start-core/execution-model

Use createServerFn to define a server function; on the client it becomes a network request while on the server it executes directly. For utilities that must never run on the client, use createServerOnlyFn, which throws an error if called in the browser.

No, route loaders are isomorphic and run on both the server during SSR and the client during navigation. Any secret access or server-only logic inside a loader must be wrapped in createServerFn to avoid leaking values into the client bundle.

Only VITE_-prefixed variables are exposed to the client bundle via import.meta.env. Server secrets must have no prefix and be accessed via process.env only inside createServerFn or createServerOnlyFn handlers.

Hydration mismatches happen when server and client render different content, such as dates or random values. Use the useHydrated hook or the ClientOnly component to defer browser-specific rendering until after hydration completes.

Use createIsomorphicFn when you need different implementations per environment, chaining .server() and .client() handlers. Use createServerFn when the logic must execute only on the server, such as database access or secret usage.