Thanks, although the fact that “Content-Length: 0” doesn’t work kills my theory about the proxy.

The “close_connection” flag is also a documented public API of Python: <https://docs.python.org/3/library/http.server.html#http.server.BaseHTTPRequestHandler.close_connection>. According to the rules for framing the message body in <https://tools.ietf.org/html/rfc7230#section-3.3.3>, the presence of “Connection: close” is not important. The server actually shutting down the connection is the key. That is why I prefer to explicitly set the flag (if it works).

Anyway, your current proposal makes the server send a valid full HTTP response.