Vercel says the capability can be used to exchange messages with stdio MCP servers, including agent-browser, or to feed other command-line programs interactively.
In the TypeScript SDK, a command is started with stdin: true and detached: true in runCommand(). The returned command supports writeStdin() for sending data and closeStdin() for ending input. A caller can then wait for the command and read its output.
The Python SDK supports creating a process with stdin=subprocess.PIPE, sending bytes through server.stdin.send(), and using communicate() to close input, collect remaining output, and wait for exit. For commands receiving all input at once, run_process() also accepts input.
The source does not specify pricing, rollout scope, performance characteristics, or compatibility across MCP servers beyond the cited example.