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This paper formalizes 'compositional behavioral leakage' (CBL), a failure mode in prompt-composed agentic systems where editing one prompt module silently shifts the behavior of others due to transformer self-attention lacking module-level isolation. It presents an operational definition, a reusable three-channel protocol, and empirical evidence from 144 trials on a Claude Sonnet 4.6 agent, finding sub-threshold interference that could compound across thousands of decisions.