{"cik":2022626,"taxonomy":"us-gaap","tag":"OtherAssetsCurrent","label":"Other Assets, Current","description":"Amount of current assets classified as other.","entityName":"UPSTREAM BIO, INC.","units":{"USD":[{"end":"2025-12-31","val":727000,"accn":"0001193125-26-343417","fy":2026,"fp":"Q2","form":"10-Q","filed":"2026-08-11","frame":"CY2025Q4I"},{"end":"2026-06-30","val":1174000,"accn":"0001193125-26-343417","fy":2026,"fp":"Q2","form":"10-Q","filed":"2026-08-11","frame":"CY2026Q2I"}]}}