stdio

Stdio (standard input/output) is a way for one program to write something and another program to read it directly. Think of two paper cups connected by a string, like a homemade telephone. Speak into one cup (output) and the words travel along the string to be heard in the other cup (input). There's no need to open a network port or set up an internet address — just run two programs side by side on the same computer, and this string-like connection forms naturally.

So stdio isn't communication that leaves the computer; it's more like programs whispering to each other inside a single machine. Talking to a distant server still requires going through a network, but once that conversation reaches its final leg inside the local computer, it often finishes over this simple pipe.

As described in the article, when a cloud-based AI agent wants to talk to a tool program on a user's local computer, the signal has to travel through several stages. That final stage — where a relay program and the actual tool program talk to each other inside the local computer — is exactly where stdio comes in.

The article explains that a relay program running on the local machine "communicates with the MCP server via stdio." A common point of confusion: stdio isn't communication over the internet — it's a local-only method where two programs on the same computer exchange text directly. That means there's no need to open a port or change firewall settings.