Multi-agent LLM systems place several agents on shared infrastructure, where each agent holds permissions far broader than the task it is delegated -- so every agent is routinely permitted to act on a peer's resources it was never assigned. We ask: under competitive pressure, what determines whether an agent crosses from its delegated scope into a peer's resources, and can such crossings be enforced away? We study this across eight LLMs in two simulated environments -- a CI/CD release pipeline and a security-operations incident queue -- using a deterministic, no-model-in-the-loop detector over hash-chained, replay-verified logs. In a controlled 2x2x2 factorial over competitive incentive, candidate visibility, and payoff information (three models, n=52 per cell), we find that action representation, not permission, governs the behavior: no agent crosses when the peer action is unlisted, whereas listing it under competitive pressure with a stated payoff raises crossings sharply -- up to 52 of 52 episodes for Claude Sonnet 4.5. We also observe a model-dependent split: two newer models (Claude Sonnet 5.5 and Opus 5.5) refuse the identical attack in 0 of 100 episodes, where prior-generation models cross in 9 of 10. We then evaluate two defenses -- a deterministic reference monitor and a model-based pre-execution judge -- both block every crossing and, when the task is winnable, cut avoidable peer deadline misses from 15/40 and 14/40 to at most 1/40. But scope enforcement is not harm prevention: an agent can still set a peer back through an authorized action on its own resources -- misreporting its incident's severity -- which neither defense detects. Our results establish action representation, not permission alone, as a controllable driver of inter-agent scope violations, and show that enforcing delegated scope is distinct from preventing harm, motivating effect-aware authorization.

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