Many English verbs typically used agentively allow non-agentive uses. Some recent approaches assume such verbs are unspecified for agentivity, although in principle, polysemy is also an option. We demonstrate the complexity of variable agentivity through an in-depth examination of the English verb sweep, which shows variable agentivity due to both underspecification and motivated polysemy. Drawing on corpus data, we identify two senses for sweep, each with unique argument realization options and interpretive properties. The prototypical uses of this verb, which involve the use of a broom, are obligatorily agentive; however, we claim they instantiate a specialized sense. We argue that the verb’s basic sense, which underlies all its other uses, is unspecified for agentivity, being found with agentive and non-agentive subjects. We formulate event structures for both senses that encode the grammatically relevant components of meaning common across all the verb’s uses: an entity moves along a surface while imparting a force via contact with it. We show that the event structure for the specialized sense, which fixes the moving entity to be a broom, is derived via established processes involving the lexicalization of instruments and routine goal-oriented activities. We demonstrate how the argument realization options and interpretive properties that the verb shows in each sense emerge from applying established principles of argument realization and semantic composition to these event structures.