fix(live): mark all agents' Event as from other agents

This will allow agent transfer to work smoothly in live workflow. See the comment added for details.

Closes issue #5238

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 900993889

Author: wuliang229

src/google/adk/flows/llm_flows/contents.py

@@ -572,6 +572,28 @@ def _get_current_turn_contents(
 
 def _is_other_agent_reply(current_agent_name: str, event: Event) -> bool:
   """Whether the event is a reply from another agent."""
+  # In live/bidi mode, all events from any agents, including the current
+  # agent, will be marked as other agent's reply. When agent transfers,
+  # the conversation history will be sent to the Live API. If the current
+  # agent previously used `transfer_to_agent` to transfer to another agent,
+  # when the conversation is sent back to the current agent, the history will
+  # contain a `transfer_to_agent` function call event from the current agent.
+  # The Live API marks anything after the function response as model response.
+  # This will confuse the model and cause the model to not respond.
+  #
+  # E.g. when the conversation is transferred from agent A to agent B, then
+  # back to agent A, the history in the last transfer will be:
+  #   User: "Some message that triggers transfer to agent B"
+  #   Model: transfer_to_agent(B)
+  #   User: transfer_to_agent(B) response
+  #   User: "Some message that triggers transfer to agent A"
+  #   User: "For context: [agent B] called transfer_to_agent(A)"
+  #   User: "For context: [agent B] tool transfer_to_agent(A) returned result:"
+  #
+  # In this case, the last three events are marked as model response by the
+  # Live API, instead of user input.
+  if event.live_session_id:
+    return event.author != 'user'
   return bool(
       current_agent_name
       and event.author != current_agent_name

tests/unittests/flows/llm_flows/test_contents.py

@@ -1068,3 +1068,30 @@ async def test_adk_function_call_ids_preserved_for_interactions_model():
   user_fr_part = llm_request.contents[2].parts[0]
   assert user_fr_part.function_response is not None
   assert user_fr_part.function_response.id == function_call_id
+
+
+def test_is_other_agent_reply_live_session():
+  """Test _is_other_agent_reply when live_session_id is present."""
+  event = Event(author="another_agent", live_session_id="session_123")
+  assert contents._is_other_agent_reply("current_agent", event) is True
+
+  event = Event(author="user", live_session_id="session_123")
+  assert contents._is_other_agent_reply("current_agent", event) is False
+
+  event = Event(author="current_agent", live_session_id="session_123")
+  assert contents._is_other_agent_reply("current_agent", event) is True
+
+
+def test_is_other_agent_reply_non_live_session():
+  """Test _is_other_agent_reply when live_session_id is not present."""
+  event = Event(author="another_agent")
+  assert contents._is_other_agent_reply("current_agent", event) is True
+
+  event = Event(author="user")
+  assert contents._is_other_agent_reply("current_agent", event) is False
+
+  event = Event(author="current_agent")
+  assert contents._is_other_agent_reply("current_agent", event) is False
+
+  event = Event(author="another_agent")
+  assert contents._is_other_agent_reply("", event) is False