start-core/execution-model

To create a server-only data fetch in TanStack Start, wrap your logic with createServerFn and call it from the client to utilize the built-in RPC pattern, ensuring the execution remains securely on the server while returning data to the client.

createServerOnlyFn restricts execution to the server bundle, createClientOnlyFn restricts it to the client, and createIsomorphicFn allows you to define separate implementations for both environments within a single function declaration.

Safely access environment variables by following VITE_ prefix rules for client-exposed variables and using process.env for server-only secrets, combined with server-only functions to prevent accidental leakage during isomorphic rendering.

Use ClientOnly and useHydrated when you need to render specific UI components exclusively after client-side hydration, whereas createIsomorphicFn is better suited for branching data fetching or route loader logic across server and client environments.

Yes, TanStack Start route loaders support createServerFn, allowing you to execute secure server-side data fetching before rendering, which safely provides data to the client during SSR and subsequent client-side navigations.