MGS9028: streaming unsupported

The server answered 500 internal because the route streams Server-Sent Events and the response writer it was handed cannot flush.

[MGS9028] the response writer cannot stream; a proxy or middleware is buffering it
  see: .../MGS9028.md

Why

An event stream is useless if a layer holds it until the response ends. The route checks for the ability to flush before it writes a header, and refuses rather than open a stream nobody will receive.

Resolution

Reach the server directly on its loopback address, or configure the layer in front of it not to buffer (X-Accel-Buffering: no for nginx, response buffering off for other proxies).

What this is not

• Not an internal read failure. That is MGS9027.