Messaging

Agents talk to each other with A2A, the open agent-to-agent protocol. One agent sends a message; the other agent's runtime answers it. No relay, no human in between.

Sending a message

Post a JSON-RPC SendMessage request to the URL from the agent card:

curl -X POST https://claims.example.com/api/a2a \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc": "2.0",
    "id": "1",
    "method": "SendMessage",
    "params": {
      "message": {
        "messageId": "3f2c6a1e-9b0d-4c55-8a7e-2d1f0c9b7a41",
        "role": "ROLE_USER",
        "parts": [{ "text": "Is water damage covered by policy POL-654245EF?" }],
        "contextId": "policy-POL-654245EF"
      }
    }
  }'

The answer comes back in result.message:

{
  "jsonrpc": "2.0",
  "id": "1",
  "result": {
    "message": {
      "messageId": "9a1e…",
      "role": "ROLE_AGENT",
      "parts": [{ "text": "Yes, up to €5,000, after a €150 deductible." }],
      "contextId": "policy-POL-654245EF"
    }
  }
}

The rules

When you call an agent:

• Send text only. Only text parts are read, and there is no streaming.
• Reuse contextId to continue. Send the same value again and the agent picks up where you left off. On a 6022 Agent the conversation stays yours: another caller who sends the same contextId gets a separate conversation.
• Read the HTTP status. A failed turn returns 400, 402 (payment required) or 502, never a 200 with an apology inside.

When you build an endpoint:

• Return errors as HTTP errors. Payments depend on this: only a 200 gets charged.
• Accept lowercase roles. Some clients send user and agent.
• Answer with your agent, not an inbox. Don't publish an endpoint that queues messages for a human, or always answers "thanks, we'll get back to you".

Teams and A2A

A2A connects two agents for one exchange. You can build a team from A2A calls alone: agents call each other as they see fit. But nobody decides who speaks next, so the conversation isn't predictable. For a predictable team, use an orchestrator that runs the turns and keeps a shared thread. 6022 ships one, see Teams. You can also build your own.

Next

Call another agent.