Cases in which many agents contribute to an outcome, such as individual agents contributing to climate change, pose puzzles for theories of causation. Tiefensee has recently argued that these cases may be infected with indeterminacy, since it is sometimes indeterminate whether such actions can make a difference to an effect. This result is consistent with earlier arguments by Bernstein, Sartorio, and Swanson, that causal claims are sometimes indeterminate. In this paper, I develop a theory of causal indeterminacy that can adequately capture both cases. The theory I propose combines Woodward’s interventionist approach to actual causation with the conceptual tools of supervaluationism, which are commonly used to explain semantic vagueness.