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All agents stream by default — astream() yields Event objects as they occur: 
from langchain_adk.events.event import Event, EventType

async for event in agent.astream("Write about distributed systems"):
    if event.type == EventType.AGENT_MESSAGE and event.partial:
        # Token-by-token streaming
        print(event.text, end="", flush=True)
    elif event.is_final_response():
        print(f"\n[DONE] {event.text}")
Partial events are suppressed automatically if the LLM decides to call a tool instead of answering — only real text chunks are streamed.

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Sub-agent streaming via AgentTool

When using AgentTool, sub-agent events stream through the parent in real-time via ctx.event_callback. Events carry branch and agent_name fields so you can distinguish which sub-agent is producing output: 
from langchain_adk import LlmAgent
from langchain_adk.tools.agent_tool import AgentTool

weather_agent = LlmAgent(name="WeatherAgent", llm=llm, tools=[get_weather])
travel_agent = LlmAgent(name="TravelAgent", llm=llm, tools=[get_attractions])

planner = LlmAgent(
    name="TripPlanner",
    llm=llm,
    tools=[AgentTool(weather_agent), AgentTool(travel_agent)],
    instructions="Use the sub-agents to plan a trip.",
)

async for event in planner.astream("Plan a trip to Copenhagen"):
    if event.branch:
        # Event from a sub-agent (e.g. branch="WeatherAgent")
        print(f"  [{event.agent_name}] {event.text}", end="")
    elif event.is_final_response():
        print(f"\nFinal: {event.text}")
Sub-agents run in parallel when the LLM calls multiple tools at once — their events are interleaved in the stream as they arrive.

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How it works

  1. Before tool execution, LlmAgent creates an asyncio.Queue and sets ctx.event_callback = queue.put_nowait on the tool context.
  2. AgentTool calls ctx.event_callback(event) for each child event as it streams.
  3. LlmAgent yields events from the queue concurrently while tools run, so events appear in real-time.
  4. The event_callback propagates through ctx.derive(), so nested sub-agents also push events up to the root.
Any custom tool can use ctx.event_callback to push events — it’s not limited to AgentTool.

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With Runner

async for event in runner.astream(
    user_id="user-1",
    session_id="session-1",
    new_message="Write me a long essay about distributed systems.",
):
    if event.is_final_response():
        print(f"\n[DONE] {event.text}")
    elif event.type == EventType.AGENT_MESSAGE and event.partial:
        print(event.text, end="", flush=True)