SSE "net::ERR_INCOMPLETE_CHUNKED_ENCODING" in CI

Browsers report "net::ERR_INCOMPLETE_CHUNKED_ENCODING" when a chunked HTTP response (such as an SSE stream) is cut off before the final zero-length chunk. The stream ended abnormally, so the EventSource errors.

What this error means

A browser-based SSE test logs "net::ERR_INCOMPLETE_CHUNKED_ENCODING 200 (OK)" and the EventSource stops receiving events. It appears when the server or a proxy terminates the stream without a clean end.

SSE

GET http://localhost:8080/events net::ERR_INCOMPLETE_CHUNKED_ENCODING 200 (OK)

Common causes

The server killed the stream without flushing an end

A worker that exits, or a handler that stops writing without properly ending the chunked response, leaves the chunked encoding incomplete.

A proxy buffered and truncated the stream

A reverse proxy that buffers responses can cut a long-lived SSE stream, producing an incomplete chunked encoding on the client.

How to fix it

End the stream cleanly and disable proxy buffering

Flush data and end the response properly when the stream closes.
Disable response buffering for the SSE route in any proxy.
Send periodic keep-alive comments so the stream stays open.

nginx.conf

location /events {
  proxy_pass http://app;
  proxy_buffering off;
  proxy_read_timeout 3600s;
}

Send keep-alive comments

Periodic comment lines keep the chunked stream alive and prevent idle truncation.

server.mjs

setInterval(() => res.write(':keep-alive\n\n'), 15000);

How to prevent it

Disable proxy buffering on SSE routes.
Send periodic keep-alive comments on long-lived streams.
End the chunked response cleanly when the stream closes.