A2A
A2A — the Agent2Agent protocol — is an open standard for communication between AI agents that know nothing about each other's internals. It is not a payment protocol: it is the substrate two of the payment rails ride on, which is why the same words turn up in both places.
The protocol describes itself as an open standard for communication and collaboration between AI agents, enabling interoperability between opaque agentic applications. Opaque is the load-bearing word: the point is that two agents can work together without either exposing how it works inside.
It carries payment by being extended rather than by defining one. AP2 is built as an extension of it, and x402 publishes a transport for it beside the HTTP and MCP transports — so the same payment fields can travel between agents instead of over a request to a server.
That is also why it changes what can be observed. A payment carried between two agents leaves no HTTP 402 response at a public address for anybody to read, which is a difference of channel rather than of protocol.
Its stewardship is documented, and it has moved. The protocol was originally developed at Google. It was donated to the Linux Foundation, and it is maintained by a technical steering committee with representatives from several member companies, listed on its own site.
→ probe402 probes the HTTP transport, where a challenge is readable from outside, and the tracked set is at the endpoint list.
Sources
- The Agent2Agent ProtocolAgent2Agent Protocol
- The AP2 specificationAgent Payments Protocol
- The transport specifications directoryx402 Foundation