The parties hereby agree
You ought not need a Latin maxim to persuade you of this, but in case a good aphorism floats your boat, try contractum signo ergo consentio
If you are occupied with the task of drawing up a contract, what you are writing down is, Q.E.D. what the parties are agreeing to do. So you don’t need to say, “the parties hereby agree...”
But - what the hell - don't let me stop you.