Income Taxes (Tables)

12 Months Ended

Dec. 31, 2025

Income Tax Disclosure [Abstract]

Summary of Reconciliation of Statutory Income Tax Rate To Effective Income Tax Rate

A reconciliation of the Company’s statutory income tax rate to its effective income tax rate in 2025 and 2024 is as follows:

	2025						2024
	Amount			Percentage			Amount			Percentage
Federal statutory tax rate	$	(46,175	)		21.00	%	$	(35,313	)		21.00	%
State and local income taxes, net of federal income tax effect(1)		—			—	%		2,132			(1.27	)%
Research and development credits		(3,342	)		1.52	%		(3,700	)		2.20	%
Nontaxable or nondeductible items
Other		2,174			(0.99	)%		1,427			(0.85	)%
Other adjustments		13,870			(6.31	)%		2,130			(1.26	)%
Valuation allowance		33,473			(15.22	)%		33,441			(19.89	)%
Effective tax rate	$	—			—	%	$	117			(0.07	)%

(1) Massachusetts makes up the majority (greater than 50 percent) of the State taxes, net of federal benefit category.

Summary of Net Deferred Income Tax Asset

As of December 31, 2025 and 2024, the net deferred income tax asset balance related to the following (in thousands):

	December 31,
	2025			2024
Deferred tax assets:
Net operating losses	$	85,763		$	32,148
Capitalized research and development		57,378			65,606
Research and development credits		11,414			6,970
Equity-based compensation		9,562			21,035
Licenses		5,723			694
Accrued expenses		2,280			2,699
Basis difference on gain on 2022 sale of zipalertinib development subsidiary		1,638			1,700
Lease liability		709			590
Capitalized organizational and start-up expenses		83			101
Gross deferred tax assets		174,550			131,543
Valuation allowance		(173,785	)		(130,939	)
Net deferred tax asset		765			604
Deferred tax liability
ROU asset		695			457
Depreciation and amortization		70			147
Net deferred tax asset	$	—		$	—

Summary of Valuation Allowance

The following table summarizes activity in Cullinan’s valuation allowance during 2025 and 2024 (in thousands):

	2025		2024
Valuation allowance at beginning of year	$	130,939	$	87,371
Increases recorded to income tax provision		42,846		43,568
Increases (decreases) recorded to equity		—		—
Valuation allowance at end of year	$	173,785	$	130,939