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Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Stock option plans excluding the 2024 POP |  |  |
Options Outstanding |  |  |
Options outstanding at beginning of the period (shares) | 7,670,908 |  |
Stock options assumed in connection with the ACELYRIN Merger (shares) | 4,712,186 |  |
Granted (shares) | 6,983,120 |  |
Forfeited or expired (shares) | (956,679) |  |
Options outstanding, vested and expected to vest at end of the period (shares) | 18,409,535 |  | 7,670,908
Exercisable at end of the period (shares) | 10,661,758 |  |
Weighted- Average Exercise Price Per Share |  |  |
Weighted-average exercise price, Outstanding at beginning of the period ($ per shares) | $ 9.59 |  |
Weighted-average exercise price, Stock options assumed in connection with the ACELYRIN Merger ($ per shares) | 12.99 |  |
Weighted-average exercise price, Granted ($ per shares) | 4.6 |  |
Weighted-average exercise price, Forfeited or expired ($ per shares) | 9.07 |  |
Weighted-average exercise price, Outstanding, vested and expected to vest at end of the period ($ per shares) | 8.6 |  | $ 9.59
Exercisable at end of the period ($ per shares) | $ 10.66 |  |
Weighted- Average Remaining Contractual Term |  |  |
Weighted-average remaining contractual term, Outstanding, vested and expected to vest (in years) | 7 years |  | 8 years 2 months 12 days
Weighted-average remaining contractual term, Exercisable at end of the period (in years) | 5 years 2 months 12 days |  |
Aggregate Intrinsic Value |  |  |
Aggregate Intrinsic Value, Outstanding at beginning of the period | $ 1,777 |  |
Aggregate Intrinsic Values, Outstanding, vested and expected to vest at the end of period | 388 |  | $ 1,777
Aggregate Intrinsic Values, Exercisable at the end of period | 116 |  |
Vested, fair value | $ 22,100 | $ 5,400 |
2021 Plan |  |  |
Aggregate Intrinsic Value |  |  |
Unvested and early exercisable shares | 2,304,921 |  |
2024 POP |  |  |
Options Outstanding |  |  |
Options outstanding at beginning of the period (shares) | 1,787,880 |  |
Forfeited or expired (shares) | (107,056) |  |
Options outstanding, vested and expected to vest at end of the period (shares) | 1,680,824 |  | 1,787,880
Weighted- Average Exercise Price Per Share |  |  |
Weighted-average exercise price, Outstanding at beginning of the period ($ per shares) | $ 10.19 |  |
Weighted-average exercise price, Forfeited or expired ($ per shares) | 10.19 |  |
Weighted-average exercise price, Outstanding, vested and expected to vest at end of the period ($ per shares) | $ 10.19 |  | $ 10.19
Weighted- Average Remaining Contractual Term |  |  |
Weighted-average remaining contractual term, Outstanding, vested and expected to vest (in years) | 8 years 7 months 6 days |  | 9 years 3 months 18 days

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- Definition Weighted average price at which the grantees can acquire the shares of common stock for the stock options that were assumed in connection with the merger. + References No definition available. + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Weighted average price of options that were either forfeited or expired. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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