Cloudflare doesn't officially support SSE, but if you send keepalives events every 15 or 20sec or so you can reliably use SSE for 40 min + in my experiance.

No server traffic for 100+ sec officially results in a 524, so you could possibly make that keepalive interval longer, but I haven't tested it.

Make sure to have the new style cache rule with Bypass cache selected and absolutely make sure you are using HTTP/2 all the way to the origin.

The 6 connections per browser limit of HTTP/1.1 SSE was painful, and I am pretty sure auto negotiation breaks, often in unexpected ways with a HTTP/1.1 origin.

I have a demo of this for CF workers https://github.com/hntrl/eventkit/tree/main/examples/workers...

(it's not SSE in particular, but it demonstrates that you can have a long running stream like SSE)

On top of the comments below about SSE, I'd also point out that Cloudflare is doing some interesting stuff around serverless resumable websockets. They also have stuff for WebRTC.

Yes, they are