The correctness of a system operating in time-constrained scenarios leverages on both precision and delivery time of its outcome. This paper presents SEAMLESS, a system enabling the design, simulation, and in-depth analysis of Multi-Agent Systems (MAS). In particular, SEAMLESS allows defining in detail the agents’ knowledge (set of tasks it might execute), needs (set of tasks to be negotiated), local scheduler (execution of the task-set), negotiation protocols, possible communication delays, and heuristics related to the parameters mentioned above. This tool is pivotal in the strive to study and realize real-time MAS.