Airbag with releasable secondary chambers

An assembly includes a first seat and a second seat spaced from the first seat. The assembly includes an airbag supported by the first seat. The airbag is inflatable to an inflated position between the seats. The airbag has a main chamber, a pair secondary chambers supported by the main chamber, and a tether releasably connected to the secondary chambers. The secondary chambers are maintained by the tether in an undeployed position relative to the main chamber and the secondary chambers are releasable by the tether to a deployed position in a cross-vehicle direction relative to the main chamber. The assembly includes a computer including a processor and a memory storing instructions executable by the processor to identify that an occupant is seated in the first seat, identify that an occupant is seated in the second seat and release the tether based on the identification of the occupants seated seats.