---
name: start-core/execution-model
description: >-
  Isomorphic-by-default principle, environment boundary functions
  (createServerFn, createServerOnlyFn, createClientOnlyFn,
  createIsomorphicFn), ClientOnly component, useHydrated hook,
  import protection, dead code elimination, environment variable
  safety (VITE_ prefix, process.env).
type: sub-skill
library: tanstack-start
library_version: '1.166.2'
requires:
  - start-core
sources:
  - TanStack/router:docs/start/framework/react/guide/execution-model.md
  - TanStack/router:docs/start/framework/react/guide/environment-variables.md
---

# 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 runs in both environments. Secret values leak into the client bundle. Access secrets ONLY inside `createServerFn` or `createServerOnlyFn`.
> **CRITICAL**: `VITE_` prefixed environment variables are exposed to the client bundle. Server secrets must NOT have the `VITE_` prefix.

## Execution Control APIs

| API                      | Use Case                  | Client Behavior           | Server Behavior       |
| ------------------------ | ------------------------- | ------------------------- | --------------------- |
| `createServerFn()`       | RPC calls, data mutations | Network request to server | Direct execution      |
| `createServerOnlyFn(fn)` | Utility functions         | Throws error              | Direct execution      |
| `createClientOnlyFn(fn)` | Browser utilities         | Direct execution          | Throws error          |
| `createIsomorphicFn()`   | Different impl per env    | Uses `.client()` impl     | Uses `.server()` impl |
| `<ClientOnly>`           | Browser-only components   | Renders children          | Renders fallback      |
| `useHydrated()`          | Hydration-dependent logic | `true` after hydration    | `false`               |

## Server-Only Execution

### createServerFn (RPC pattern)

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

```tsx
// 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:

```tsx
// 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

```tsx
// 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

```tsx
// 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

```tsx
// 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

```tsx
// 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 }))
```

## Environment Variables

### Server-Side (inside createServerFn)

Access any variable via `process.env`:

```tsx
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:

```tsx
// 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:

```tsx
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

```tsx
// 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

```tsx
// 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: Exposing secrets via module-level process.env

```tsx
// WRONG — runs in both environments, value in client bundle
const apiKey = process.env.SECRET_KEY
export function fetchData() {
  /* uses apiKey */
}

// CORRECT — access inside server function only
const fetchData = createServerFn({ method: 'GET' }).handler(async () => {
  const apiKey = process.env.SECRET_KEY
  return fetch(url, { headers: { Authorization: apiKey } })
})
```

### 3. CRITICAL: Using VITE\_ prefix for server secrets

```bash
# 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

```tsx
// 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](../server-functions/SKILL.md) — the primary server boundary
- [start-core/deployment](../deployment/SKILL.md) — deployment target affects execution