This paper presents a context-aware middleware for multimodal dialogue applications. The middleware has the context tracing feature, which is the possibility of the middleware to explain why and how a situation occurs (or occurred). The middleware consists of several agents communicating with each other and an ontology is used to describe various concepts such as resources, situations, plans, structure of context history, and data to be exchanged between the agents. Categories and Subject Descriptors D.2.10 [Software Engineering]: Design—Methodologies; I.2.11 [Artificial Intelligence]: Distributed Artificial Intelligence General Terms Design Keywords Middleware, multimodal dialogue, ontology, context tracing