v3.25.2
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2025
Jul. 31, 2025
Cover [Abstract]    
Amendment Flag false  
Entity Central Index Key 0001782303  
Current Fiscal Year End Date --12-31  
Document Type 10-Q  
Document Period End Date Jun. 30, 2025  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Entity Registrant Name BOUNDLESS BIO, INC.  
Entity Incorporation, State or Country Code DE  
Entity File Number 001-41989  
Entity Tax Identification Number 83-0751369  
Entity Address, Address Line One 10955 Alexandria Way  
Entity Address, Address Line Two Suite 100  
Entity Address, City or Town San Diego  
Entity Address, Postal Zip Code 92121  
Entity Address, State or Province CA  
City Area Code (858)  
Local Phone Number 766-9912  
Title of 12(b) Security Common stock, par value $0.0001 per share  
Trading Symbol BOLD  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Shell Company false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   22,385,611
Document Transition Report false  
Document Quarterly Report true