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Stockholders' Equity - Stock reserved for issuance (Details) - shares	Sep. 30, 2025	Dec. 31, 2024
Stockholders' Equity
Common stock reserved for future issuance	31,348,966	17,095,105
Stock options
Stockholders' Equity
Common stock reserved for future issuance	20,090,359	9,458,788
RSUs issued and outstanding
Stockholders' Equity
Common stock reserved for future issuance	2,670,905
Shares available for grant | Shares available for grant under the 2024 Equity Incentive Plan
Stockholders' Equity
Common stock reserved for future issuance	930,301	6,893,517
Shares available for grant | Shares available for grant under the 2024 Employee Stock Purchase Plan
Stockholders' Equity
Common stock reserved for future issuance	1,009,440	650,000
Shares available for grant | Shares available for grant under the ACELYRIN, Inc. 2023 Equity Incentive Plan
Stockholders' Equity
Common stock reserved for future issuance	6,448,105
Shares available for grant | Shares available for grant under the 2024 Performance Option Plan
Stockholders' Equity
Common stock reserved for future issuance	199,856	92,800

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