How does MCP organize messages by sender direction?

The MCP specification organizes messages by who sends them. Client messages include requests that clients send to servers, such as tool calls, prompt listings, and resource reads, plus notifications that clients send like the Initialized Notification. Server messages include requests that servers send to clients, such as Create Message requests for sampling and List Roots requests for file access, plus notifications that servers broadcast like progress updates and log messages. Both clients and servers can initiate communication, making MCP a bidirectional protocol.