My roommate agreed that I don't need to pay rent for the days I am not in apartment. I have to go to Texas for a 6 month period. I informed my roommate before adding him as roommate. We had this communication in WhatsApp chat. Can I use this chat in court in case if he harasses me to pay half rent?
Online conversations are generally allowed evidence.
However, EVERYTHING in the chat should be considered, as well as any later actions.
Were there later conversations that said something different?
Did the lease you signed have different terms?
One line in a WhatsApp does not make your case a "slam dunk". It may only be one piece of evidence in a much larger context.
Generally speaking, yes.
This issue arose in the trial of Michael Avenatti. There, Avenatti tried to exclude screenshots of his Whatsapp messages with Donald Trump, claiming that the government needed to use the original, electronic version of the messages that had been stored on his phone, and that the government had never produced that version to him. But the Court denied his motion and permitted the government to use the screenshots:
Avenatti's first and third requests — to suppress the WhatsApp messages evidence and to compel immediate production of certain witnesses' prior statements — require comparatively little discussion. With respect to the first, the law is clear that the Government has an obligation to produce only what is in its possession, custody, or control — and the Government complied with that obligation here. Additionally, so long as the WhatsApp messages are properly authenticated at trial, they can be admitted even if they are not in their "original, electronically stored" format.
So the main question for you would be whether you can authenticate those messages. The rules for authentication vary some from one jurisdiction to the next, but Federal Rule of Evidence 901 is similar to what you'll see in most places:
To satisfy the requirement of authenticating or identifying an item of evidence, the proponent must produce evidence sufficient to support a finding that the item is what the proponent claims it is.
So this just means that you have to have some evidence that the messages you're introducing in court are actually messages between you and the roommate. In most cases, you can just say under oath, "These are screenshots of messages between me and my roommate."