A2A vs MCP: What’s the Actual Difference?
Ask five agent builders to distinguish A2A from MCP and you’ll get five muddy answers. The confusion is understandable — both are “agent protocols,” both came from big labs (Google and Anthropic respectively), and both show up in the same architecture diagrams. But they solve different problems:
- MCP (Model Context Protocol) connects a model to tools and data.
- A2A (Agent-to-Agent) connects an agent to other agents.
One is a function call; the other is a conversation. And they monetize differently.
The 30-second answer
- Use MCP when your agent needs to do something: query a database, call an API, read a file. MCP is the USB-C port between an LLM and its capabilities — see the MCP payments category.
- Use A2A when your agent needs to delegate something: hire another agent to research, negotiate, or execute a task end-to-end — see the A2A payments category.
- Use both in any real system: an orchestrator talks to sub-agents over A2A; each sub-agent uses MCP tools to do its job.
What they share
Both are open standards with strong backing, both are transport-explicit about capability discovery (an MCP server lists its tools; an A2A agent publishes an Agent Card), and both became payment surfaces the moment agents started transacting: paid MCP servers charge per tool call, while A2A payments settle per completed task — usually via the A2A x402 extension.
What’s different
| Dimension | MCP | A2A |
|---|---|---|
| Backer | Anthropic | |
| Relationship modeled | Model ↔ tool | Agent ↔ agent |
| Interaction shape | Synchronous function calls | Long-running, stateful tasks |
| Discovery | Server tool/prompt/resource lists | Agent Cards (capabilities, endpoints) |
| State | Mostly stateless calls | Task lifecycle with status updates |
| Payment unit | Per tool call (cents) | Per task or deliverable (dollars) |
| Typical monetization | x402 paywall on the MCP server | x402 settlement inside the A2A flow |
MCP: the tool-access layer
MCP gives an LLM a uniform way to discover and invoke tools: list_tools, call_tool, done. It’s deliberately simple, which is why it won — thousands of servers expose everything from Postgres to Playwright.
Monetizing MCP is the most proven pattern in agent payments: wrap the server in an x402 paywall, price each tool call, and any agent with a wallet can pay per use — no API keys, no subscriptions. Real example: blockrun-mcp sells live data (search, markets, X/Twitter) per call in USDC. Our guide: MCP server payments.
A2A: the delegation layer
A2A models something bigger: one agent hiring another. The hiring agent discovers a remote agent’s Agent Card, opens a task, streams status updates, and receives artifacts. Tasks can run for minutes or days and involve multi-turn negotiation — a conversation, not a function call.
That changes the payment shape too. You don’t meter an A2A task per call; you price the outcome — research delivered, code reviewed, booking completed. Google’s A2A x402 extension standardizes exactly this: the seller agent demands payment mid-task, the buyer pays, work continues. SDKs are already multi-language, e.g. a2a-go for Go services.
How they compose
The canonical 2026 agent architecture uses both layers plus a settlement rail:
- A2A — the orchestrator delegates a paid task to a specialist agent.
- MCP — the specialist calls its paid tools (search, data, compute).
- x402 — every payment, at both layers, settles in USDC inside the HTTP flow.
The orchestrator pays the specialist per task; the specialist pays its tool providers per call. Two payment granularities, one rail.
Which should you build first?
- Have a useful API or dataset? Wrap it as a paid MCP server — fastest path to agent revenue today (tutorial, paid MCP guide).
- Have a full agent that completes jobs? Expose it over A2A with an Agent Card and price the deliverable.
- Building an orchestrator? You’ll consume both: budget per-task spend over A2A, and give sub-agents wallets with limits for their MCP tool calls.
Bottom line
MCP is how agents use tools; A2A is how agents use each other. They’re complements stacked at different altitudes, and x402 is the payment rail that lets both layers charge. Browse the live project lists in MCP and A2A, or see every protocol side by side in the comparison.