v3.25.3

Long-Term Debt - Narrative (Details) - USD ($)		9 Months Ended
Long-Term Debt - Narrative (Details) - USD ($)	Mar. 26, 2024	Sep. 30, 2025	Sep. 30, 2024
Debt Instrument [Line Items]
Proceeds from issuance of term debt		$ 0	$ 10,000,000
2024 Loan and Security Agreement \| Secured Debt
Debt Instrument [Line Items]
Aggregate principal amount	$ 10,000,000
Basis spread on variable rate	0.25%
Debt instrument, interest rate, stated percentage	6.00%
Final payment, percentage of principal	2.00%
Proceeds from issuance of term debt	$ 10,000,000
Final payment	$ 200,000
Effective interest rate		7.99%
2024 Loan and Security Agreement \| Secured Debt \| Prior to March 26, 2026
Debt Instrument [Line Items]
Prepayment fee percentage	1.50%
2024 Loan and Security Agreement \| Secured Debt \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 25,000,000
Proceeds from line of credit draw		$ 0

Debt Instrument, Periodic Payment Terms, Balloon Payment To Be Paid, Percentage Of Principal

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Face (par) amount of debt instrument at time of issuance.

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CASTLE BIOSCIENCES INC 10-Q