spec is very simple

disable buffering on your proxy

Content-Type: text/event-stream

each line has a prefix + data (followed by a newline), like so:

prefix: data 1

prefix2: data 2

two newlines = end of message

types:

- event: once per message, defines event type

- id: event ID to be given to consumer

- data: data for event, multiple data lines can exist (they are joined by newline)

- retry: send by itself with nothing else, number in milliseconds to try to reconnect after connection loss