| Document And Entity Information | May 13, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 13, 2024 |
| 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 | 9880 Campus Point Drive, Suite 120, |
| Entity Address, City or Town | San Diego |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92121 |
| 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 |
