Checked, Not Just Cited: Source-Grounded Verification for Multi-Agent Handoffs

Multi-agent systems let agents use one another's findings without rereading the underlying material. In handoffs where the receiving agent lacks source access, however, a convincing report may contain genuine quotations yet draw unsupported conclusions. We study a source-grounded verification signal that combines quotation matching with model-based reasoning checks at the handoff boundary, where the source remains. We test it on BrowseComp-Plus by selecting reports that increasingly persuade the receiver. Across nine sender–receiver pairings, verification loses precision more slowly than receiver adoption, retaining 78–84% under the strongest selection compared with 54–67% for adoption, although it accepts only 14–36% of reports. The reasoning checks catch unsupported conclusions even when quotations match the source. When fed back to receivers, the signal improves their decisions, especially under stronger selection. We incorporate this signal into AgentCite, a handoff protocol that informs receiver decisions and triggers source-side re-investigation when quotations are missing. On three tool-based delegation tasks across three model families, AgentCite without re-investigation matches the strongest baseline within 0.3 points with 23–55% shorter receiver messages; re-investigation lifts mean accuracy 1.8–4.4 points above it at extra tool cost. These results support source-grounded verification for agent handoffs without transferring the full source context to the receiver.