Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2022	Nov. 11, 2022
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q3
Entity Registrant Name	Cogent Biosciences, Inc.
Entity Central Index Key	0001622229
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		69,893,434
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity File Number	001-38443
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5308248
Entity Address, Address Line One	275 Wyman Street
Entity Address, Address Line Two	3rd Floor
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	(617)
Local Phone Number	945-5576
Title of 12(b) Security	Common Stock, $0.001 Par Value
Trading Symbol	COGT
Security Exchange Name	NASDAQ
Former Address
Document Information [Line Items]
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