Astrocytes secrete neurocan (NCAN), a protein that regulates the formation and function of inhibitory synapses in the developing cerebral cortex. NCAN is cleaved into N- and C-terminal fragments, with the C-terminal fragment playing a key role in inhibitory synaptogenesis. The C-terminal fragment localizes to synapses and is necessary and sufficient for the formation of SST+ inhibitory synapses. NCAN knockout mice show reduced inhibitory synapse numbers and function, while restoring NCAN expression in astrocytes rescues these deficits. The study reveals that astrocytes control specific GABAergic synapse types through distinct molecular mechanisms, highlighting the role of astrocyte-secreted molecules in regulating synaptic connectivity and brain function. The findings provide insights into the mechanisms by which astrocytes influence circuit-specific inhibitory synapse development in the mammalian brain.Astrocytes secrete neurocan (NCAN), a protein that regulates the formation and function of inhibitory synapses in the developing cerebral cortex. NCAN is cleaved into N- and C-terminal fragments, with the C-terminal fragment playing a key role in inhibitory synaptogenesis. The C-terminal fragment localizes to synapses and is necessary and sufficient for the formation of SST+ inhibitory synapses. NCAN knockout mice show reduced inhibitory synapse numbers and function, while restoring NCAN expression in astrocytes rescues these deficits. The study reveals that astrocytes control specific GABAergic synapse types through distinct molecular mechanisms, highlighting the role of astrocyte-secreted molecules in regulating synaptic connectivity and brain function. The findings provide insights into the mechanisms by which astrocytes influence circuit-specific inhibitory synapse development in the mammalian brain.