Here's a custom MCP tool I use to run commands and parse stdout / stderr all the time:

    try {
        const execPromise = promisify(exec);
        const { stdout, stderr } = await execPromise(command);
        if (stderr) {
            return {
                content: [{
                    type: "text",
                    text: `Error: ${stderr}`
                }],
                isError: true
            };
        }
        return {
            content: [{
                type: "text",
                text: stdout
            }],
            isError: false
        };
    } catch (error: any) {
        return {
            content: [{
                type: "text",
                text: `Error executing command: ${error.message}`
            }],
            isError: true
        };
    }

Yeah, if you want to be super technical, it's Node that does the actual command running, but in my opinion, that's as good as saying the MCP client is...