  Large sequence model (SM) such as GPT series and BERT has displayed
outstanding performance and generalization capabilities on vision, language,
and recently reinforcement learning tasks. A natural follow-up question is how
to abstract multi-agent decision making into an SM problem and benefit from the
prosperous development of SMs. In this paper, we introduce a novel architecture
named Multi-Agent Transformer (MAT) that effectively casts cooperative
multi-agent reinforcement learning (MARL) into SM problems wherein the task is
to map agents' observation sequence to agents' optimal action sequence. Our
goal is to build the bridge between MARL and SMs so that the modeling power of
modern sequence models can be unleashed for MARL. Central to our MAT is an
encoder-decoder architecture which leverages the multi-agent advantage
decomposition theorem to transform the joint policy search problem into a
sequential decision making process; this renders only linear time complexity
for multi-agent problems and, most importantly, endows MAT with monotonic
performance improvement guarantee. Unlike prior arts such as Decision
Transformer fit only pre-collected offline data, MAT is trained by online
trials and errors from the environment in an on-policy fashion. To validate
MAT, we conduct extensive experiments on StarCraftII, Multi-Agent MuJoCo,
Dexterous Hands Manipulation, and Google Research Football benchmarks. Results
demonstrate that MAT achieves superior performance and data efficiency compared
to strong baselines including MAPPO and HAPPO. Furthermore, we demonstrate that
MAT is an excellent few-short learner on unseen tasks regardless of changes in
the number of agents. See our project page at
https://sites.google.com/view/multi-agent-transformer.
