MemResidue: Measuring Risk Propagation in Evolving Agent Memory

Persistent memory reuses experience across tasks, creating a channel for risk to travel across memory updates. Safety evaluation of task outcomes can miss this process because a safe current result can coexist with contaminated memory that threatens later tasks. We call the memory left by risk propagation memory residue. The MemResidue Test tracks this residue to measure the extent of propagation across memory evolution. Its paired protocol follows three rounds of memory evolution in Answer and Execution tracks. Across 18 models and 191 cases, residue forms in every tested model, averaging 65.1% in Answer and 32.6% in Execution. Paired interventions show that retaining prior risky memory increases residue in the next update and can increase unsafe outcomes. Generic SafePrompt reduces residue formation. Surviving residue remains highly retrievable, and the phenomenon persists in a representative general memory architecture and under a dedicated memory safety mechanism. Risk propagation in evolving agent memory therefore constitutes an urgent safety dimension for agents.