Two strangers need to do business, but they have never met and speak different internal languages. They manage anyway, because they share a few agreed conventions: how to introduce themselves, how to ask a question, how to make an offer. Agentic advertising works the same way, on top of three general protocols.
None of these three protocols is about advertising. They are the general machinery agents use to talk. The ad-specific meaning sits in a layer on top, which Track H covers.
MCP: a model calling tools
MCP (Model Context Protocol) is how an AI model uses tools that someone else exposes. A server publishes a set of tools (think "get available products" or "create a media buy"). A client, driven by a model, calls them. A publisher that wants to be buyable by agents stands up an MCP server that exposes its inventory as tools. If you have followed how Prebid exposes an auction through a clean interface, this is the same idea for an agent.
Key idea
MCP turns a system into a set of callable tools an AI can use. A sales agent is, in large part, an MCP server that exposes a publisher’s inventory as tools a buyer’s model can call.
A2A: two agents negotiating
A2A (Agent-to-Agent) is how two independent agents coordinate a task directly, without a human relaying messages. Where MCP is a model reaching for a tool, A2A is two peers agreeing on a job: one asks, the other accepts, declines, or counters, and they track the task to completion. A negotiation between a buyer agent and a sales agent is an A2A exchange.
gRPC: the fast pipe
gRPC is a high-throughput way for services to call each other using compact binary messages instead of bulky text. It is a transport, not a conversation style. Some agentic systems use it where speed and volume matter, for example signal exchange at machine speed. You will see it named in the IAB Tech Lab stack in Track I.
| Protocol | In one line | Role in a buy |
|---|---|---|
| MCP | A model calls tools a server exposes | A buyer’s model calls a publisher’s "get products" or "create buy" tools |
| A2A | Two agents negotiate a task directly | Buyer agent and sales agent agree on terms |
| gRPC | Fast binary transport between services | Carries high-volume traffic such as signal exchange |
Note
Hold onto the key point: these are general protocols. They do not know what a CPM or a creative is. The advertising meaning comes from a thin layer defined on top, which is exactly what AdCP is.
Quick check
A buyer’s model calls a publisher tool named get_products to see what is for sale. Which protocol is that, and which would carry a back-and-forth negotiation to agree on the deal?