| Document and Entity Information | Dec. 24, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001622229 |
| Document Type | 8-K |
| Document Period End Date | Dec. 24, 2021 |
| Entity Registrant Name | COGENT BIOSCIENCES, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38443 |
| Entity Tax Identification Number | 46-5308248 |
| Entity Address, Address Line One | 200 Cambridge Park Drive |
| Entity Address, Address Line Two | Suite 2500 |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02140 |
| City Area Code | (617) |
| Local Phone Number | 945-5576 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.001 Par Value |
| Trading Symbol | COGT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
