Four patterns. Three protocols. One rule: pick the pattern first, then the protocol that fits it.
Once you've decided multi-agent is the right shape (and the multi-agent guide covers the cases where it isn't), the next question is how the agents coordinate. There are four orchestration patterns that recur in production, and the protocol choice (A2A, LangGraph, OpenAI Swarm, raw HTTP) is largely downstream of which pattern fits your problem.
This guide covers the four patterns, the three protocols, and the rule that usually picks one. Full treatment in Chapters 3-4 of A2A in Production.
One agent receives the request, decides which specialist should handle it, dispatches, returns the specialist's result.
One agent owns the task end-to-end. It plans, decomposes, dispatches subtasks to specialists, integrates results, decides next steps.
Multiple agents work in parallel on independent subtasks; an aggregator combines results. No central orchestrator after the initial dispatch.
Agents read from and write to a shared workspace. The next agent to act picks up wherever the previous agent left off. Coordination is via the shared state, not via a central orchestrator.
The first attempt at a standardized agent-to-agent protocol. Cards (capability descriptions), tasks (the work unit), and messaging primitives. Best when you want cross-organization interop and the other side speaks it.
An orchestration framework, not a protocol. Agents are nodes in a graph; hand-offs are edges. Best when you're inside the LangChain ecosystem and want to keep state management batteries-included.
A lightweight pattern (not really a protocol) for agent hand-offs. Best when you're already using OpenAI's tools and want the minimum surface.
Agents talk over JSON-over-HTTP with whatever shape you define. Most production multi-agent systems start here. Best when you want full control and don't need cross-org interop.
The decision order:
Patterns compose -- a supervisor whose specialists are themselves routers, for example -- but compositions multiply the failure surface. Start with one pattern; add composition only when measurement says you need it.
A2A in Production Chapters 3-4 cover the four patterns with worked examples, the protocol comparison with the spec-vs-pattern distinction, and the "when not to use A2A" cases that often turn out to be the right answer.
A2A in Production
The book on multi-agent systems and agent orchestration. Twelve chapters drafted; readable end to end as of v1.0.0-early.2. PDF + EPUB. Free updates through v1.0 and beyond. $39 (early access), secure checkout.
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