Callbacks and Event Listeners

AgentEnsemble provides an event listener API that lets you observe task and tool execution lifecycle events without modifying your agent or workflow configuration.

Quick Start

EnsembleOutput output = Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .onTaskStart(event -> log.info("[{}] Starting: {}", event.taskIndex(), event.taskDescription()))
    .onTaskComplete(event -> log.info("[{}] Done in {}", event.taskIndex(), event.duration()))
    .onTaskFailed(event -> alertService.notify(event.cause()))
    .onToolCall(event -> metrics.increment("tool." + event.toolName()))
    .onDelegationStarted(event -> log.info("Delegating to {} [{}]", event.workerRole(), event.delegationId()))
    .onDelegationCompleted(event -> metrics.record("delegation.latency", event.duration()))
    .onDelegationFailed(event -> log.warn("Delegation failed: {}", event.failureReason()))
    .build()
    .run();

Event Types

TaskStartEvent

Fired immediately before an agent begins executing a task.

Field	Type	Description
`taskDescription()`	`String`	The description of the task
`agentRole()`	`String`	The role of the agent executing the task
`taskIndex()`	`int`	1-based index of this task in the workflow run
`totalTasks()`	`int`	Total number of tasks in this run

TaskCompleteEvent

Fired immediately after an agent successfully completes a task.

Field	Type	Description
`taskDescription()`	`String`	The description of the completed task
`agentRole()`	`String`	The role of the agent
`taskOutput()`	`TaskOutput`	The full output produced by the agent
`duration()`	`Duration`	Time from task start to completion
`taskIndex()`	`int`	1-based index of this task
`totalTasks()`	`int`	Total number of tasks

TaskFailedEvent

Fired when an agent fails to complete a task, before the exception propagates. This lets you observe failures (for alerting, metrics, etc.) without needing to wrap run() in try-catch.

Field	Type	Description
`taskDescription()`	`String`	The description of the failed task
`agentRole()`	`String`	The role of the agent
`cause()`	`Throwable`	The exception that caused the failure
`duration()`	`Duration`	Time from task start to failure
`taskIndex()`	`int`	1-based index of this task
`totalTasks()`	`int`	Total number of tasks

ToolCallEvent

Fired after each tool execution within an agent’s ReAct loop.

Field	Type	Description
`toolName()`	`String`	The name of the tool that was called
`toolArguments()`	`String`	JSON string of arguments passed to the tool
`toolResult()`	`String`	The result returned by the tool
`agentRole()`	`String`	The role of the agent that invoked the tool
`duration()`	`Duration`	Time taken for the tool execution

TokenEvent

Fired for each token received during streaming generation of the final agent response. Only fires when a StreamingChatModel is resolved for the agent (see Streaming Output below).

Field	Type	Description
`token()`	`String`	The text fragment emitted by the streaming model
`agentRole()`	`String`	The role of the agent generating the response

Token events are fired during the direct LLM-to-answer path only. Tool-loop iterations remain synchronous because the full response must be seen to detect tool-call requests.

DelegationStartedEvent

Fired immediately before a delegation is handed off to a worker agent. Only fired for delegations that pass all built-in guards and registered policy evaluations.

Field	Type	Description
`delegationId()`	`String`	Unique correlation ID; matches the completed or failed event
`delegatingAgentRole()`	`String`	Role of the agent initiating the delegation
`workerRole()`	`String`	Role of the agent that will execute the subtask
`taskDescription()`	`String`	Description of the subtask
`delegationDepth()`	`int`	Depth in the chain (1 = first delegation, 2 = nested, etc.)
`request()`	`DelegationRequest`	The full typed delegation request

DelegationCompletedEvent

Fired immediately after a delegation completes successfully.

Field	Type	Description
`delegationId()`	`String`	Matches the corresponding `DelegationStartedEvent`
`delegatingAgentRole()`	`String`	Role of the agent that initiated the delegation
`workerRole()`	`String`	Role of the worker that executed
`response()`	`DelegationResponse`	Full typed response with output and metadata
`duration()`	`Duration`	Elapsed time from delegation start to completion

DelegationFailedEvent

Fired when a delegation fails, whether due to a guard violation, policy rejection, or worker exception. Guard/policy failures have cause() == null; worker exceptions carry the thrown exception. Guard and policy failures do not have a corresponding DelegationStartedEvent.

Field	Type	Description
`delegationId()`	`String`	Matches `DelegationRequest.getTaskId()`
`delegatingAgentRole()`	`String`	Role of the initiating agent
`workerRole()`	`String`	Role of the intended target
`failureReason()`	`String`	Human-readable description of the failure
`cause()`	`Throwable`	Exception if worker threw; `null` for guard/policy failures
`response()`	`DelegationResponse`	FAILURE response with error messages
`duration()`	`Duration`	Elapsed time from delegation start to failure

Full Interface Implementation

For listeners that handle multiple event types, implement EnsembleListener directly:

public class MetricsListener implements EnsembleListener {

    private final MeterRegistry registry;

    public MetricsListener(MeterRegistry registry) {
        this.registry = registry;
    }

    @Override
    public void onTaskStart(TaskStartEvent event) {
        registry.counter("task.started", "agent", event.agentRole()).increment();
    }

    @Override
    public void onTaskComplete(TaskCompleteEvent event) {
        registry.timer("task.duration", "agent", event.agentRole())
                .record(event.duration());
    }

    @Override
    public void onTaskFailed(TaskFailedEvent event) {
        registry.counter("task.failed", "agent", event.agentRole()).increment();
    }

    @Override
    public void onToolCall(ToolCallEvent event) {
        registry.counter("tool.calls", "tool", event.toolName()).increment();
    }
}

// Register with the builder
Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .listener(new MetricsListener(meterRegistry))
    .build()
    .run();

All methods have default no-op implementations, so you only need to override the events you care about. The three delegation event methods (onDelegationStarted, onDelegationCompleted, onDelegationFailed) are also available on EnsembleListener and can be overridden in the same way.

Registering Multiple Listeners

All registration methods accumulate — calling them multiple times adds each listener to the list. Listeners are called in registration order.

Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .listener(new MetricsListener())      // Full implementation
    .listener(new AuditLogger())          // Another full implementation
    .onTaskFailed(event -> alertPager())  // Lambda for just this one event
    .build()
    .run();

Exception Safety

If a listener throws an exception, the exception is caught and logged at WARN level. Execution continues normally and all subsequent listeners are still called. A misbehaving listener cannot abort task execution or prevent other listeners from receiving events.

Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .onTaskComplete(event -> {
        // If this throws, it is logged and the next listener still runs
        externalService.send(event);
    })
    .onTaskComplete(event -> {
        // This always runs, even if the previous listener threw
        localLog.record(event);
    })
    .build()
    .run();

Thread Safety

ExecutionContext (which carries the listener list) is immutable. When using Workflow.PARALLEL, the onTaskStart, onTaskComplete, onTaskFailed, and onToolCall events may be fired concurrently from multiple virtual threads.

Listener implementations must be thread-safe when used with a parallel workflow.

For example, use ConcurrentLinkedQueue instead of ArrayList if you are collecting events in a listener:

ConcurrentLinkedQueue<TaskCompleteEvent> events = new ConcurrentLinkedQueue<>();

Ensemble.builder()
    .agents(List.of(a1, a2, a3))
    .tasks(List.of(t1, t2, t3))
    .workflow(Workflow.PARALLEL)
    .onTaskComplete(events::add)  // ConcurrentLinkedQueue is thread-safe
    .build()
    .run();

Practical Examples

Logging Task Progress

Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .onTaskStart(e -> log.info("Task {}/{} starting: {}", e.taskIndex(), e.totalTasks(), e.agentRole()))
    .onTaskComplete(e -> log.info("Task {}/{} done in {}", e.taskIndex(), e.totalTasks(), e.duration()))
    .onTaskFailed(e -> log.error("Task {}/{} failed: {}", e.taskIndex(), e.totalTasks(), e.cause().getMessage()))
    .build()
    .run();

Collecting Metrics

The example below uses ArrayList, which is safe for Workflow.SEQUENTIAL and Workflow.HIERARCHICAL. For Workflow.PARALLEL, callbacks fire from concurrent virtual threads — use thread-safe collections instead (see the Thread Safety section above).

// Safe for sequential/hierarchical workflows. For parallel, replace ArrayList
// with ConcurrentLinkedQueue or CopyOnWriteArrayList.
List<Duration> taskDurations = new ArrayList<>();
List<String> toolsUsed = new ArrayList<>();

Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .onTaskComplete(e -> taskDurations.add(e.duration()))
    .onToolCall(e -> toolsUsed.add(e.toolName()))
    .build()
    .run();

log.info("Average task duration: {}", average(taskDurations));
log.info("Tools used: {}", toolsUsed);

Alerting on Failure

Ensemble.builder()
    .agent(researcher)
    .task(criticalTask)
    .onTaskFailed(event -> {
        alertService.sendAlert(
            "Task failed: " + event.taskDescription(),
            "Agent: " + event.agentRole(),
            event.cause()
        );
    })
    .build()
    .run();

Auditing Tool Usage

Ensemble.builder()
    .agent(researcher)
    .task(researchTask)
    .onToolCall(event -> auditLog.record(
        "Agent '" + event.agentRole() + "' called tool '" + event.toolName() + "'" +
        " with args: " + event.toolArguments() +
        " -> result: " + event.toolResult() +
        " (" + event.duration().toMillis() + "ms)"
    ))
    .build()
    .run();

Streaming Output

Agents can stream their final response token-by-token using LangChain4j’s StreamingChatModel. When streaming is configured, each token fires an onToken(TokenEvent) callback to all registered listeners.

Configuration

Streaming is opt-in and off by default. Configure it at one of three levels; the first non-null value in the chain wins:

Level	How to set	Priority
Agent-level	`Agent.builder().streamingLlm(model)`	Highest
Task-level	`Task.builder().streamingChatLanguageModel(model)`	Middle
Ensemble-level	`Ensemble.builder().streamingChatLanguageModel(model)`	Lowest

Only the final answer path is streamed. When an agent has tools, the tool-calling iterations use the synchronous ChatModel (full responses are needed to detect tool-call requests). Streaming fires only when executeWithoutTools is active.

Example: printing tokens to the console

StreamingChatModel streamingModel = OpenAiStreamingChatModel.builder()
    .apiKey(System.getenv("OPENAI_API_KEY"))
    .modelName("gpt-4o")
    .build();

EnsembleOutput output = Ensemble.builder()
    .chatLanguageModel(syncModel)              // used for tool-loop iterations
    .streamingChatLanguageModel(streamingModel) // used for final answers
    .task(Task.builder()
        .description("Write a haiku about Java")
        .expectedOutput("A three-line haiku")
        .build())
    .onToken(event -> System.out.print(event.token()))  // typewriter effect
    .build()
    .run();

System.out.println(); // newline after streaming

Example: streaming with the live dashboard

When webDashboard() is registered alongside streamingChatLanguageModel(), tokens are broadcast over WebSocket as token messages. The viz dashboard displays the text live in the Live Output section of the task detail panel.

WebDashboard dashboard = WebDashboard.onPort(7329);

Ensemble.builder()
    .chatLanguageModel(syncModel)
    .streamingChatLanguageModel(streamingModel)
    .webDashboard(dashboard)
    .task(Task.of("Summarize the state of AI in 2025"))
    .build()
    .run();

Thread safety note

In a parallel workflow, onToken may be called concurrently from multiple virtual threads (one per running agent). Use thread-safe accumulators when collecting tokens across tasks:

ConcurrentHashMap<String, StringBuffer> tokenBuffers = new ConcurrentHashMap<>();

Ensemble.builder()
    .workflow(Workflow.PARALLEL)
    .streamingChatLanguageModel(streamingModel)
    .onToken(event -> tokenBuffers
        .computeIfAbsent(event.agentRole(), k -> new StringBuffer())
        .append(event.token()))
    .tasks(List.of(task1, task2))
    .build()
    .run();