RAG, Agents, MCP, A2A: Four Layers of the Agentic Stack — and Which One Is Actually Shipping
They're not competitors. They're four layers of the same stack.
RAG → what the system knows. Retrieve, rerank, answer with sources. Most failures here are chunking failures, not retrieval ones.
Agents → what the system decides. Plan, act, observe, reflect. The real design choice is the autonomy slider: reads free, writes behind approval.
MCP → what the system can touch. Write the tool server once, every compliant client gets access. This is the layer actually working today.
A2A → who the system can talk to. Agents delegating to partner agents. Still mostly conference slides, because nobody can answer who is liable when a partner agent gets it wrong.
Three are shipping. One isn't. Build the layer that's blocking you, not the layer that demos well.
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Confused by RAG, Agents, MCP, and A2A? They're not competitors — they're four layers of the same stack.
RAG → what the system knows. Retrieve, rerank, answer with sources. Most failures here are chunking failures, not retrieval failures.
Agents → what the system decides. Plan, act, observe, reflect. The real design choice is the autonomy slider: reads free, writes behind approval.
MCP → what the system can touch. Write the tool server once, every compliant client gets access. This layer has real multi-vendor adoption today.
A2A → who the system can talk to. Agents delegating to partner agents. As of 2026 this has moved past the demo stage — over 150 organizations back it, with production deployments across several industries and a v1.0 spec adding cryptographic Agent Card signing and multi-tenancy. The open question isn't "does it work," it's governance: who's liable when a partner agent gets it wrong, and how do you verify a partner agent's identity before trusting its output.
All four layers now have real production use — the honest question for a team is which one is currently blocking your system, not which one demos best.