EcoExec is a host-assisted interactive execution environment for wireless sensing systems. Users can interact with sensor nodes by viewing attributes and invoking functions via a command-line interface. Functions that are not resident in the node's firmware are automatically compiled on the host, packaged up and downloaded to the node, linked, and executed, all seamlessly and transparently to the user. By packaging these features in a dynamically object-oriented programming environment such as Python, EcoExec enables programmers to experiment with features of the wireless sensor nodes and to rapidly develop application software. Most importantly, EcoExec empowers resource-constrained wireless sensor platforms with rich functionalities that would otherwise be prohibitive, thanks to its host-assisted execution feature with code swapping over the network. Experimental results on actual wireless sensor platforms show EcoExec to perform effectively with negligible observed overhead to the user.