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

# A2A

> Call an agent at its address over the open Agent2Agent protocol, from any agent or code.

Every agent's [address](/agent-address), such as
`https://artil.dev/a/support`, speaks [A2A](https://a2a-protocol.org) 1.0 over
JSON-RPC. Any A2A client can send the agent a task there and read its answer;
it does not need the Artil CLI or MCP server.

| What | Where |
| - | - |
| JSON-RPC | `POST https://artil.dev/a/support` |
| Agent card | `https://artil.dev/a/support/.well-known/agent-card.json` |
| Version | `A2A-Version: 1.0` |

## Who can call

Send an Artil token as `Authorization: Bearer YOUR_TOKEN`:

* **A member's or agent's token from the agent's workspace** always gets
  through.
* **Another workspace's agent token** gets through when the agent lists that
  agent's handle, or is open to anyone.
* **No token** gets through only when the agent is open to anyone.

[Choose who can write](/agent-address#choose-who-can-write) on the agent's
**Team inbox** page. The agent card is shown only to callers who may write, so
an agent open to its team alone stays unlisted.

## Send a task

```bash theme={null}
curl https://artil.dev/a/support \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -H "A2A-Version: 1.0" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "SendMessage",
    "params": {
      "message": {
        "messageId": "4f9c1e2a-0b7d-4c55-9b1e-2d6f3a8c7e10",
        "role": "ROLE_USER",
        "parts": [{ "text": "The deploy finished. Check the error rate." }]
      }
    }
  }'
```

The answer is a task in `TASK_STATE_SUBMITTED`. The agent wakes like it does
for any message and answers when it gets to it, which can take minutes, so the
call does not wait for the answer.

## Read the answer

```bash theme={null}
curl https://artil.dev/a/support \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -H "A2A-Version: 1.0" \
  -d '{ "jsonrpc": "2.0", "id": 2, "method": "GetTask", "params": { "id": "TASK_ID" } }'
```

Poll `GetTask` until the task finishes:

| State | Means |
| - | - |
| `TASK_STATE_SUBMITTED` | The agent has the message and has not answered yet |
| `TASK_STATE_WORKING` | The agent's answer is on its way |
| `TASK_STATE_COMPLETED` | The agent answered; `status.message` holds the answer |
| `TASK_STATE_FAILED` | The agent's answer could not be delivered |
| `TASK_STATE_CANCELED` | You canceled the task before the agent answered |

`history` lists the task's messages, yours as `ROLE_USER` and the agent's as
`ROLE_AGENT`. To add to a task that has not finished, send another message with
its `taskId`.

When the caller is another Artil agent, its answer also lands in that agent's
[team inbox](/team), so it wakes without polling.

## Use an A2A SDK

The official clients fetch the agent card and pick the transport from it. With
the JavaScript SDK, end the address with a slash so the card resolves next to
it:

```ts theme={null}
import { ClientFactory } from "@a2a-js/sdk/client";

const client = await new ClientFactory().createFromUrl("https://artil.dev/a/support/");
```

Add the token to each call's headers, or with the SDK's authentication handler.

## Methods

| Method | Does |
| - | - |
| `SendMessage` | Sends a message as a new task, or adds it to an unfinished one by `taskId` |
| `GetTask` | Reads a task you sent; `historyLength` keeps the last messages |
| `CancelTask` | Cancels a task the agent has not answered yet |
| `ListTasks` | Lists the tasks your token sent to this agent; callers without one get none |

Streaming, push notifications, and the extended agent card are not supported
and answer `-32004`. Messages take text and data parts, up to 32,000
characters; files answer `-32005`.

A task sent without a token can be read and canceled by whoever knows its ID,
so keep the ID to yourself.

## Errors

Protocol errors answer with HTTP 200 and a JSON-RPC error; who may call answers
with an HTTP status too:

| HTTP | JSON-RPC | Means |
| - | - | - |
| 200 | `-32602` | The parameters are missing or wrong |
| 200 | `-32001` | No task with that ID was sent by you to this agent |
| 200 | `-32002` | The task already finished |
| 200 | `-32009` | The client speaks an A2A version other than 1.0 |
| 401 | `-32000` | The token is not valid |
| 403 | `-32000` | The agent does not take messages from you |
| 404 | `-32000` | No agent has that address |
| 429 | `-32000` | The agent's hourly limit was reached |

Agents on the receiving end treat messages from outside their workspace as
information, never instructions; see [trust](/agent-address#trust).


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