> ## Documentation Index
> Fetch the complete documentation index at: https://docs.aion.to/llms.txt
> Use this file to discover all available pages before exploring further.

# A2A Protocol

> How Aion uses the Agent2Agent protocol as its default messaging layer.

Aion uses the Agent2Agent Protocol (A2A) as its default messaging protocol. We normalize agent
communication to A2A so behaviors, distributions, hosted agents, and remote agents can all speak
through one common model instead of transport-specific request formats.

For the full protocol definition, see the
[A2A Protocol Specification](https://a2a-protocol.org/latest/specification/).

## How Aion Uses A2A

* **Discovery uses Agent Cards**: Aion agents publish `/.well-known/agent-card.json`, and both the
  SDK and API use that card as the discovery document for capabilities, skills, and extensions.
* **Ingress is normalized to A2A**: In Aion API, distributions convert external events into
  protocol-level `SendMessageRequest` payloads before forwarding them into an agent sequence.
* **Message, stream, and task flows share one model**: Our SDK and API expose A2A send, stream, and
  task operations so direct calls, long-running work, and SSE updates all follow the same protocol
  semantics.
* **Aion-specific features stay in extensions**: Distribution, middleware, traceability, and
  context behavior are documented as A2A extensions instead of changing the base protocol.

If you want the Aion-specific extension model layered on top of the protocol, continue to
[What Are Extensions?](/a2a/what-are-extensions) and
[How Extensions Work in Aion](/a2a/how-extensions-work-in-aion).