start-core/execution-model

To prevent environment variable leaks during SSR, access process.env and secret values exclusively through server functions rather than client bundles. Keep runtime variables safe by using server-bound calls and avoid exposing VITE_ prefixed vars to the client.

Enforce server and client boundaries by applying creation APIs like createServerFn, createServerOnlyFn, createClientOnlyFn, and createIsomorphicFn to separate server-only logic from client-safe operations during SSR and hydration.

You need ClientOnly components and hydration guards like useHydrated when implementing isomorphic logic that should only execute or render after client hydration, preventing runtime errors and mismatched server-client markup.

The best way to handle process.env in an isomorphic application is to route secret access through server functions, ensuring the client never receives sensitive data while maintaining safe access to runtime vars via server-bound calls.

Isomorphic-by-default development can cause data leaks in server-side rendering if server-only logic is not separated from client-safe operations, making it crucial to apply execution model boundaries to protect secret values.