Methods, systems, and apparatus, including computer programs encoded on computer storage media, for communications between user equipment. One of the methods includes receiving, by a first device and from a second device, sidelink control information indicating a resource reservation for a single-slot resource for the second device to use to communicate with the first device; determining, by the first device and based at least in part on the sidelink control information, whether a potential resource collision exists when the first device is scheduled to communicate with another device using the single-slot resource or when the single-slot resource is reserved by another device; and in response to determining that the potential resource collision exists, sending, by the first device and within a predetermined number of slots from a reference slot, a collision message to cause the second device to determine another single-slot resource to use to communicate with the first device.