execution-model

Use createServerFn to define an RPC-style function whose handler executes only on the server; client calls become network requests. For utilities 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.

createServerFn creates an RPC boundary: client calls become network requests to the server. createServerOnlyFn wraps a utility that executes directly on the server but throws an error if called from the client, with no network fallback.

No. VITE_ prefixed variables are inlined into the client bundle and publicly visible. Keep server secrets unprefixed and access them via process.env inside server functions; reserve VITE_ for public client values like app names.

Render browser-dependent content only after hydration using the ClientOnly component with a fallback, or gate logic with the useHydrated hook, which returns false during SSR and true after hydration. Avoid rendering values like new Date() directly.

Use createIsomorphicFn when one symbol needs different implementations per environment, defined via .server() and .client() branches. Use createServerFn when the logic must only ever execute on the server, such as database access or secret usage.