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Income taxes - Reconciliation of the differences between PRC statutory income tax rate and the Group's effective income tax rates (Details)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Computed expected income tax benefit	(25.00%)	(25.00%)	(25.00%)
Effect of preferential tax rate	7.00%	4.00%	(17.00%)
Effect of different tax jurisdiction	4.00%	2.00%	2.00%
Change in tax rate	4.00%	18.00%	0.00%
Prior year return-to-provision true up	0.00%	(2.00%)	0.00%
Non-deductible expenses and non-taxable income	1.00%	4.00%	40.00%
Research and development expenses additional deduction	(5.00%)	(6.00%)	(5.00%)
Change in valuation allowance	14.00%	5.00%	7.00%
Actual income tax expense	0.00%	0.00%	2.00%

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Percentage of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations attributable to preferential tax rate.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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