fix: Treat empty-but-defined text responses as a text response

Handles an edge case in structured type generation where the JSON content was previously completed but a final, empty response is sent with a STOP finish reason.

PiperOrigin-RevId: 774917500

Author: google-genai-bot

src/google/adk/models/google_llm.py

@@ -124,7 +124,7 @@ async def generate_content_async(
         if (
             llm_response.content
             and llm_response.content.parts
-            and llm_response.content.parts[0].text
+            and llm_response.content.parts[0].text is not None
         ):
           part0 = llm_response.content.parts[0]
           if part0.thought:

tests/unittests/models/test_google_llm.py

@@ -318,6 +318,108 @@ async def mock_coro():
     mock_client.aio.models.generate_content_stream.assert_called_once()
 
 
+@pytest.mark.asyncio
+async def test_generate_content_async_stream_handles_empty_text(
+    gemini_llm, llm_request
+):
+  with mock.patch.object(gemini_llm, "api_client") as mock_client:
+
+    class MockAsyncIterator:
+
+      def __init__(self, seq):
+        self._iter = iter(seq)
+
+      def __aiter__(self):
+        return self
+
+      async def __anext__(self):
+        try:
+          return next(self._iter)
+        except StopIteration:
+          raise StopAsyncIteration
+
+    response1 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=Content(
+                    role="model",
+                    parts=[Part(text="Think1", thought=True)],
+                ),
+                finish_reason=None,
+            )
+        ]
+    )
+    response2 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=Content(
+                    role="model",
+                    parts=[Part(text="", thought=True)],
+                ),
+                finish_reason=None,
+            )
+        ]
+    )
+    response3 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=Content(
+                    role="model",
+                    parts=[Part(text="Think2", thought=True)],
+                ),
+                finish_reason=None,
+            )
+        ]
+    )
+    response4 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=Content(
+                    role="model",
+                    parts=[Part.from_text(text="Answer.")],
+                ),
+                finish_reason=None,
+            )
+        ]
+    )
+    response5 = types.GenerateContentResponse(
+        candidates=[
+            types.Candidate(
+                content=Content(
+                    role="model",
+                    parts=[Part.from_text(text="")],
+                ),
+                finish_reason=types.FinishReason.STOP,
+            )
+        ]
+    )
+
+    async def mock_coro():
+      return MockAsyncIterator(
+          [response1, response2, response3, response4, response5]
+      )
+
+    mock_client.aio.models.generate_content_stream.return_value = mock_coro()
+
+    responses = [
+        resp
+        async for resp in gemini_llm.generate_content_async(
+            llm_request, stream=True
+        )
+    ]
+
+    assert len(responses) == 4
+    assert responses[0].partial is True
+    assert responses[1].partial is True
+    assert responses[2].partial is True
+    assert responses[3].partial is True
+    assert responses[4].partial is True
+    assert responses[5].content.parts[0].text == "Think1Think2"
+    assert responses[5].content.parts[0].thought is True
+    assert responses[5].content.parts[1].text == "Answer."
+    mock_client.aio.models.generate_content_stream.assert_called_once()
+
+
 @pytest.mark.asyncio
 async def test_connect(gemini_llm, llm_request):
   # Create a mock connection
@@ -619,8 +721,7 @@ def test_preprocess_request_handles_backend_specific_fields(
     expected_inline_display_name: Optional[str],
     expected_labels: Optional[str],
 ):
-  """
-  Tests that _preprocess_request correctly sanitizes fields based on the API backend.
+  """Tests that _preprocess_request correctly sanitizes fields based on the API backend.
 
   - For GEMINI_API, it should remove 'display_name' from file/inline data
     and remove 'labels' from the config.