| Document And Entity Information | Nov. 07, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 07, 2024 |
| Entity Registrant Name | MBX Biosciences, Inc. |
| Entity Central Index Key | 0001776111 |
| Entity Emerging Growth Company | true |
| Entity File Number | 001-42272 |
| 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 |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Ex Transition Period | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Trading Symbol | MBX |
| Security Exchange Name | NASDAQ |
