fix: improve partial event handling and streaming aggregation

- Allow run_async to break on partial events instead of raising ValueError
- Generate aggregated streaming content regardless of finish_reason
- Add error_code and error_message to final streaming responses if model response is interrupted.

PiperOrigin-RevId: 781377328

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

@@ -283,14 +283,10 @@ class BaseLlmFlow(ABC):
       async for event in self._run_one_step_async(invocation_context):
         last_event = event
         yield event
-      if not last_event or last_event.is_final_response():
+      if not last_event or last_event.is_final_response() or last_event.partial:
+        if last_event and last_event.partial:
+          logger.warning('The last event is partial, which is not expected.')
         break
-      if last_event.partial:
-        # TODO: handle this in BaseLlm level.
-        raise ValueError(
-            f"Last event shouldn't be partial. LLM max output limit may be"
-            f' reached.'
-        )
 
   async def _run_one_step_async(
       self,

src/google/adk/models/google_llm.py

@@ -150,12 +150,10 @@ class Gemini(BaseLlm):
           thought_text = ''
           text = ''
         yield llm_response
-      if (
-          (text or thought_text)
-          and response
-          and response.candidates
-          and response.candidates[0].finish_reason == types.FinishReason.STOP
-      ):
+
+      # generate an aggregated content at the end regardless the
+      # response.candidates[0].finish_reason
+      if (text or thought_text) and response and response.candidates:
         parts = []
         if thought_text:
           parts.append(types.Part(text=thought_text, thought=True))
@@ -163,6 +161,8 @@ class Gemini(BaseLlm):
           parts.append(types.Part.from_text(text=text))
         yield LlmResponse(
             content=types.ModelContent(parts=parts),
+            error_code=response.candidates[0].finish_reason,
+            error_message=response.candidates[0].finish_message,
             usage_metadata=usage_metadata,
         )
 

tests/unittests/flows/llm_flows/test_base_llm_flow_partial_handling.py

@@ -0,0 +1,164 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from google.adk.agents import Agent
+from google.adk.flows.llm_flows.base_llm_flow import BaseLlmFlow
+from google.adk.models.llm_response import LlmResponse
+from google.genai import types
+import pytest
+
+from ... import testing_utils
+
+
+class BaseLlmFlowForTesting(BaseLlmFlow):
+  """Test implementation of BaseLlmFlow for testing purposes."""
+
+  pass
+
+
+@pytest.mark.asyncio
+async def test_run_async_breaks_on_partial_event():
+  """Test that run_async breaks when the last event is partial."""
+  # Create a mock model that returns partial responses
+  partial_response = LlmResponse(
+      content=types.Content(
+          role='model', parts=[types.Part.from_text(text='Partial response')]
+      ),
+      partial=True,
+  )
+
+  mock_model = testing_utils.MockModel.create(responses=[partial_response])
+
+  agent = Agent(name='test_agent', model=mock_model)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+
+  flow = BaseLlmFlowForTesting()
+  events = []
+
+  # Collect events from the flow
+  async for event in flow.run_async(invocation_context):
+    events.append(event)
+
+  # Should have one event (the partial response)
+  assert len(events) == 1
+  assert events[0].partial is True
+  assert events[0].content.parts[0].text == 'Partial response'
+
+
+@pytest.mark.asyncio
+async def test_run_async_breaks_on_final_response():
+  """Test that run_async breaks when the last event is a final response."""
+  # Create a mock model that returns a final response
+  final_response = LlmResponse(
+      content=types.Content(
+          role='model', parts=[types.Part.from_text(text='Final response')]
+      ),
+      partial=False,
+      error_code=types.FinishReason.STOP,
+  )
+
+  mock_model = testing_utils.MockModel.create(responses=[final_response])
+
+  agent = Agent(name='test_agent', model=mock_model)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+
+  flow = BaseLlmFlowForTesting()
+  events = []
+
+  # Collect events from the flow
+  async for event in flow.run_async(invocation_context):
+    events.append(event)
+
+  # Should have one event (the final response)
+  assert len(events) == 1
+  assert events[0].partial is False
+  assert events[0].content.parts[0].text == 'Final response'
+
+
+@pytest.mark.asyncio
+async def test_run_async_breaks_on_no_last_event():
+  """Test that run_async breaks when there is no last event."""
+  # Create a mock model that returns an empty response (no content)
+  empty_response = LlmResponse(content=None, partial=False)
+
+  mock_model = testing_utils.MockModel.create(responses=[empty_response])
+
+  agent = Agent(name='test_agent', model=mock_model)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+
+  flow = BaseLlmFlowForTesting()
+  events = []
+
+  # Collect events from the flow
+  async for event in flow.run_async(invocation_context):
+    events.append(event)
+
+  # Should have no events because empty responses are filtered out
+  assert len(events) == 0
+
+
+@pytest.mark.asyncio
+async def test_run_async_breaks_on_first_partial_response():
+  """Test run_async breaks on the first partial response."""
+  # Create responses with mixed partial states
+  partial_response = LlmResponse(
+      content=types.Content(
+          role='model', parts=[types.Part.from_text(text='Partial response')]
+      ),
+      partial=True,
+  )
+
+  # These won't be reached because the flow breaks on the first partial
+  non_partial_response = LlmResponse(
+      content=types.Content(
+          role='model',
+          parts=[types.Part.from_text(text='Non-partial response')],
+      ),
+      partial=False,
+  )
+
+  final_partial_response = LlmResponse(
+      content=types.Content(
+          role='model',
+          parts=[types.Part.from_text(text='Final partial response')],
+      ),
+      partial=True,
+  )
+
+  mock_model = testing_utils.MockModel.create(
+      responses=[partial_response, non_partial_response, final_partial_response]
+  )
+
+  agent = Agent(name='test_agent', model=mock_model)
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent, user_content='test message'
+  )
+
+  flow = BaseLlmFlowForTesting()
+  events = []
+
+  # Collect events from the flow
+  async for event in flow.run_async(invocation_context):
+    events.append(event)
+
+  # Should have only one event, breaking on the first partial response
+  assert len(events) == 1
+  assert events[0].partial is True
+  assert events[0].content.parts[0].text == 'Partial response'