An evaluation of the performance and characteristics of tool-based covert communication in cooperative agentic settings

Multi-agent systems deployed in critical domains often risk performance degradation if colluding agents end up prioritising internal sub-rewards over shared system goals. While agentic collusion is typically supported by secret text communication channels, we investigate whether a coalition can coordinate without it and instead rely on publicly visible tool parameters and execution sequences. Utilising a modified COLOSSEUM (Terrarium)-based environment, we evaluate the performance and suspicion of covert coalitions across benign, private-channel and tool-mediated communication conditions with varying pre-coordination rounds. Our empirical results show that while agents readily implement and execute tool-based signaling protocols such as resource type allocation, scheduling slot offsets and call sequencing, they fail to demonstrate a significant coalition advantage over benign baselines due to the architectural constraints of the environment. Furthermore, these tool-mediated strategies trigger significantly higher suspicion rates among non-adversarial agents due to observable misuse of tools, such as low-value resource ping-ponging and deceptive rationales. We conclude that while agents can establish implicit tool-based communication protocols, such schemes heavily sacrifice coalition secrecy, likely constrained by incomplete environmental knowledge and architectural bottlenecks.