fix: async streaming chunk indices in GoogleGenAIChatGenerator start at 0, not 1 (#3410)

Author: abhay23-AI

integrations/google_genai/src/haystack_integrations/components/generators/google_genai/chat/chat_generator.py

@@ -361,15 +361,14 @@ async def _handle_streaming_response_async(
             i = 0
             chunk = None
             async for chunk in response_stream:
-                i += 1
-
                 streaming_chunk = _convert_google_chunk_to_streaming_chunk(
                     chunk=chunk, index=i, component_info=component_info, model=self._model
                 )
                 chunks.append(streaming_chunk)
 
                 # Stream the chunk
                 await streaming_callback(streaming_chunk)
+                i += 1
 
             # Use custom aggregation that supports reasoning content
             message = _aggregate_streaming_chunks_with_reasoning(chunks)

integrations/google_genai/tests/test_chat_generator.py

@@ -501,6 +501,30 @@ async def callback(c):
         assert "Hello async" in results["replies"][0].text
         assert len(callback_chunks) == 1
 
+    @pytest.mark.asyncio
+    async def test_run_async_streaming_chunk_indices_start_at_zero(self, monkeypatch, mock_streaming_chunk):
+        monkeypatch.setenv("GOOGLE_API_KEY", "test-api-key")
+        component = GoogleGenAIChatGenerator()
+
+        chunks = [mock_streaming_chunk(text="Hello "), mock_streaming_chunk(text="world", finish_reason="STOP")]
+
+        async def mock_stream():
+            for chunk in chunks:
+                yield chunk
+
+        component._client.aio.models.generate_content_stream = AsyncMock(return_value=mock_stream())
+
+        callback_chunks = []
+
+        async def callback(c):
+            callback_chunks.append(c)
+
+        await component.run_async([ChatMessage.from_user("Hi")], streaming_callback=callback)
+
+        # Async indices must match the sync path (0-based); the first text chunk must mark the stream start
+        assert [c.index for c in callback_chunks] == [0, 1]
+        assert callback_chunks[0].start is True
+
     @pytest.mark.asyncio
     async def test_run_async_extracts_system_message(self, monkeypatch, mock_response):
         monkeypatch.setenv("GOOGLE_API_KEY", "test-api-key")