---
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
  - TanStack/router:docs/start/framework/react/guide/import-protection.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 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.

## 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`               |
| `import '@tanstack/<fw>-start/server-only'` | Mark whole file server-only | Import denied             | Allowed               |
| `import '@tanstack/<fw>-start/client-only'` | Mark whole file client-only | Allowed                   | Import denied         |

## 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 }))
```

## 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:

```ts
// 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
}
```

```ts
// 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`:

```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: 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.

```tsx
// 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

```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