For anyone seeing ERR_INTERNET_DISCONNECTED on localhost:5173 in Edge with Vite/React: check Edge DevTools → Network. Mine was set to Offline for that tab, so all requests (including Vite’s HMR/WebSocket and my /health fetch) were blocked. Changing it back to No throttling and hard-refreshing fixed it. This was a DevTools setting, not a Vite or backend issue.