Description of Business and Summary of Significant Accounting Policies - Summary of Stock-Based Compensation Expense Classification (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 760	$ 1,820	$ 2,027	$ 5,987
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	179	1,143	778	3,108
Selling, general and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 581	$ 677	$ 1,249	$ 2,879

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

X4 Pharmaceuticals, Inc 10-Q