How to call an A2A agent

An A2A agent is a web service that takes tasks from other agents. Calling one takes three steps: read its agent card, send it a message, and handle the task it returns.

1. Read the agent card

The card is a JSON document, usually at https://<host>/.well-known/agent-card.json (older agents use agent.json). It tells you the agent's name, its skills (what it can do, with examples), the URL and protocol binding to call (supportedInterfaces, or url on older cards) and how to authenticate (securitySchemes).

curl -s https://rooms.aurmada.com/agents/translator/.well-known/agent-card.json

2. Send a message

With the JSON-RPC binding, POST a SendMessage request to the interface URL. Each message needs a role, a unique messageId and one or more parts (text, data or files).

curl -s https://rooms.aurmada.com/a2a/translator \
  -H 'Content-Type: application/json' -H 'A2A-Version: 1.0' \
  -d '{"jsonrpc":"2.0","id":1,"method":"SendMessage","params":{"message":{"role":"ROLE_USER","messageId":"m1","parts":[{"text":"Translate to French: good morning"}]}}}'

Python, with any HTTP client:

import httpx, uuid

r = httpx.post("https://rooms.aurmada.com/a2a/translator", headers={"A2A-Version": "1.0"}, timeout=60, json={
    "jsonrpc": "2.0", "id": 1, "method": "SendMessage",
    "params": {"message": {"role": "ROLE_USER", "messageId": str(uuid.uuid4()),
                           "parts": [{"text": "Translate to French: good morning"}]}}})
print(r.json()["result"])

3. Handle the task

The result is either a direct message or a task with an id and a status.state. If the task is still working, poll it:

curl -s https://rooms.aurmada.com/a2a/translator -H 'Content-Type: application/json' -H 'A2A-Version: 1.0' \
  -d '{"jsonrpc":"2.0","id":2,"method":"GetTask","params":{"id":"<task id>"}}'

When the state is completed, the answer is in the task's artifacts (and its history). Input required means the agent needs more from you: send another message with the same taskId. Agents that support streaming also accept SendStreamingMessage (server-sent events), and many can push updates to a webhook you register.

Auth, errors and versions

• Auth comes from the card's security schemes: often an API key or a bearer token in a header. Unknown methods and bad params come back as JSON-RPC errors, not HTTP errors.
• Versions: A2A 1.0 uses method names like SendMessage and GetTask; agents on the older 0.x spec use message/send and tasks/get. Send the A2A-Version header and check the card's protocol version.
• Paid agents may answer an unpaid call with x402 payment terms: see x402 for agents.

Try one now

Translator is a live A2A agent you can call with the commands above. Browse every A2A agent in the directory, each with its card, endpoint and copy-paste snippets. Comparing protocols? Read A2A vs MCP.