Compressing KV-Cache Communication in Multi-Agent Systems with Collective Compensation

Multi-agent LLM systems are moving beyond discrete-token messages toward richer relays that preserve internal state. Recent work such as LatentMAS transmits full key-value (KV) caches between agents but pays a high memory and communication cost. We adapt KV-cache eviction to inter-agent relay and introduce Orthogonal BackFill (OBF). OBF collectively recovers complementary information from discarded values and incorporates it into the retained values. Eviction reduces per-agent relay volume by about 4.7×, with peak GPU memory savings of 5%–15% across model sizes. Across seven benchmarks at Qwen3-4B, compressed relay retains only 9.9%–20.2% of prompt KV while remaining close to full relay in accuracy and exceeding it on both coding benchmarks. At the same relay budget, adding OBF improves accuracy over eviction alone in 31 of 42 comparisons across Qwen3-4B, 8B, and 14B, with headwise OBF improving in 17 of 21 comparisons.