Tanstack Start Client Core Start Core Execution Model

Execution Model

Understanding where code runs is fundamental to TanStack Start. This skill covers the isomorphic execution model and how to control environment boundaries.

CRITICAL: ALL code in TanStack Start is isomorphic by default — it runs in BOTH server and client bundles. Route loaders run on BOTH server (during SSR) AND client (during navigation). Server-only operations MUST use createServerFn. CRITICAL: Module-level process.env access is wrong on two axes — security (values leak into the client bundle) AND runtime correctness (on Cloudflare Workers and other edge runtimes, env is injected per-request, so module-level reads evaluate to undefined even on the server). Read env inside .handler() or another per-request function, never at module scope. CRITICAL: VITE_ prefixed environment variables are exposed to the client bundle. Server secrets must NOT have the VITE_ prefix. CRITICAL: Relative URLs belong to browser-only code. An isomorphic loader also runs during SSR, where fetch('/api/...') may have no base URL. Call a server function from the loader, or keep the fetch inside an explicit environment boundary.

Execution Control APIs

Server-Only Execution

createServerFn (RPC pattern)

The primary way to run server-only code. On the client, calls become fetch requests:

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

const fetchUser = createServerFn().handler(async () => {
  const secret = process.env.API_SECRET // safe — server only
  return await db.users.find()
})

// Client calls this via network request
const user = await fetchUser()

createServerOnlyFn (throws on client)

For utility functions that must never run on client:

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

const getSecret = createServerOnlyFn(() => process.env.DATABASE_URL)

// Server: returns the value
// Client: THROWS an error

Client-Only Execution

createClientOnlyFn

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

const saveToStorage = createClientOnlyFn((key: string, value: string) => {
  localStorage.setItem(key, value)
})

ClientOnly Component

// Use @tanstack/<framework>-router for your framework (react, solid, vue)
import { ClientOnly } from '@tanstack/react-router'

function Analytics() {
  return (
    <ClientOnly fallback={null}>
      <GoogleAnalyticsScript />
    </ClientOnly>
  )
}

useHydrated Hook

// Use @tanstack/<framework>-router for your framework (react, solid, vue)
import { useHydrated } from '@tanstack/react-router'

function TimeZoneDisplay() {
  const hydrated = useHydrated()
  const timeZone = hydrated
    ? Intl.DateTimeFormat().resolvedOptions().timeZone
    : 'UTC'

  return <div>Your timezone: {timeZone}</div>
}

Behavior: SSR → false, first client render → false, after hydration → true (stays true).

Environment-Specific Implementations

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

const getDeviceInfo = createIsomorphicFn()
  .server(() => ({ type: 'server', platform: process.platform }))
  .client(() => ({ type: 'client', userAgent: navigator.userAgent }))

Import Protection: File Markers

Experimental.

The .server.* and .client.* filename suffixes (e.g. db.server.ts) opt a file into Start's import protection — it can't be imported from the wrong environment. When you can't or don't want to rename the file, add a side-effect import at the top of the file to apply the same protection by marker:

// src/lib/secrets.ts (filename can't be *.server.ts)
import '@tanstack/react-start/server-only'
// (or @tanstack/solid-start/server-only, @tanstack/vue-start/server-only)

export function getApiKey() {
  return process.env.API_KEY
}

// src/lib/storage.ts
import '@tanstack/react-start/client-only'
// (or @tanstack/solid-start/client-only, @tanstack/vue-start/client-only)

export function savePreferences(prefs: Record<string, string>) {
  localStorage.setItem('prefs', JSON.stringify(prefs))
}

Rules:

• Both markers in the same file is an error.
• Type-only imports are ignored (they erase to nothing at runtime).
• Default behavior is error in production builds and mock in dev. The mock returns a recursive Proxy so dev keeps running while you fix the import graph.

Pick the right tool:

• File should never run on the wrong side and has no client API → *.server.ts filename or import '@tanstack/<fw>-start/server-only'.
• One symbol needs to behave differently per environment → createIsomorphicFn().client(...).server(...).
• One function should error if called from the wrong side → createServerOnlyFn / createClientOnlyFn.
• Component renders only after hydration → <ClientOnly> or useHydrated().

Environment Variables

Server-Side (inside createServerFn)

Access any variable via process.env:

const connectDb = createServerFn().handler(async () => {
  const url = process.env.DATABASE_URL // no prefix needed
  return createConnection(url)
})

Client-Side (components)

Only VITE_ prefixed variables are available:

// Framework-specific component type (React.ReactNode, JSX.Element, etc.)
function ApiProvider({ children }: { children: React.ReactNode }) {
  const apiUrl = import.meta.env.VITE_API_URL // available
  // import.meta.env.DATABASE_URL → undefined (security)
  return (
    <ApiContext.Provider value={{ apiUrl }}>{children}</ApiContext.Provider>
  )
}

Runtime Client Variables

If you need server-side variables on the client without VITE_ prefix, pass them through a server function:

const getRuntimeVar = createServerFn({ method: 'GET' }).handler(() => {
  return process.env.MY_RUNTIME_VAR
})

export const Route = createFileRoute('/')({
  loader: async () => {
    const foo = await getRuntimeVar()
    return { foo }
  },
  component: () => {
    const { foo } = Route.useLoaderData()
    return <div>{foo}</div>
  },
})

Type Safety for Environment Variables

// src/env.d.ts
/// <reference types="vite/client" />

interface ImportMetaEnv {
  readonly VITE_APP_NAME: string
  readonly VITE_API_URL: string
}

interface ImportMeta {
  readonly env: ImportMetaEnv
}

declare global {
  namespace NodeJS {
    interface ProcessEnv {
      readonly DATABASE_URL: string
      readonly JWT_SECRET: string
    }
  }
}

export {}

Common Mistakes

1. CRITICAL: Assuming loaders are server-only

// WRONG — loader runs on BOTH server and client
export const Route = createFileRoute('/dashboard')({
  loader: async () => {
    const secret = process.env.API_SECRET // LEAKED to client
    return fetch(`https://api.example.com/data`, {
      headers: { Authorization: secret },
    })
  },
})

// CORRECT — use createServerFn
const getData = createServerFn({ method: 'GET' }).handler(async () => {
  const secret = process.env.API_SECRET
  return fetch(`https://api.example.com/data`, {
    headers: { Authorization: secret },
  })
})

export const Route = createFileRoute('/dashboard')({
  loader: () => getData(),
})

2. CRITICAL: Reading process.env at module scope

Module-level process.env reads are wrong for two reasons, not one:

1. Security: the value can be inlined into the client bundle, leaking secrets.
2. Runtime correctness (edge runtimes): Cloudflare Workers and other edge SSR runtimes inject env at request time. Module-level code runs at module load, before the env exists, so the read evaluates to undefined even on the server. The bug only surfaces at deploy time.

// WRONG — leaks to client AND is undefined on Workers
const apiKey = process.env.SECRET_KEY
export function fetchData() {
  /* uses apiKey, which is undefined under Worker SSR */
}

// CORRECT — read per-request, inside the handler
const fetchData = createServerFn({ method: 'GET' }).handler(async () => {
  const apiKey = process.env.SECRET_KEY
  return fetch(url, { headers: { Authorization: apiKey } })
})

The same rule applies to middleware .server() callbacks, server-route handlers, and any function that runs per request — read env there, not at the top of the file.

3. CRITICAL: Using VITE_ prefix for server secrets

# WRONG — exposed to client bundle
VITE_SECRET_API_KEY=sk_live_xxx

# CORRECT — no prefix for server secrets
SECRET_API_KEY=sk_live_xxx

# CORRECT — VITE_ only for public client values
VITE_APP_NAME=My App

4. HIGH: Hydration mismatches

// WRONG — different content server vs client
function CurrentTime() {
  return <div>{new Date().toLocaleString()}</div>
}

// CORRECT — consistent rendering
function CurrentTime() {
  const [time, setTime] = useState<string>()
  useEffect(() => {
    setTime(new Date().toLocaleString())
  }, [])
  return <div>{time || 'Loading...'}</div>
}

Architecture Decision Framework

Server-Only (createServerFn / createServerOnlyFn):

• Sensitive data (env vars, secrets)
• Database connections, file system
• External API keys

Client-Only (createClientOnlyFn / <ClientOnly>):

• DOM manipulation, browser APIs
• localStorage, geolocation
• Analytics/tracking

Isomorphic (default / createIsomorphicFn):

• Data formatting, business logic
• Shared utilities
• Route loaders (they're isomorphic by nature)

Cross-References

• start-core/server-functions — the primary server boundary
• start-core/deployment — deployment target affects execution