Multi-Agent Coordination: Message-Bus Patterns That Keep Agents Sane
Book: Agents in Production — Building, Tracing, and Shipping Multi-Step AI You Can Trust Also by me: Observability for LLM Applications — the companion book in The AI Engineer's Library (2-book series) My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub In June 2025 two engineering teams posted opposite advice in the same week. Anthropic shipped How we built our multi-agent research system : an orchestrator dispatching subtasks to worker agents, beating a single agent by 90.2% on breadth-first research. A few days later Cognition, the team behind Devin, published Don't Build Multi-Agents , arguing that parallel subagents without shared context produce fragile systems. Both were right. They were describing different workloads. Anthropic's research agent is embarrassingly parallel: four workers go read four things and come back with four small summaries. Cognition's target is writing code, where every edit depends on every other edit and context cannot be sliced. Most people get the plumbing wrong, not the decision. Once you have two agents that need to coordinate, you have to choose how they talk. That choice decides your failure modes long before the models do. Handoffs vs a shared bus There are two ways to wire agents together, and they fail differently. A handoff transfers control. Agent A finishes, hands the whole conversation to Agent B, and steps out. This reads well in a demo. In production it means the transcript grows on every hop, and by the fourth agent you are paying to re-read a conversation nobody trimmed. Handoffs also lose the parent: once A hands off to B, nobody is holding the original task to check the final answer against it. A shared bus keeps a supervisor in charge. Workers never talk to each other. They receive a small typed task, do the work, and return a small typed artifact to the supervisor, which composes the result. This is the shape of Anthropic's research