TL;DR: A2A protocol refers to the communications standards that allow software agents to exchange structured messages directly, without a human intermediary initiating each exchange. In advertising, A2A protocols define how a buy-side agent and a sell-side agent represent themselves, communicate deal proposals, and reach bilateral agreement. The primary advertising-specific implementation is AdCP (Agent Communication Protocol), published by AgenticAdvertising.org.

The term "A2A" stands for agent-to-agent. It describes a class of protocol designed for machine-to-machine communication where both parties are autonomous software agents operating under human-defined instructions, rather than humans operating software manually.

A2A protocols are not new to computing. Distributed systems have used agent communication protocols for decades. What is new in advertising is the application of A2A communication to bilateral deal negotiation at the media-buying layer, where previously the communication between a buyer's system and a publisher's system was mediated by a series of intermediary platforms, each with its own proprietary interface.

What A2A protocol does in an advertising context

An advertising A2A protocol defines four things.

How agents identify themselves. A buy-side agent needs to communicate who it represents, under what mandate authority, and with what credentials. A sell-side agent needs to communicate which publisher it represents and what inventory it is authorised to offer. The protocol specifies the format for these identity claims so that agents from different systems can verify each other's credentials without a human introduction.

How proposals are structured. A deal proposal from a buy-side agent needs to carry the buyer's CPM range, inventory requirements, audience data conditions, and approval scope in a machine-readable format that the sell-side agent can evaluate against the publisher's mandate. The protocol specifies what fields a proposal must contain, in what format, so that any compliant sell-side agent can parse it.

How counterproposals and acceptances are communicated. A bilateral negotiation involves multiple exchanges: initial proposal, counterproposal, revised proposal, and final acceptance. The protocol specifies the message types for each step and the conditions under which a deal is considered agreed.

How the deal record is written. Once both parties reach agreement, the protocol specifies how the agreed terms are recorded, who holds the record, and how the record can be verified by either party after the fact.

AdCP: the advertising implementation

AdCP (Agent Communication Protocol), published by AgenticAdvertising.org is the primary A2A standard for advertising. It was developed under the IAB Tech Lab standards process and is designed to be interoperable: any buy-side agent built to the AdCP specification can negotiate with any sell-side agent built to the same specification, regardless of which platform each party's agent runs on.

AdCP covers agent identity, deal proposal structure, negotiation message types, and deal record format. It is designed to work alongside existing programmatic standards rather than replace them. A publisher that has integrated AdCP can accept agent-negotiated bilateral deals through the same infrastructure that handles their programmatic guaranteed and open auction traffic.

AdCP is the foundation for agent-to-agent negotiation in advertising as of 2026. Platforms building agentic buying and selling capabilities are expected to be AdCP-compliant for cross-platform negotiations to work.

How A2A protocol differs from programmatic RTB

Real-time bidding (RTB) is also a machine-to-machine protocol, but it operates differently. In RTB, a publisher sends a bid request to many buyers simultaneously, buyers respond with a bid price within milliseconds, and the highest bid wins the impression. There is no negotiation: the buyer either bids or does not. The outcome is a price, not a bilateral agreement.

A2A negotiation for media buying is asynchronous and bilateral. The buy-side agent proposes terms to a specific sell-side agent; the sell-side agent evaluates the proposal against the publisher's parameters; either party can counter-propose; and the result, if agreement is reached, is a deal record that both parties hold. This takes longer than an RTB auction but produces a different outcome: a documented bilateral agreement rather than a transient price signal.

The two protocols address different buying scenarios. RTB addresses commodity impression buying where price discovery through auction is appropriate. A2A addresses bilateral deal negotiation where terms, not prices alone, are the subject of agreement.

What buyers need to use A2A protocol

A buyer wanting to negotiate via A2A protocol needs three things: a buy-side agent that is AdCP-compliant, a mandate that the agent can operate under, and access to a marketplace or publisher sell-side agent that is also AdCP-compliant. The marketplace provides the infrastructure for the agents to communicate; the protocol ensures they can understand each other.

The mandate is the buyer's responsibility. The protocol handles the communication format. The marketplace handles the deal record infrastructure. A buyer that has those three elements in place can execute bilateral agent-to-agent deals with any publisher whose sell-side agent is also AdCP-compliant.