Why is the STDIO transport considered the baseline for understanding MCP communication?

STDIO represents the ideal case where bidirectional communication is completely seamless. Both client and server can initiate messages at any time using stdin and stdout, with no restrictions on direction or timing. This makes all MCP features work perfectly including sampling, notifications, progress reports, and roots. When you move to HTTP transports, the server cannot always initiate requests to the client. Understanding STDIO as the full-capability baseline helps you recognize what gets lost with other transport methods.