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Organization and Nature of the Business (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
May 21, 2025
USD ($)
shares
Jul. 17, 2024
USD ($)
shares
Jul. 01, 2024
USD ($)
$ / shares
shares
Sep. 30, 2025
USD ($)
item
Jun. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Sep. 30, 2024
USD ($)
Jun. 30, 2024
USD ($)
Mar. 31, 2024
USD ($)
Sep. 30, 2025
USD ($)
item
Sep. 30, 2024
USD ($)
Dec. 31, 2024
USD ($)
Mar. 13, 2024
item
Organization and Nature of the Business                          
Number of wholly owned subsidiaries | item       1           1     2
Preferred stock conversion ratio     0.2139                    
Issuance of common stock and equity awards in connection with the ACELYRIN Merger         $ 238,077                
Cash used in operating activities                   $ 297,616 $ 180,314    
Net Income (Loss)       $ (110,752) $ 59,321 $ (98,963) $ (93,117) $ (56,508) $ (49,849) (150,394) $ (199,474)    
Accumulated deficit       808,945           808,945   $ 658,551  
Cash, cash equivalents and marketable securities       $ 377,700           $ 377,700      
ACELYRIN                          
Organization and Nature of the Business                          
Issuance of common stock and equity awards in connection with the ACELYRIN Merger $ 238,100                        
Issuance of common stock and equity awards in connection with the ACELYRIN Merger (in shares) | shares 48,653,549                        
Fair value of net assets $ 425,984                        
Class A common stock                          
Organization and Nature of the Business                          
Redeemable convertible preferred stock | shares     28,855,656                    
Class B common stock                          
Organization and Nature of the Business                          
Redeemable convertible preferred stock | shares     7,184,908                    
IPO                          
Organization and Nature of the Business                          
Number of shares issued and sold | shares     13,125,000                    
Issue price per share | $ / shares     $ 16                    
Net proceeds after deducting underwriting discounts and commissions     $ 193,300                    
Payments of underwriting discounts and commission     $ 16,700                    
Concurrent Private Placement                          
Organization and Nature of the Business                          
Percentage of ownership held by investor   5.00%                      
Additional number of shares purchased | shares   2,500,000                      
Aggregate gross and net proceeds   $ 40,000