Treasury bills?
If you sell a Treasury bill (T-bill) before its maturity, the resulting gain or loss is considered a capital gain or loss, not interest income1. Here’s how it works:
Capital Gain or Loss: If you sell the T-bill before maturity, the difference between the sale price and the purchase price (adjusted for any accrued interest) determines whether you have a capital gain or capital loss.
anything else ask the broker. the sale should be reported on a 1099-B. sometines a consolidated 1099 with 1099-B info.