GASNet is a portable, open-source, high-performance communication library designed to efficiently support the networking requirements of PGAS runtime systems and other alternative models on future exascale machines. This paper investigates strategies for efficient implementation of GASNet’s “AM Long” API that couples an RMA (Remote Memory Access) transfer with an Active Message (AM) delivery. We discuss several network-level protocols for AM Long and propose a new target-side reassembly protocol. We present a microbenchmark evaluation on the Cray XC Aries network hardware. The target-side reassembly protocol on this network improves AM Long end-to-end latency by up to 33%, and the effective bandwidth by up to 49%, while also enabling asynchronous source completion that drastically reduces injection overheads. The improved AM Long implementation for Aries is available in GASNet-EX release v2019.9.0 and later.