0002022626 2026-09-01 2026-09-01 0002022626 false 8-K 2026-09-01 UPSTREAM BIO, INC. DE 001-42366 38-4187694 890 Winter Street Suite 200 Waltham MA 02451 (781) 208-2466 Not Applicable false false false false Common Stock, $0.001 par value per share UPB NASDAQ true false