What Drives Successful Latent Communication? Tracing Sender Information Through Latent Working Memory

Latent multi-agent systems can communicate through internal working memory rather than explicit text. In KV-cache relays, however, the transmitted working memory can contain both states produced while the sender processes its private input and states generated later during latent reasoning. Successful whole-relay communication therefore leaves a key attribution question unresolved: which source actually governs the receiver's sender-specific behavior? We study this behavioral provenance attribution problem using paired decision worlds with mutually exclusive sender-private requirements under the same public context. Our Causal Provenance Decomposition (CPD) first verifies sender-specific communication with native relays, then crosses the requirement source of sender-input states and sender-generated states while keeping both components available. Across Qwen3-4B and Llama-3.1-8B-Instruct on 150 paired worlds per Airline and Retail domain, receiver decisions under provenance conflict follow the sender-input source in approximately 63–85% of outputs, versus 1–10% for the sender-generated source. The resulting source-following margin is 52.7–84.0 percentage points and remains positive under conservative reassignment of all unmatched outputs. These results show that successful whole-relay communication does not imply comparable following of the generated-state donor under provenance conflict, motivating latent relays that transfer sender-private information without direct access to sender-input KV states.