SvelteKit 2 breaking changes: error/redirect no longer thrown, cookies require path, top-level promises not auto-awaited, goto rejects external URLs, paths relative by default, preloadCode takes single base-prefixed arg, resolvePath→resolveRoute, improved error handling with status/message, dynamic env vars forbidden during prerendering, use:enhance removes form/data, file forms need multipart/form-data, requires Node 18.13+/Svelte 4/Vite 5/TypeScript 5, $app/stores deprecated for $app/state.