v3.26.1
Cover Page
 6 Months Ended
Jun. 30, 2026
Document Information [Line Items]  
Document Type S-4
Amendment Flag false
Entity Registrant Name FULCRUM THERAPEUTICS, INC.
Entity Central Index Key 0001680581
Entity Filer Category Non-accelerated Filer
Entity Small Business true
Entity Emerging Growth Company false
Entity Incorporation, State or Country Code DE
Entity Tax Identification Number 47-4839948
Entity Address, Address Line One 26 Landsdowne Street
Entity Address, City or Town Cambridge
Entity Address, State or Province MA
City Area Code 617
Local Phone Number 651-8851
Entity Primary SIC Number 2834
Business Contact [Member]  
Document Information [Line Items]  
Entity Address, Address Line One 26 Landsdowne Street
Entity Address, City or Town Cambridge
Entity Address, State or Province MA
Entity Address, Postal Zip Code 02139
City Area Code 617
Local Phone Number 651-8851
Contact Personnel Name Alex C. Sapir