v3.26.1
Organization and Basis of Presentation - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 23, 2026
Jun. 22, 2026
Apr. 01, 2025
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Entity Incorporation, State or Country Code           DE    
Aggregate offering price of common stock       $ 2   $ 2   $ 2
Cash, cash equivalents, and short-term investments       72,600   72,600    
Payment to landlord on lease termination       10,000   $ 10,000    
Lease termination effective date           May 31, 2026    
Security deposit       500   $ 500    
Accumulated deficit       (296,917)   (296,917)   (259,669)
Net loss       (23,693) $ (15,675) (37,248) $ (31,433)  
Net cash used in operating activities           (35,945) $ (26,359)  
Serapha Bio, Inc. | Merger Agreement                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Percentage of voting interest acquired   96.31%            
Percentage of common stock owned by Boundless Bio's existing stockholders   3.69%            
Gross proceeds from issuance of common stock   $ 200,000            
Termination fee amount payable by terminating party   $ 1,000            
Percentage of reduction in workforce committed 75.00%              
Restructruing charges recognized       2,800   2,800    
Accrued compensation liability       2,800   2,800   $ 0
Serapha Bio, Inc. | Merger Agreement | Minimum                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Estimated aggregate one-time charges reminder to be recognized       3,000   3,000    
Serapha Bio, Inc. | Merger Agreement | Maximum                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Estimated aggregate one-time charges reminder to be recognized       5,000   5,000    
Location, Statement of Income, Balance [Axis]: us-gaap:GeneralAndAdministrativeExpense | Serapha Bio, Inc. | Merger Agreement                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Restructruing charges recognized       1,300   1,300    
Location, Statement of Income, Balance [Axis]: us-gaap:ResearchAndDevelopmentExpense | Serapha Bio, Inc. | Merger Agreement                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Restructruing charges recognized       $ 1,500   $ 1,500    
Open Market Sale Agreement                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Shares issued           0    
Open Market Sale Agreement | Maximum                
Organization, Consolidation and Presentation of Financial Statements [Line Items]                
Aggregate offering price of common stock     $ 14,500          
Percentage of agent commission on gross proceeds of common stock sold     3.00%