Tether (USDT) sells all reserve commercial paper at zero loss
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At this point it feels as though Tether is essentially "unauditable", even if they somehow managed to have actual 1:1 cash reserves there'd still be a huge question mark as to how they got to that point (and whether or not it can be sustained).
For the record, I am not suggesting that USDT should be 1:1 cash backed. There's plenty of good reasons to use other instruments, like government bonds and the like.