Does Modularity Coordinate? Disentangling Decoupling, Interfaces, and Design Rules via Simon’s Theory of the Artifact

Modularity undergirds every contemporary platform, yet the contestation of its coordinative capacity conflates complements with substitutes, either undermining app evolution or squandering coordination resources. Without theory to guide them, platforms navigate by costly trial-and-error. Drawing on Simon’s theory of the artifact, we disentangle modularity into three facets—decoupling (minimizing app-platform dependencies), interfaces (enabling app-platform integration), and “design rules” (circumscribing app internals)—theorizing that coordination complements the separability-fostering facets while integration-fostering facets substitute for it. Design rules, ubiquitous in platforms yet theoretically overlooked, generate equivocality through prescriptive fluidity that coordination must resolve. We advance modularity theory by introducing design rules as its third facet—Simon’s missing “inner environment”—showing that coordination’s role is facet-specific. For practice, we equip platform owners to calibrate coordination effort to each facet, particularly at agentic artificial intelligence and hybrid platform frontiers.