What is the STDIO transport in MCP and how does it work?

The STDIO transport is the most commonly used transport for local MCP development. The client launches the MCP server as a subprocess and communicates through standard input and output streams. The client sends messages to the server using the server's stdin, and the server responds by writing to stdout. Either party can send a message at any time, making it truly bidirectional. The main limitation is that both client and server must run on the same machine since they share a process pipe. This transport is ideal for development and testing.