how to get deleted messages back

Once a message has been deleted, it's not possible to get it back.

they come back everywhere, ive tried to delete them all on every platform but theres hundreds of them

what do you mean reset the app? uninstall and reinstall or close and open again? ive tried signing out and in on every device but it didnt help

I don't think so, but contact MS support.

This is why they need a "are you sure?" when you select delete.

Try contacting Xbox Customer Service (not Xbox Ambassadors) but it's probably gone for good.

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Once it's deleted there's nothing you can really do about it except giving it a try through the Xbox Customer Service Support.

Do they come back on the Xbox itself, or just on your phone/PC via the apps?

I've had lots of problems with the Xbox app on Windows 10 being out of sync when it comes to messages. Sometimes it'll be missing conversations that I can see on the console. Sometimes messages will be missing from conversations. Sometimes conversations
I've deleted on the console will still show up in the app.

The solution - albeit, likely a temporary one - is to reset the app. That will reset all of the app's settings to defaults and force a resync of your conversations and messages.

If you're seeing deleted conversations reappearing on the console itself, you could try removing and re-adding your account. That will remove your local saves, but they will be resynced via Xbox Live as you launch each game/app, so as long as your console
has been connected to Live you won't lose any data/saves.

If that still doesn't work, I'd report the issue via the Xbox Insider Hub and/or Feedback Hub apps.