Who Else Is In? Multi-Agent Collusion Initiation via Strategic Vagueness

Multi-agent LLM systems raise the risk of collusion among agents pursuing objec- tives hidden from their overseers. When potential partners’ identities are unknown, collusion first requires initiation: agents must discover one another, often through a public channel observed by ordinary agents and monitors. Strategic vagueness offers a way to navigate this mixed audience, conveying intent to a prospective partner while preserving a plausible ordinary interpretation. We introduce a mutual- identification game in which two schemers must identify each other through public interaction, while jointly solving an ordinary task. We find that some models dis- cover successful handshake strategies without explicit signalling guidance, using task-compatible exchanges that monitors often fail to flag at low false-positive rates. To characterize the communicative capabilities underlying these exchanges, we study a controlled coordination setting . We find an asymmetric response to message explicitness: as messages become more indirect, monitor punishment can fall faster than partner coordination, creating a profitable intermediate regime of strategic vagueness. Together, these findings illustrate a Blackstone-style evidential asymmetry: communication sufficient for coordination between agents may remain insufficient for confident detection by monitor, opening a new surface of collusion.