Streaming response in adk

[rajib76]

Hi, I just wanted to clarify something. From what I understand, ADK appears to return responses in a streaming manner by default. If I don’t want streaming behavior, would I need to handle this in my code by checking whether the event is marked as final before displaying the response? Is my understanding correct?

[Paurakh977]

No — ADK does not stream by default. You must explicitly opt in by passing RunConfig(streaming_mode=StreamingMode.SSE) to run_async(). Without it, you only ever receive the final aggregated event and event.partial is never True.

Once SSE mode is enabled, ADK emits incremental chunks as the model generates them. The distinction between streaming and non-streaming behavior is then purely about which events you choose to act on.

---

The Two Event Types You'll See

event.partial	What it contains
True	Only the new text added since the last chunk
False	The full merged text — ADK accumulated all partials into this

ADK accumulates all partial=True chunks internally and hands you the complete merged text when partial=False arrives. You never need to concatenate manually.

Edge case: Some model backends (e.g. certain LiteLLM providers) never emit partial=True at all — they go straight to partial=False. The patterns below handle this correctly.

---

If You Don't Want Streaming — Skip partial=True, act on partial=False

saw_partial_text = False

async for event in runner.run_async(
    user_id=user_id,
    session_id=session.id,
    new_message=content,
    run_config=RunConfig(streaming_mode=StreamingMode.SSE),
):
    if not event.content:
        continue

    text = "".join(part.text for part in event.content.parts if part.text)
    if not text:
        continue

    if event.partial:
        saw_partial_text = True
        continue  # skip all incremental chunks

    # partial=False — the final merged text.
    # Guard: only print if we actually saw partials first.
    # If the model never emitted partials, this is the only event we get.
    if not saw_partial_text:
        print(text)  # model emitted no partials — this is all we have
    # else: partials were already seen/skipped — nothing to do

---

If You Do Want Streaming — Forward partial=True chunks live

saw_partial_text = False

async for event in runner.run_async(
    user_id=user_id,
    session_id=session.id,
    new_message=content,
    run_config=RunConfig(streaming_mode=StreamingMode.SSE),
):
    if not event.content:
        continue

    text = "".join(part.text for part in event.content.parts if part.text)
    if not text:
        continue

    if event.partial:
        await websocket.send_json({"type": "chunk", "text": text})  # stream each chunk live
        saw_partial_text = True
        continue

    # partial=False — final merged text arrives after all partials.
    # Fallback: if the model never emitted partials, send this as the only chunk.
    if not saw_partial_text:
        await websocket.send_json({"type": "chunk", "text": text})

await websocket.send_json({"type": "done"})

---

The Typical Event Sequence for One Turn

Event 1:  partial=True,  text="The answer is"       ← incremental chunk
Event 2:  partial=True,  text=" 42."                ← incremental chunk
Event 3:  partial=False, text="The answer is 42."   ← full merged text (ADK accumulated)
Event 4:  partial=False, text="", turn_complete=True ← turn finished signal

---

Summary

Goal	What to do
Enable streaming at all	Pass RunConfig(streaming_mode=StreamingMode.SSE) — it is not on by default
Non-streaming (full answer at once)	Skip event.partial == True; act on event.partial == False
Streaming (text as it arrives)	Forward event.partial == True chunks to your UI
Fallback for no-partial models	Check saw_partial_text before acting on the final partial=False event

The distinction between streaming and non-streaming is purely about which events you choose to act on — you always use StreamingMode.SSE.

You can have a look at some of the code examples in

https://github.dev/google/adk-python/contributing/samples/litellm_streaming/main.py

Or if you want to explore about BI-Directional live streaming you can have a look at https://github.com/google/adk-samples/blob/main/python/agents/bidi-demo/app/main.py

Hope this helps.