start-core-execution-model

Wrap the logic in createServerFn, which executes directly on the server and becomes a network request when called from the client. For utility functions that must never run on the client, use createServerOnlyFn, which throws an error if invoked client-side.

Edge runtimes like Cloudflare Workers inject environment variables per request, so module-level process.env reads evaluate to undefined. Read env inside a createServerFn .handler() or another per-request function instead of at module scope.

Loaders are isomorphic: they run on the server during SSR and on the client during navigation. Any secret or server-only operation inside a loader must be moved into a createServerFn call to avoid leaking into the client bundle.

Only variables with the VITE_ prefix are exposed to the client bundle via import.meta.env. For non-prefixed server variables, pass them through a createServerFn handler and return the value to the loader or component.

Render browser-dependent content only after hydration using the ClientOnly component with a fallback, or the useHydrated hook which returns false during SSR and true after hydration. This keeps server and first client render output identical.

Use createIsomorphicFn when one symbol needs different implementations per environment, such as reading navigator.userAgent on the client and process.platform on the server. Use createServerFn when the logic must only ever execute on the server.