Beginner's Guide to Agent2Agent Streaming and Events¶

Does an Agent Have the Ability to Send Messages Based on Events?¶

Yes! In the Agent2Agent (A2A) protocol, agents have the ability to send event-based messages to provide real-time updates and asynchronous communication. This capability enables agents to proactively notify clients about state changes, data updates, or important occurrences without requiring constant polling.

Event-based messaging is a core feature that distinguishes reactive, streaming agents from simple request-response agents. When an agent advertises streaming or notification capabilities in its Agent Card, clients can subscribe to receive automatic updates whenever relevant events occur.

How A2A Streaming Works¶

A2A streaming enables continuous, real-time communication between agents through an event-driven architecture. Rather than requiring clients to repeatedly poll for updates, streaming allows servers to push information as it becomes available.

Core Streaming Concepts¶

1. Server-Sent Events (SSE) Protocol

A2A streaming commonly uses the Server-Sent Events (SSE) standard, a simple HTTP-based protocol designed for one-way server-to-client communication. SSE provides:

Automatic reconnection on connection loss
Event IDs for tracking message delivery
UTF-8 text-based data format
Built-in browser support

2. Persistent Connections

Once established, a streaming connection remains open indefinitely, allowing the server to send multiple messages over time:

Client                          Agent Server
  |                                  |
  |------ HTTP GET /stream -------->|
  |                                  |
  |<===== Connection Open ==========|
  |                                  |
  |<----- Event: price_update ------| 
  |<----- Event: price_update ------|
  |<----- Event: alert --------------|
  |        ... continuous ...        |

3. Subscribe/Notify Pattern

Agents implement a publish-subscribe model:

Subscribe: Client registers interest in specific event types
Notify: Server sends events to all subscribed clients
Unsubscribe: Client terminates subscription when done

Main Event Messages an Agent Can Send¶

The A2A protocol defines several standard event message types that agents use to communicate asynchronously. These messages enable real-time interactions between agents.

1. Subscribe/Notify Messages¶

Purpose: Establish and manage event subscriptions

Subscribe Message

{
  "message_type": "SUBSCRIBE",
  "sender_id": "client-12345",
  "recipient_id": "crypto-agent-001",
  "payload": {
    "event_types": ["price_update", "alert"],
    "filters": {
      "currencies": ["BTC", "ETH"]
    }
  }
}

Notify Message

{
  "message_type": "NOTIFY",
  "sender_id": "crypto-agent-001",
  "recipient_id": "client-12345",
  "payload": {
    "event_type": "price_update",
    "data": {
      "currency": "BTC",
      "price": 125430.50,
      "timestamp": "2025-10-22T14:30:00Z"
    }
  }
}

2. Data Update Events¶

Purpose: Push real-time data changes to subscribers

Examples: - price_update: Cryptocurrency price changes - status_change: Agent or service status updates - data_refresh: New data available - metrics_update: Performance or analytics data

3. State Change Events¶

Purpose: Notify clients when agent state changes

Examples: - agent_online: Agent becomes available - agent_offline: Agent going offline - capability_changed: Agent capabilities modified - maintenance_mode: Agent entering maintenance

4. Alert Events¶

Purpose: Communicate important conditions requiring attention

Examples: - threshold_exceeded: Value crosses configured threshold - error_occurred: Error condition detected - warning: Non-critical issue - anomaly_detected: Unusual pattern identified

5. Progress Events¶

Purpose: Report progress on long-running operations

Examples: - task_started: Operation initiated - progress_update: Percentage complete - task_completed: Operation finished - task_failed: Operation encountered error

6. Heartbeat Events¶

Purpose: Maintain connection health and detect failures

Example:

{
  "message_type": "HEARTBEAT",
  "sender_id": "crypto-agent-001",
  "timestamp": "2025-10-22T14:30:00Z",
  "payload": {
    "status": "healthy",
    "uptime_seconds": 86400
  }
}

Message Format Examples¶

Event messages in A2A streaming follow the Server-Sent Events (SSE) specification. Each event consists of one or more lines, with each line having a specific purpose. Let's examine both the raw SSE format and human-readable interpretations.

Basic Event Structure¶

An SSE event message consists of fields, each on its own line:

event: - The event type/name
data: - The event payload (can span multiple lines)
id: - Optional unique identifier for the event
retry: - Optional reconnection time in milliseconds
Empty line - Marks the end of an event