What is stdio transport?

In the stdio transport, an MCP client starts the server as a child process. The server reads JSON-RPC messages from its standard input and writes its own messages to standard output. Each message sits on one line, so messages are delimited by newlines and must not contain embedded newlines. Nothing else may be written to those two streams: the server must not print anything to stdout that is not a valid MCP message, and the client must not send anything to stdin that is not one.

Standard error is free for logging. A server may write informational, debug, or error text there, and the client may capture, forward, or ignore it, but should not treat it as proof that something failed.

Shutdown is built on stream closure. The client closes the server's input, waits for it to exit, and forcibly terminates the process if it does not. Servers should exit promptly when their input closes. If a server dies unexpectedly, the client should restart it.

Because the client starts the server as its own subprocess, this transport suits servers that run locally on the same machine. The MCP authorization page says stdio implementations should not follow its OAuth flow and should instead take credentials from the environment.