Spawn & Announce - PraisonAI

Spawn & Announce lets a parent agent delegate work to sub-agents without blocking — fire tasks in parallel and collect results via callbacks or the event bus.

from praisonaiagents import Agent, AgentTeam, Task

team = AgentTeam(agents=[], name="research_team")

researcher = Agent(name="Researcher", instructions="Research the topic briefly.")
task = Task(description="What is quantum computing?")

team.spawn_sub_agent(researcher, task)
# Parent continues immediately — no blocking

The user delegates parallel work; the parent spawns sub-agents and keeps running without blocking.

Quick Start

Simple Spawn with Callback

Fire-and-forget pattern with immediate feedback.

from praisonaiagents import Agent, AgentTeam, Task

team = AgentTeam(agents=[], name="my_team")

researcher = Agent(name="Researcher", instructions="Research the topic briefly.")
task = Task(description="What is quantum computing?")

def on_done(event):
    print(f"Sub-agent {event.agent_id} done: {event.result}")

team.spawn_sub_agent(researcher, task, completion_callback=on_done)
# Parent continues immediately — no blocking

Parallel Fan-out Pattern

Spawn multiple sub-agents, do other work, then collect results.

from praisonaiagents import Agent, AgentTeam, Task

team = AgentTeam(agents=[], name="research_team")

agents = [
    Agent(name="Researcher", instructions="Research briefly."),
    Agent(name="Writer", instructions="Write briefly."),
    Agent(name="Analyst", instructions="Analyze briefly."),
]
tasks = [
    Task(description="Research AI trends"),
    Task(description="Write a tagline about AI"),
    Task(description="List one risk of AI"),
]

for agent, task in zip(agents, tasks):
    team.spawn_sub_agent(agent, task)

# Do other work here while sub-agents run in parallel...

completions = team.wait_for_completions(timeout=60.0)
for c in completions:
    print(c.agent_id, "->", c.result)

How It Works

The spawn-announce pattern uses an event-driven architecture to coordinate between parent and sub-agents.

Component	Role
Parent Agent	Spawns sub-agents, continues work non-blocking
Sub-Agent	Executes task, announces completion when done
AgentTeam	Orchestrates spawning and event coordination
EventBus	Delivers completion events to callbacks

Choosing Your Pattern

Different use cases require different coordination patterns.

Common Patterns

Fire-and-Forget with Callback

Best for immediate processing of sub-agent results.

from praisonaiagents import Agent, AgentTeam, Task

team = AgentTeam(agents=[], name="callback_team")

def process_result(event):
    if event.success:
        print(f"✅ {event.agent_id}: {event.result}")
    else:
        print(f"❌ {event.agent_id}: {event.error}")

team.spawn_sub_agent(
    Agent(name="Worker", instructions="Answer in one line."),
    Task(description="Say hello."),
    completion_callback=process_result
)

Parallel Fan-out with Join

Scale work across multiple sub-agents, then combine results.

from praisonaiagents import Agent, AgentTeam, Task

team = AgentTeam(agents=[], name="parallel_team")
topics = ["AI", "blockchain", "quantum computing"]

# Spawn all sub-agents
for topic in topics:
    agent = Agent(name=f"{topic}_expert", instructions=f"Expert in {topic}.")
    task = Task(description=f"Explain {topic} in one sentence.")
    team.spawn_sub_agent(agent, task)

# Collect all results
results = team.wait_for_completions(timeout=120.0)
summary = "\n".join([f"• {r.result}" for r in results if r.success])
print("Research Summary:\n" + summary)

Event-Driven Coordination

React to sub-agent events without callbacks.

from praisonaiagents import Agent, AgentTeam, Task
from praisonaiagents.bus import EventBus
from praisonaiagents.bus.event import EventType

bus = EventBus()

def on_subagent_event(event):
    print(f"[{event.type}] {event.data['agent_id']} - {event.data.get('result', 'spawned')}")

bus.subscribe(on_subagent_event, [
    EventType.SUBAGENT_SPAWNED.value,
    EventType.SUBAGENT_COMPLETED.value,
    EventType.SUBAGENT_ERROR.value,
])

team = AgentTeam(agents=[], name="event_team")
team._event_bus = bus  # share bus

team.spawn_sub_agent(
    Agent(name="Worker", instructions="Answer briefly."),
    Task(description="What is 2+2?"),
)

Configuration

spawn_sub_agent Parameters

Parameter	Type	Default	Description
`agent`	`Agent`	required	The agent that will execute the task.
`task`	`Task` or `str`	required	Task to run. If it has `.description`, that string is sent to `agent.start()`.
`completion_callback`	`Callable[[SubAgentCompletionEvent], Any]`	`None`	Called once the sub-agent finishes.
`metadata`	`Dict[str, Any]`	`None`	Free-form metadata stored on the `SpawnedSubAgent`.

announce_completion Parameters

Parameter	Type	Default	Description
`agent_id`	`str`	required	ID returned by `spawn_sub_agent`.
`task_id`	`str`	required	Task ID from `SpawnedSubAgent.task_id`.
`result`	`Any`	required	The sub-agent’s output.
`success`	`bool`	`True`	Whether the task succeeded.
`error`	`Optional[str]`	`None`	Error message if `success=False`.
`metadata`	`Dict[str, Any]`	`None`	Free-form metadata on the completion event.

wait_for_completions Parameters

Parameter	Type	Default	Description
`timeout`	`Optional[float]`	`None`	Max seconds to wait. `None` = wait forever (not recommended).
`agent_ids`	`Optional[List[str]]`	`None`	Specific IDs to wait for. `None` = wait for all currently spawned.

Async Usage

For async code, use the async variants with proper event-driven waiting.

import asyncio
from praisonaiagents import Agent, AgentTeam, Task

async def main():
    team = AgentTeam(agents=[], name="async_team")
    
    agent = Agent(name="Researcher", instructions="Research briefly.")
    task = Task(description="What is RAG?")

    await team.aspawn_sub_agent(agent, task)
    completions = await team.await_for_completions(timeout=60.0)
    
    print(f"Result: {completions[0].result}")

asyncio.run(main())

Best Practices

Always Set Timeouts

Prevent indefinite waiting by setting reasonable timeout values.

# Good: Set timeout to prevent hanging
completions = team.wait_for_completions(timeout=60.0)

# Bad: No timeout - may wait forever
completions = team.wait_for_completions()

Use Async Variants for Async Code

Inside async def functions, use aspawn_sub_agent and await_for_completions.

async def process_requests():
    # Good: Async variant
    await team.aspawn_sub_agent(agent, task)
    results = await team.await_for_completions(timeout=30.0)
    
    # Bad: Blocking sync calls in async context
    team.spawn_sub_agent(agent, task)
    results = team.wait_for_completions(timeout=30.0)

Keep Callbacks Fast

Completion callbacks should be lightweight to avoid blocking the event loop.

# Good: Fast processing
def quick_callback(event):
    print(f"Done: {event.agent_id}")
    results_queue.put(event.result)

# Bad: Heavy processing in callback
def slow_callback(event):
    heavy_analysis(event.result)  # Blocks other events
    expensive_db_save(event.result)

Use Metadata for Correlation

Attach metadata to track spawned sub-agents back to your domain context.

metadata = {"request_id": "req_123", "user_id": "user_456"}
spawned = team.spawn_sub_agent(agent, task, metadata=metadata)

# Later, correlate completion back to request
def on_completion(event):
    request_id = event.metadata.get("request_id")
    notify_user_of_completion(request_id, event.result)

Subagent Delegation

Core multi-agent coordination

Event Bus

Event-driven architecture

Agentic Orchestrator Worker

Multi-Agent Hooks

​Quick Start

​How It Works

​Choosing Your Pattern

​Common Patterns

​Fire-and-Forget with Callback

​Parallel Fan-out with Join

​Event-Driven Coordination

​Configuration

​spawn_sub_agent Parameters

​announce_completion Parameters

​wait_for_completions Parameters

​Async Usage

​Best Practices

​Related