refactor: Extract a utility for aggregating partial streaming responses and emitting LlmResponses for them as needed

src/google/adk/models/gemini_llm_connection.py

@@ -144,6 +144,8 @@ async def receive(self) -> AsyncGenerator[LlmResponse, None]:
 
     text = ''
     async with Aclosing(self._gemini_session.receive()) as agen:
+      # TODO(b/440101573): Reuse StreamingResponseAggregator to accumulate
+      # partial content and emit responses as needed.
       async for message in agen:
         logger.debug('Got LLM Live message: %s', message)
         if message.server_content:

src/google/adk/models/google_llm.py

@@ -33,6 +33,7 @@
 
 from .. import version
 from ..utils.context_utils import Aclosing
+from ..utils.streaming_utils import StreamingResponseAggregator
 from ..utils.variant_utils import GoogleLLMVariant
 from .base_llm import BaseLlm
 from .base_llm_connection import BaseLlmConnection
@@ -133,68 +134,23 @@ async def generate_content_async(
           contents=llm_request.contents,
           config=llm_request.config,
       )
-      response = None
-      thought_text = ''
-      text = ''
-      usage_metadata = None
+
       # for sse, similar as bidi (see receive method in gemini_llm_connecton.py),
       # we need to mark those text content as partial and after all partial
       # contents are sent, we send an accumulated event which contains all the
       # previous partial content. The only difference is bidi rely on
       # complete_turn flag to detect end while sse depends on finish_reason.
+      aggregator = StreamingResponseAggregator()
       async with Aclosing(responses) as agen:
         async for response in agen:
           logger.debug(_build_response_log(response))
-          llm_response = LlmResponse.create(response)
-          usage_metadata = llm_response.usage_metadata
-          if (
-              llm_response.content
-              and llm_response.content.parts
-              and llm_response.content.parts[0].text
-          ):
-            part0 = llm_response.content.parts[0]
-            if part0.thought:
-              thought_text += part0.text
-            else:
-              text += part0.text
-            llm_response.partial = True
-          elif (thought_text or text) and (
-              not llm_response.content
-              or not llm_response.content.parts
-              # don't yield the merged text event when receiving audio data
-              or not llm_response.content.parts[0].inline_data
-          ):
-            parts = []
-            if thought_text:
-              parts.append(types.Part(text=thought_text, thought=True))
-            if text:
-              parts.append(types.Part.from_text(text=text))
-            yield LlmResponse(
-                content=types.ModelContent(parts=parts),
-                usage_metadata=llm_response.usage_metadata,
-            )
-            thought_text = ''
-            text = ''
-          yield llm_response
-
-      # 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))
-        if text:
-          parts.append(types.Part.from_text(text=text))
-        yield LlmResponse(
-            content=types.ModelContent(parts=parts),
-            error_code=None
-            if response.candidates[0].finish_reason == FinishReason.STOP
-            else response.candidates[0].finish_reason,
-            error_message=None
-            if response.candidates[0].finish_reason == FinishReason.STOP
-            else response.candidates[0].finish_message,
-            usage_metadata=usage_metadata,
-        )
+          async with Aclosing(
+              aggregator.process_response(response)
+          ) as aggregator_gen:
+            async for llm_response in aggregator_gen:
+              yield llm_response
+      if (close_result := aggregator.close()) is not None:
+        yield close_result
 
     else:
       response = await self.api_client.aio.models.generate_content(

src/google/adk/utils/streaming_utils.py

@@ -0,0 +1,112 @@
+# 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 __future__ import annotations
+
+from typing import AsyncGenerator
+from typing import Optional
+
+from google.genai import types
+
+from ..models.llm_response import LlmResponse
+
+
+class StreamingResponseAggregator:
+  """Aggregates partial streaming responses.
+
+  It aggregates content from partial responses, and generates LlmResponses for
+  individual (partial) model responses, as well as for aggregated content.
+  """
+
+  def __init__(self):
+    self._text = ''
+    self._thought_text = ''
+    self._usage_metadata = None
+    self._response = None
+
+  async def process_response(
+      self, response: types.GenerateContentResponse
+  ) -> AsyncGenerator[LlmResponse, None]:
+    """Processes a single model response.
+
+    Args:
+      response: The response to process.
+
+    Yields:
+      The generated LlmResponse(s), for the partial response, and the aggregated
+      response if needed.
+    """
+    # results = []
+    self._response = response
+    llm_response = LlmResponse.create(response)
+    self._usage_metadata = llm_response.usage_metadata
+    if (
+        llm_response.content
+        and llm_response.content.parts
+        and llm_response.content.parts[0].text
+    ):
+      part0 = llm_response.content.parts[0]
+      if part0.thought:
+        self._thought_text += part0.text
+      else:
+        self._text += part0.text
+      llm_response.partial = True
+    elif (self._thought_text or self._text) and (
+        not llm_response.content
+        or not llm_response.content.parts
+        # don't yield the merged text event when receiving audio data
+        or not llm_response.content.parts[0].inline_data
+    ):
+      parts = []
+      if self._thought_text:
+        parts.append(types.Part(text=self._thought_text, thought=True))
+      if self._text:
+        parts.append(types.Part.from_text(text=self._text))
+      yield LlmResponse(
+          content=types.ModelContent(parts=parts),
+          usage_metadata=llm_response.usage_metadata,
+      )
+      self._thought_text = ''
+      self._text = ''
+    yield llm_response
+
+  def close(self) -> Optional[LlmResponse]:
+    """Generate an aggregated response at the end, if needed.
+
+    This should be called after all the model responses are processed.
+
+    Returns:
+      The aggregated LlmResponse.
+    """
+    if (
+        (self._text or self._thought_text)
+        and self._response
+        and self._response.candidates
+    ):
+      parts = []
+      if self._thought_text:
+        parts.append(types.Part(text=self._thought_text, thought=True))
+      if self._text:
+        parts.append(types.Part.from_text(text=self._text))
+      candidate = self._response.candidates[0]
+      return LlmResponse(
+          content=types.ModelContent(parts=parts),
+          error_code=None
+          if candidate.finish_reason == types.FinishReason.STOP
+          else candidate.finish_reason,
+          error_message=None
+          if candidate.finish_reason == types.FinishReason.STOP
+          else candidate.finish_message,
+          usage_metadata=self._usage_metadata,
+      )

tests/unittests/utils/test_streaming_utils.py

@@ -0,0 +1,181 @@
+# 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 __future__ import annotations
+
+from google.adk.utils import streaming_utils
+from google.genai import types
+import pytest
+
+
+class TestStreamingResponseAggregator:
+
+  @pytest.mark.asyncio
+  async def test_process_response_with_text(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[types.Part(text="Hello")])
+            )
+        ]
+    )
+    results = []
+    async for r in aggregator.process_response(response):
+      results.append(r)
+    assert len(results) == 1
+    assert results[0].content.parts[0].text == "Hello"
+    assert results[0].partial
+
+  @pytest.mark.asyncio
+  async def test_process_response_with_thought(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(
+                    parts=[types.Part(text="Thinking...", thought=True)]
+                )
+            )
+        ]
+    )
+    results = []
+    async for r in aggregator.process_response(response):
+      results.append(r)
+    assert len(results) == 1
+    assert results[0].content.parts[0].text == "Thinking..."
+    assert results[0].content.parts[0].thought
+    assert results[0].partial
+
+  @pytest.mark.asyncio
+  async def test_process_response_multiple(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response1 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[types.Part(text="Hello ")])
+            )
+        ]
+    )
+    response2 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[types.Part(text="World!")])
+            )
+        ]
+    )
+    async for _ in aggregator.process_response(response1):
+      pass
+    results = []
+    async for r in aggregator.process_response(response2):
+      results.append(r)
+    assert len(results) == 1
+    assert results[0].content.parts[0].text == "World!"
+
+    closed_response = aggregator.close()
+    assert closed_response is not None
+    assert closed_response.content.parts[0].text == "Hello World!"
+
+  @pytest.mark.asyncio
+  async def test_process_response_interleaved_thought_and_text(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response1 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(
+                    parts=[types.Part(text="I am thinking...", thought=True)]
+                )
+            )
+        ]
+    )
+    response2 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(
+                    parts=[types.Part(text="Okay, I have a result.")]
+                )
+            )
+        ]
+    )
+    response3 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(
+                    parts=[types.Part(text=" The result is 42.")]
+                )
+            )
+        ]
+    )
+
+    async for _ in aggregator.process_response(response1):
+      pass
+    async for _ in aggregator.process_response(response2):
+      pass
+    async for _ in aggregator.process_response(response3):
+      pass
+
+    closed_response = aggregator.close()
+    assert closed_response is not None
+    assert len(closed_response.content.parts) == 2
+    assert closed_response.content.parts[0].text == "I am thinking..."
+    assert closed_response.content.parts[0].thought
+    assert (
+        closed_response.content.parts[1].text
+        == "Okay, I have a result. The result is 42."
+    )
+    assert not closed_response.content.parts[1].thought
+
+  def test_close_with_no_responses(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    closed_response = aggregator.close()
+    assert closed_response is None
+
+  @pytest.mark.asyncio
+  async def test_close_with_finish_reason(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[types.Part(text="Hello")]),
+                finish_reason=types.FinishReason.STOP,
+            )
+        ]
+    )
+    async for _ in aggregator.process_response(response):
+      pass
+    closed_response = aggregator.close()
+    assert closed_response is not None
+    assert closed_response.content.parts[0].text == "Hello"
+    assert closed_response.error_code is None
+    assert closed_response.error_message is None
+
+  @pytest.mark.asyncio
+  async def test_close_with_error(self):
+    aggregator = streaming_utils.StreamingResponseAggregator()
+    response = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=types.Content(parts=[types.Part(text="Error")]),
+                finish_reason=types.FinishReason.RECITATION,
+                finish_message="Recitation error",
+            )
+        ]
+    )
+    async for _ in aggregator.process_response(response):
+      pass
+    closed_response = aggregator.close()
+    assert closed_response is not None
+    assert closed_response.content.parts[0].text == "Error"
+    assert closed_response.error_code == types.FinishReason.RECITATION
+    assert closed_response.error_message == "Recitation error"