| Document and Entity Information | May 13, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | BOLT BIOTHERAPEUTICS, INC. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001641281 |
| Document Period End Date | May 13, 2021 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity File Number | 001-39988 |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 47-2804636 |
| Entity Address Address Line1 | 900 Chesapeake Drive |
| Entity Address City Or Town | Redwood City |
| Entity Address State Or Province | CA |
| Entity Address Postal Zip Code | 94063 |
| City Area Code | 650 |
| Local Phone Number | 665-9295 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Trading Symbol(s) | BOLT |
| Title of 12(b) Security | Common Stock, par value $0.00001 per share |
| Security Exchange Name | NASDAQ |
