v3.26.3
Cover
 6 Months Ended
Jun. 30, 2026
Document Information [Line Items]  
Document Type S-4
Entity Registrant Name BOUNDLESS BIO, INC.
Entity Central Index Key 0001782303
Amendment Flag false
Entity Tax Identification Number 83-0751369
Entity Address, Address Line One 11099 North Torrey Pines Road
Entity Address, Address Line Two Suite 150 La Jolla
Entity Address, State or Province CA
Entity Incorporation, State or Country Code DE
City Area Code 858
Local Phone Number 766-9912
Entity Filer Category Non-accelerated Filer
Entity Small Business true
Entity Emerging Growth Company true
Entity Ex Transition Period false
Entity Primary SIC Number 2834
Business Contact [Member]  
Document Information [Line Items]  
Entity Address, Address Line One 11099 North Torrey Pines Road
Entity Address, Address Line Two Suite 150 La Jolla
Entity Address, State or Province CA
City Area Code 858
Local Phone Number 766-9912
Contact Personnel Name Jessica Oien