caddy: Server Sent Events don't work

1. What version of Caddy are you running (caddy -version)?

0.9.4

2. What are you trying to do?

Proxy SSE through Caddy

3. What is your entire Caddyfile?

:80 
gzip {
    not /application/events/
}
maxrequestbody 200MB

log stdout
errors stderr

root html/

proxy /application 127.0.0.1:9000 {
    transparent
    header_upstream X-Real-IP {remote}
    header_upstream X-Forwarded-Server {host}
    header_upstream X-Forwarded-Host {host}
}

4. How did you run Caddy (give the full command and describe the execution environment)?

caddy

5. What did you expect to see?

I expected my SSE requests to establish a long-running connection that can send data to the client.

6. What did you see instead (give full error messages and/or log)?

The HTTP request does seem to work (returns a 200, and headers), and the connection is kept alive, but SSE frames don’t arrive to the client.

7. How can someone who is starting from scratch reproduce this behavior as minimally as possible?

I’ve tried the https://github.com/jraedisch/go_sse_example to confirm the bug, but streaming does work with that setup. I suspect this error may be specific to interacting with the Java upstream I’m using. nginx, however, works well in the place of caddy, without additional changes to the upstream config. I’m using a python requests script as reference client.