Messages, tasks, and events

A quick introduction to A2A messages, tasks, and events, and how to follow them through Bifrost.

This guide introduces the A2A concepts you need to send work to an agent and follow its progress through Bifrost. Start with messages, tasks, and contexts, then learn how to read updates and results.

A message is what you send. A task is the work the agent does in response. A context keeps related work in the same conversation.

For example, you ask a travel agent to plan a trip. It may need your dates before it can finish. That is still one task, even though it takes more than one message.

Follow the IDs

We used a travel-planner agent for the examples below. To try these request patterns with your own agent, replace the registration name and gateway address, and set BIFROST_VK to a key with access to your registration. Our requests used the JSON-RPC endpoint with x-bf-vk, Content-Type: application/json, and A2A-Version: 1.0 headers.

ID	What it identifies	When to reuse it
Message ID	One message	Give each new message its own ID.
Task ID	One unit of work	Continue, retrieve, cancel, or subscribe to that task.
Context ID	A conversation	Send related work in the same conversation.
Bifrost request ID	One gateway operation	Use it to investigate that particular call and its events in Bifrost.

Message, task, and context IDs belong to A2A. The Bifrost request ID is gateway metadata, separate from the JSON-RPC id used to match a protocol request with its response.

The agent creates and owns task and context IDs. Bifrost preserves them while routing requests. A task ID is also the value you pass as the agent correlation ID on the agent’s LLM and MCP requests.

Read the task state

State	What to do
submitted	The agent has accepted the task.
working	Wait for progress, retrieve the task, or subscribe to updates.
input-required	Read the agent’s message and send the missing information using the same task ID.
auth-required	Follow the agent’s authentication instructions before continuing.
completed	Read the result and artifacts.
failed	Inspect the status message and underlying model or tool calls.
rejected	The agent declined the work. Read its reason.
canceled	The cancellation completed.

A successful HTTP or JSON-RPC operation can return a task in failed state. The request reached the agent successfully; the work itself failed. Check both the gateway operation status and the task state when debugging.

Choose how to receive updates

One response

Call SendMessage through the agent’s Bifrost endpoint:

curl http://localhost:8080/agents/a2a/travel-planner/json-rpc \
  -H "x-bf-vk: $BIFROST_VK" \
  -H "Content-Type: application/json" \
  -H "A2A-Version: 1.0" \
  -d '{
    "jsonrpc": "2.0",
    "id": "trip-1",
    "method": "SendMessage",
    "params": {
      "message": {
        "messageId": "trip-message-1",
        "role": "user",
        "parts": [{"text": "Plan a trip to Lisbon."}]
      }
    }
  }'

Read result.message for a direct reply or result.task for task-based work. For a task, save its id and contextId, then inspect status.state and status.message.

If it is still working, send this body to the same endpoint with the same headers, replacing TASK_ID with the returned task ID:

{
  "jsonrpc": "2.0",
  "id": "trip-status-1",
  "method": "GetTask",
  "params": {"id": "TASK_ID"}
}

GetTask reads the current snapshot. It does not start another task. When the task completes, read its artifacts.

Streaming

Use SendStreamingMessage to receive events as the agent works. You may receive a task, status updates, and artifact updates. Bifrost groups those event records beneath the request in Agent Logs.

Use the SendMessage example above with "method": "SendStreamingMessage" and add -N to curl so it displays events as they arrive. The HTTP response uses server-sent events. Each data: payload contains a JSON-RPC result with a task, statusUpdate, or artifactUpdate.

To observe an existing task, send this body to the same endpoint using curl -N and the same headers:

{
  "jsonrpc": "2.0",
  "id": "trip-subscribe-1",
  "method": "SubscribeToTask",
  "params": {"id": "TASK_ID"}
}

Disconnecting a stream does not cancel the task. Retrieve the task or use SubscribeToTask to resume observing it. To stop the work, call CancelTask and check the result.

Push notifications

After obtaining a task ID, call CreateTaskPushNotificationConfig through the same Bifrost endpoint to register your callback. The agent sends updates to Bifrost, and Bifrost delivers them to your callback. You do not need to hold a stream open. Follow Push notifications for the request body, external URL setup, and callback delivery behavior.

Continue a task that needs input

For input-required, read task.status.message to learn what is missing. Send another SendMessage request with a new messageId, the returned taskId and contextId, and your answer:

{
  "jsonrpc": "2.0",
  "id": "trip-2",
  "method": "SendMessage",
  "params": {
    "message": {
      "messageId": "trip-message-2",
      "taskId": "TASK_ID",
      "contextId": "CONTEXT_ID",
      "role": "user",
      "parts": [{"text": "Make it three days."}]
    }
  }
}

To request cancellation, call CancelTask with "params": {"id": "TASK_ID"} and inspect the returned task state. See Transport selection and recovery for stream failure behavior.

Read artifacts

Artifacts are the agent’s outputs. One task can produce several, such as a report and a spreadsheet. Each can contain text, files, or structured data, plus a name, description, and metadata.

In Agent Logs, open the conversation or event payload to inspect the parts. File parts can contain inline bytes or an external URL. The dashboard renders supported media and provides file actions where available. Inspect the payload when you need the MIME type, filename, URI, or metadata.

An artifact update can append to an existing artifact. Several stream events do not necessarily mean several separate files.

In a task response, read result.task.artifacts[].parts[]. A text part has text; a file part includes its MIME type and either inline bytes or a URI. In a stream, use artifactUpdate.artifact.artifactId to match updates to the same artifact and its append flag to distinguish appended output from replacement output.

See a travel task's report and files

We sent our travel agent a request for a three-day Lisbon itinerary. After it completed, we opened its operation in Agent Logs and selected Conversation. The screenshot shows the returned lisbon-itinerary.txt and image artifacts for individual days. Each artifact has a name and description; Download saves the itinerary, and Metadata reveals the image’s additional fields.

Which operations appear in Agent Logs?

A2A operations appear under their protocol names:

Operation	What it records
GetAgentCard, GetExtendedAgentCard	Public or authenticated discovery.
SendMessage, SendStreamingMessage	A new message and the returned result or stream.
GetTask, ListTasks	Reading agent-owned task state.
CancelTask	Asking the agent to cancel work.
SubscribeToTask	Receiving further events for an existing task.
Push configuration create, get, list, delete	Managing where task updates should go.

Bifrost also records these gateway-specific push relay events. These labels are not A2A methods:

Bifrost log event	What it records
push_notification	An update received from the agent.
push_delivery	An attempt to deliver that update to the client.

Events have their own type, sequence, and available IDs. Some operations do not have a task or context ID, such as initial card discovery. They remain visible as individual operations.

Use CancelTask to ask the agent to stop work. DeleteTaskPushNotificationConfig removes a task’s callback configuration.

Next, Connect model and tool calls to preserve caller attribution and correlate the agent’s work.