hook when calling server functions from components (not direct calls)
6. Middleware composes — stack auth, validation, logging as reusable middleware

Living Document

This skill will grow as we build gremlin-cms. Update with patterns discovered during development.