Issue and phenomenon When I use server sent events to the react client: the event messages can be received when I access the backend by request url directly from the browser address bar, but can never be received by the front end application, and the EventStream in Chrome Dev-Tool is empty; However, once I stop the backend server, ALL the messages are flushed/sent to the client at one time. It seems that the messageS are buffered or pended somewhere. The node.js/express test code: The client javascript: Analysis I have already enabled CORS by using cors middleware at the backend; The proxy works fine, as the other requests to the endpoints are avalaible; The Cache Control option settings of the response HTTP header of server-side are exactly the same with the sample code of MDN docs and it looks right. I didn't get an answer from others or google sse, eventSource, cors or proxy. One suggested way is to disable CORS of browser: $ google-chrome --disable-web-security But I don't think this is a good way as we need to put things online eventually, it didn't work for debugging as well when I tried. Solutions This bugged me serveral hours. I luckily found two solutions by changing code line by line: Use full URL to request the events (may not be decent, but it works) from client side: const sse = new EventSource('http://localhost:8080/events'); Set the server-side Cache Control Header to no-transform: 'Cache-Control': 'no-cache', --> 'Cache-Control': 'no-transform', or 'Cache-Control': 'no-cache', --> 'Cache-Control': 'no-cache, no-transform', I cannot fully understand why the no-transform directive matters…