v3.25.2
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2025
Aug. 04, 2025
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2025  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Entity Registrant Name UPSTREAM BIO, INC.  
Trading Symbol UPB  
Entity Central Index Key 0002022626  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   53,909,427
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Entity File Number 001-42366  
Entity Tax Identification Number 38-4187694  
Entity Address, Address Line One 890 Winter Street  
Entity Address, Address Line Two Suite 200  
Entity Address, City or Town Waltham  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02451  
City Area Code 781  
Local Phone Number 208-2466  
Entity Incorporation, State or Country Code DE  
Document Quarterly Report true  
Document Transition Report false  
Title of 12(b) Security Common Stock, par value $0.001 per share  
Security Exchange Name NASDAQ