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Document and Entity Information - shares
 9 Months Ended
Sep. 30, 2025
Nov. 03, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2025  
Document Transition Report false  
Entity File Number 001-42272  
Entity Registrant Name MBX Biosciences, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 84-1882872  
Entity Address, Address Line One 11711 N. Meridian Street  
Entity Address, Address Line Two Suite 300  
Entity Address, City or Town Carmel  
Entity Address, State or Province IN  
Entity Address, Postal Zip Code 46032  
City Area Code 317  
Local Phone Number 659-0200  
Title of 12(b) Security Common Stock, par value $0.0001 per share  
Trading Symbol MBX  
Security Exchange Name NASDAQ  
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  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   44,902,302
Entity Central Index Key 0001776111  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Current Fiscal Year End Date --12-31