v3.26.1
Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2025
Mar. 31, 2026
Document And Entity Information    
Document Type 10-K  
Document Annual Report true  
Document Period End Date Dec. 31, 2025  
Document Transition Report false  
Securities Act File Number 000-56407  
Entity Registrant Name Deep Fission, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 87-4265302  
Entity Address, Address Line One 2001 Addison St.  
Entity Address, Address Line Two Suite 300  
Entity Address, City or Town Berkeley  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94704  
City Area Code 707  
Local Phone Number 400-0778  
Title of 12(g) Security Common Stock, $0.0001 par value per share  
No Trading Symbol Flag true  
Security Exchange Name NONE  
Entity Well-known Seasoned Issuer No  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
ICFR Auditor Attestation Flag false  
Document Financial Statement Error Correction [Flag] false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   56,207,422
Entity Public Float $ 0  
Auditor Firm ID 248  
Auditor Name GRANT THORNTON LLP  
Auditor Location Dallas, Texas  
Entity Central Index Key 0001918102  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus FY  
Amendment Flag false