What are the dual SSE connections in StreamableHTTP and why are they needed?

When the client makes a tool call over StreamableHTTP, the system creates two separate SSE connections. The primary SSE connection is established during initialization, stays open indefinitely, and handles server-initiated requests like sampling. The tool-specific SSE connection is created per tool call and closes when the result is sent. Progress notifications route through the primary connection, while logging and tool results go through the tool-specific connection. This separation prevents tool responses from blocking server-initiated requests.