start-core/execution-model

Isomorphic execution boundaries in TanStack Start are handled using createIsomorphicFn, createServerFn, createServerOnlyFn, and createClientOnlyFn APIs to enforce where code runs and prevent environment leaks.

Environment variable safety during SSR requires enforcing server-side access rules and using the VITE_ prefix for variables exposed to the client, preventing sensitive server-only values from leaking into client bundles.

Sharing route loaders across SSR and client navigation is achieved by applying isomorphic execution functions, allowing data fetching logic to run safely on the server initially and hydrate on the client.

TanStack Start supports hydration-aware rendering through the ClientOnly component and the useHydrated hook, which control when client-specific UI mounts after server-side hydration completes.

Use createServerOnlyFn to strictly block code from running on the client, while createIsomorphicFn allows shared logic to execute on both server and client with different implementations per environment.

Server-only functions executing on the client indicate missing environment boundaries; wrapping them with createServerOnlyFn enforces server execution and prevents client-side access during navigation.