Trace the SSE message flow

Beacon Labs runs a remote MCP server on the StreamableHTTP transport. They built this tracer so engineers can step through a single add tool call and see exactly which stream each message travels on.

• Step through the flow with the Next step button.
• Watch the persistent GET SSE stream and the per-call POST response stream fill up, and note where each message lands.

Which message would stop being delivered if the persistent GET SSE stream were removed?