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Cover - USD ($)
 12 Months Ended
Dec. 31, 2025
Feb. 09, 2026
Jun. 30, 2025
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Amendment Flag false    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Entity Registrant Name BEST SPAC I ACQUISITION CORP.    
Entity Central Index Key 0002051587    
Entity Incorporation, State or Country Code D8    
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 false    
City Area Code +852    
Local Phone Number 9828 3397    
Document Period End Date Dec. 31, 2025    
Entity Shell Company true    
Entity Interactive Data Current Yes    
Entity Current Reporting Status Yes    
Entity File Number 001-42700    
Entity Address, Address Line One 701, 7/FloorUnited Building    
Entity Address, Address Line Two 17-19 Jubilee Street    
Entity Address, City or Town Hong Kong    
Entity Address, Country HK    
Entity Address, Postal Zip Code 00000    
Entity Tax Identification Number 00-0000000    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
ICFR Auditor Attestation Flag false    
Document Financial Statement Error Correction [Flag] false    
Entity Public Float     $ 55,220,000
Documents Incorporated by Reference [Text Block] None    
Class A Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   6,024,500  
Class B Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   1,375,000  
Units, each consisting of one Class A ordinary share and one right to receive one-tenth of one Class A ordinary share      
Document Information [Line Items]      
Title of 12(b) Security Units, each consisting of one Class A ordinary share and one right    
Trading Symbol BSAAU    
Security Exchange Name NASDAQ    
Class A ordinary shares, no par value      
Document Information [Line Items]      
Title of 12(b) Security Class A ordinary shares, no par value    
Trading Symbol BSAA    
Security Exchange Name NASDAQ    
Rights      
Document Information [Line Items]      
Title of 12(b) Security Rights    
Trading Symbol BSAAR    
Security Exchange Name NASDAQ