cancellable-agent

Cancellable Agent

This sample demonstrates user-initiated A2A task cancellation. The agent runs a multi-step (5×1s) task and checks for cancellation before each step. The companion client kicks off the task and, after a short delay, calls cancelTask on the same client instance.

For details, see the A2A Specification §3.1.7 cancelTask.

Components

• agent_executor.ts — implements cancelTask(taskId, eventBus) by recording the taskId in an in-memory Set. The execute loop checks this set before each step and, if cancelled, publishes a final TaskState.TASK_STATE_CANCELED status update.
• index.ts — Express A2A server using JSON-RPC.
• client.ts — sends a streaming message, captures the taskId from the first task event, then triggers cancellation after CANCEL_AFTER_MS.

Running the sample

Open two terminals.

Terminal 1 — start the agent:

npm run agents:cancellable-agent

Terminal 2 — run the cancelling client:

npm run agents:cancellable-client

Expected output (timing-dependent):

[Client] Sending streaming message to http://localhost:41241
[Client] Task created id=<task-id> state=TASK_STATE_SUBMITTED
[Client] statusUpdate task=<task-id> state=TASK_STATE_WORKING
[Client] Sending cancelTask for <task-id> after 2500ms
[Client] statusUpdate task=<task-id> state=TASK_STATE_CANCELED
[Client] Confirmed task <task-id> was cancelled.
[Client] cancelTask returned state=TASK_STATE_CANCELED
[Client] Stream complete.

On the agent side you should see:

[CancellableAgentExecutor] Task <task-id>: step 1/5
[CancellableAgentExecutor] Task <task-id>: step 2/5
[CancellableAgentExecutor] Cancellation requested for task <task-id>
[CancellableAgentExecutor] Aborting task <task-id> at step 3.

Configuration

Variable	Default	Used by
PORT	41241	server
AGENT_URL	http://localhost:41241	client
CANCEL_AFTER_MS	2500	client