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CancelNotification to interrupt a running promptStopReason// Run prompt in a background thread
AtomicReference<PromptResponse> responseRef = new AtomicReference<>();
CompletableFuture<Void> promptFuture = CompletableFuture.runAsync(() -> {
var response = client.prompt(new PromptRequest(
sessionId,
List.of(new TextContent("Do a long task"))));
responseRef.set(response);
});
// Wait, then cancel
Thread.sleep(1500);
client.cancel(new CancelNotification(sessionId));
// Wait for prompt to finish
promptFuture.join();
System.out.println("Stop reason: " + responseRef.get().stopReason());
client.cancel() sends a one-way notification (not a request) to the agent. The agent’s cancelHandler receives it and sets a flag. The prompt handler checks this flag between steps and stops early when cancelled.
On the agent side:
// Track cancellation per session
Map<String, Boolean> cancelledSessions = new ConcurrentHashMap<>();
AcpSyncAgent agent = AcpAgent.sync(transport)
.cancelHandler(notification -> {
// Set flag — no response needed (notification, not request)
cancelledSessions.put(notification.sessionId(), true);
})
.promptHandler((req, context) -> {
cancelledSessions.put(req.sessionId(), false);
for (int i = 1; i <= 10; i++) {
// Check flag before each step
if (cancelledSessions.getOrDefault(req.sessionId(), false)) {
context.sendMessage("[Cancelled at step " + i + "]");
return PromptResponse.endTurn();
}
context.sendMessage("Step " + i + "/10... ");
Thread.sleep(500);
}
context.sendMessage("All steps completed!");
return PromptResponse.endTurn();
})
.build();
./mvnw package -pl module-10-cancellation -q
./mvnw exec:java -pl module-10-cancellation