v3.26.1
Document And Entity Information
Jun. 15, 2026
Document Information [Line Items]  
Document Type 8-K
Amendment Flag false
Document Period End Date Jun. 15, 2026
Entity Registrant Name Boundless Bio, Inc.
Entity Central Index Key 0001782303
Entity Emerging Growth Company true
Entity File Number 001-41989
Entity Incorporation, State or Country Code DE
Entity Tax Identification Number 83-0751369
Entity Address, Address Line One 11099 North Torrey Pines Road, Suite 150
Entity Address, City or Town La Jolla
Entity Address, State or Province CA
Entity Address, Postal Zip Code 92037
City Area Code (858)
Local Phone Number 766-9912
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Entity Ex Transition Period false
Title of 12(b) Security Common Stock, par value $0.0001 per share
Trading Symbol BOLD
Security Exchange Name NASDAQ
Former Address [Member]  
Document Information [Line Items]  
Entity Address, Address Line One 10955 Alexandria Way, Suite 100
Entity Address, City or Town San
Entity Address, State or Province CA
Entity Address, Postal Zip Code 92121