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//side by side

MCP vs A2A vs Agent Protocol

MCP, A2A and the Agent Protocol REST spec are the three standards teams evaluate most often. They are frequently framed as rivals, but they solve different problems. This comparison lays them out so you can choose deliberately — or combine them.

ComparisonUpdated 2026

The short answer

They operate at different layers of an agent system, so “which is best?” is usually the wrong question. A better one: which layer am I working at right now?

  • MCP — connect one agent to tools, data and context.
  • A2A — let independent agents delegate tasks to each other.
  • Agent Protocol — give clients an API-style interface to an agent.

Side-by-side comparison

Dimension
MCPModel Context Protocol
A2AAgent2Agent
Agent ProtocolREST spec
Primary layerAgent ↔ tools/dataAgent ↔ agentClient ↔ agent
TransportJSON-RPC 2.0HTTP + JSON-RPC / SSEREST / HTTP + JSON
Core primitivesTools, resources, promptsAgent cards, tasks, messagesRuns, tasks, steps, artifacts
DiscoveryList tools/resourcesAgent cardsOut of scope
Streaming~Partial✓Yes~Partial
Cross-vendor focus~Partial✓Yes~Partial
Best whenWiring tools & contextCoordinating many agentsExposing an agent as an API
Marks are directional
Support levels above reflect each protocol's general focus, not a formal conformance test. Capabilities shift between spec versions — verify against official docs for your target version.

How they combine

In a realistic system you may use all three at once. A concrete pattern:

  1. A client calls your agent through an Agent Protocol REST endpoint (start a run, poll for steps).
  2. Inside, the agent uses MCP to read a database and call internal tools.
  3. For a specialised sub-task, it delegates to a peer agent over A2A and folds the result back in.
Rule of thumb
Ask what is on each side of the connection. Tools on one side → MCP. Another agent on the other side → A2A. A client wanting an API → Agent Protocol.

Choosing for your project

If you'd like a guided recommendation, the protocol selector asks five questions and suggests MCP, A2A, Agent Protocol, a custom API, or a hybrid. For depth on any single option, see the MCP, A2A and Agent Protocol guides.

//questions

Frequently asked questions

Is MCP better than A2A?

Neither is 'better' — they target different layers. MCP is about an agent using tools and context; A2A is about agents delegating to one another. Many systems use both together.

Can I use MCP and A2A together?

Yes. A common design has each agent use MCP for its own tools and A2A to coordinate with peer agents. The Agent Protocol REST spec can sit in front as the client-facing API.

Which protocol has the most adoption?

Adoption is shifting quickly and depends on the ecosystem you're in. Rather than pick by popularity alone, choose by the layer you need and validate current maturity against each project's official documentation.

What if none of them fit?

A custom API is a legitimate choice, especially for a closed, single-vendor system with narrow needs. The selector tool flags when a custom or hybrid approach may serve you better than adopting a standard.

// AgentProtocol is independent and not the official source for any protocol named here.