String theory in the tensionless limit is expected to have a large gauge symmetry. By recasting string theory on the AdS background as a generalization of Vasiliev's theory of massless higher-spin fields, it has become possible to understand the nature of this symmetry. In this talk, I will first give an overview of three-dimensional Vasiliev theory and its dual CFT. I will then discuss the current understanding of the symmetry algebra of string theory as well as some open problems related to it.