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Acquisitions (Details)
$ / shares in Units, $ in Thousands
3 Months Ended 4 Months Ended 9 Months Ended 12 Months Ended
May 21, 2025
USD ($)
installment
lease
$ / shares
shares
Mar. 29, 2024
$ / shares
Sep. 30, 2025
USD ($)
shares
Sep. 30, 2024
USD ($)
Sep. 30, 2025
USD ($)
shares
Sep. 30, 2025
USD ($)
shares
Sep. 30, 2024
USD ($)
Dec. 31, 2025
USD ($)
Jan. 31, 2025
Dec. 31, 2024
shares
Acquisitions                    
Stock option, exercise price ($ per share) | $ / shares   $ 8.84                
Common stock reserved for future issuance | shares     31,348,966   31,348,966 31,348,966       17,095,105
Net income (loss)     $ (104,452) $ (141,667)   $ (380,973) $ (222,102)      
Gain on bargain purchase           187,907        
Severance liability     1,374   $ 1,374 $ 1,374        
RSUs                    
Acquisitions                    
Shares issued for conversion | shares           1,470,868        
ACELYRIN                    
Acquisitions                    
Number of shares issued in exchange | shares 48,653,549                  
Stock option, exercise price ($ per share) | $ / shares $ 18                  
Exchange ratio 0.4814                  
Merger transaction costs     (100)     $ 14,700        
Fair value of IPR&D asset $ 50,959                  
Credit voucher acquired, balance $ 11,400                  
Number of operating leases acquired | lease 2                  
Lease liability fair value adjustment, short term portion $ 200                  
Lease liability fair value adjustment, long term portion 500                  
Right of use asset fair value adjustment 100                  
Gain on bargain purchase 187,907   0     187,900        
Deferred income tax liability 10,701                  
Additional stock-based compensation     2,100     12,900        
Revenue of acquiree         0          
Operating losses of acquiree         14,200          
ACELYRIN | Acquisition related cost                    
Acquisitions                    
Merger transaction costs       0     14,700      
ACELYRIN | Stock based compensation                    
Acquisitions                    
Net income (loss)             13,100      
ACELYRIN | Stock based compensation | Less than                    
Acquisitions                    
Net income (loss)       100            
ACELYRIN | Gain on bargain purchase                    
Acquisitions                    
Net income (loss)       0     187,900      
ACELYRIN | General and administrative expenses                    
Acquisitions                    
Merger transaction costs     (100)     14,700        
ACELYRIN | Severance Plan                    
Acquisitions                    
Severance cash obligations $ 13,500                  
Severance cash obligations payment period 12 months                  
Severance cost     4,300     13,200        
Severance liability     1,400   $ 1,400 1,400        
Additional stock-based compensation               $ 13,100    
ACELYRIN | Severance Plan | Severance cash obligations                    
Acquisitions                    
Net income (loss)       $ 0     $ 13,500      
ACELYRIN | Severance Plan | General and administrative expenses                    
Acquisitions                    
Severance cost     1,900     7,100        
Additional stock-based compensation     1,000     8,800        
ACELYRIN | Severance Plan | Research and development expenses                    
Acquisitions                    
Severance cost     2,400     6,100        
Additional stock-based compensation     $ 1,100     $ 4,100        
ACELYRIN | IPR&D asset                    
Acquisitions                    
Fair value of IPR&D asset $ 51,000                  
Discount rate 23.00%                  
Deferred income tax liability $ 10,700                  
ACELYRIN | RSUs                    
Acquisitions                    
Shares issued for conversion | shares 1,323,905                  
ACELYRIN | Performance based RSU                    
Acquisitions                    
Shares issued for conversion | shares 146,963                  
Percentage of performance conditions achieved 100.00%                  
ACELYRIN | Performance based RSU | Merger closes before May 15, 2025                    
Acquisitions                    
Number of installments | installment 2                  
ACELYRIN | ACELYRIN Plans                    
Acquisitions                    
Common stock reserved for future issuance | shares 4,712,186                  
Legacy Alumis Inc                    
Acquisitions                    
Percentage of equity owned                 52.00%  
Legacy Acelyrin Inc                    
Acquisitions                    
Percentage of equity owned                 48.00%