Block: A2A Block — Zelaxy Docs

Interact with AI agents using the A2A (Agent-to-Agent) protocol. Use this block to send messages, retrieve or cancel task status, fetch an agent's capability card, resubscribe to a streaming task, and configure push-notification callbacks — all from within a single block that selects the right underlying tool based on the chosen operation.

Overview

Property	Value
Type	`a2a`
Category	tools
Color	`#4151B5`

When to Use

Send a message to a remote A2A-compatible agent and receive its text response and task artifacts
Poll the current status of a long-running agent task using its task ID
Cancel an in-flight agent task you no longer need results from
Discover a remote agent's capabilities by fetching its Agent Card (name, skills, streaming support, etc.)
Resubscribe to an existing task's event stream after a disconnect or polling gap
Register a webhook URL so an agent can push task-status updates to your system automatically

Configuration

Operation

Dropdown (required) — selects which A2A protocol action to run. Every operation requires Agent URL.

Label	ID	Additional required fields
Send Message	`a2a_send_message`	`message`
Get Task	`a2a_get_task`	`taskId`
Cancel Task	`a2a_cancel_task`	`taskId`
Get Agent Card	`a2a_get_agent_card`	(none beyond Agent URL)
Resubscribe	`a2a_resubscribe`	`taskId` (passed as block input; the UI shows the `message` field but the underlying tool requires `taskId`)
Set Push Notification	`a2a_set_push_notification`	`taskId`

Agent URL

Short text input (required for all operations) — the base endpoint of the target A2A agent, e.g. https://agent.example.com.

Message

Long text input — the message text to deliver to the agent. Shown only when Operation is Send Message or Resubscribe.

Task ID

Short text input — the identifier of an existing task. Shown only when Operation is Get Task, Cancel Task, or Set Push Notification.

Inputs & Outputs

Inputs (block-level)

operation (string) — Operation to perform (one of the six operation IDs above)
agentUrl (string) — Agent URL
message (string) — Message to send (used by a2a_send_message)
taskId (string) — Task ID (used by a2a_get_task, a2a_cancel_task, a2a_resubscribe, and a2a_set_push_notification)

Outputs (block-level)

The block exposes a unified set of outputs drawn from whichever tool runs. Not every output is populated for every operation — see the per-tool detail in the Tools section below.

content (string) — Text response content returned by the agent
taskId (string) — Unique task identifier
contextId (string) — Groups related tasks/messages (optional)
state (string) — Current lifecycle state: working, completed, failed, canceled, rejected, input_required, or auth_required
artifacts (json) — Task output artifacts array

Tools

Each operation maps directly to one tool. The block selects the tool via params.operation.

A2A Send Message (a2a_send_message) — Posts a new message to an A2A-compatible agent and returns the response. Required params: agentUrl, message. Optional params: taskId (continue an existing task), contextId (conversation continuity), data (structured JSON string), files (file attachments), apiKey (bearer auth). Returns content, taskId, contextId, state, artifacts, and history.

A2A Get Task (a2a_get_task) — Queries the current status of an existing task by taskId. Required params: agentUrl, taskId. Optional params: historyLength (number of history messages to include), apiKey. Returns taskId, contextId, state, artifacts, and history.

A2A Cancel Task (a2a_cancel_task) — Signals the agent to cancel a running task identified by taskId. Required params: agentUrl, taskId. Optional params: apiKey. Returns cancelled (boolean) and state.

A2A Get Agent Card (a2a_get_agent_card) — Fetches the agent's discovery document (Agent Card), which describes the agent's name, description, protocol version, supported input/output modes, skills, and capabilities (streaming, push notifications, extended card). Required params: agentUrl. Optional params: apiKey. Returns name, description, url, provider, capabilities, skills, version, defaultInputModes, and defaultOutputModes.

A2A Resubscribe (a2a_resubscribe) — Resubscribes to a streaming A2A task to retrieve new updates after a disconnect. Required params: agentUrl, taskId. Optional params: apiKey. Returns taskId, contextId, state, isRunning, artifacts, and history.

A2A Set Push Notification (a2a_set_push_notification) — Registers a webhook URL to receive push notifications for task-status changes on the given taskId. Required params: agentUrl, taskId, webhookUrl. Optional params: token (sent with webhook requests for validation), apiKey. Returns url, token, and success.

YAML Example

a2a_1:
  type: a2a
  name: "A2A"
  inputs:
    operation: "a2a_send_message"
    agentUrl: "https://agent.example.com"
    message: "{{start.input}}"
  connections:
    outgoing:
      - target: next-block-id

A2A Block