Why is understanding MCP message directionality important?

Understanding that both servers and clients can send messages is crucial when choosing a transport method. Some transports like STDIO support full bidirectional communication naturally. Others like HTTP make server-to-client communication difficult because HTTP is designed for clients to request from servers, not the reverse. When you deploy with StreamableHTTP and certain flags, server-initiated messages like sampling and progress notifications may stop working. Knowing which messages need which direction helps you design servers that work across transports. ---