fix: Remove newline when combining reasoning in litellm

Author: lorenzbaraldi

src/google/adk/models/lite_llm.py

@@ -912,7 +912,9 @@ async def _content_to_message_param(
       ):
         reasoning_texts.append(_decode_inline_text_data(part.inline_data.data))
 
-    reasoning_content = _NEW_LINE.join(text for text in reasoning_texts if text)
+    # Preserve reasoning deltas exactly as received. Injecting separators
+    # between fragments can corrupt provider-streamed thinking text.
+    reasoning_content = "".join(text for text in reasoning_texts if text)
     return ChatCompletionAssistantMessage(
         role=role,
         content=final_content,

tests/unittests/models/test_litellm.py

@@ -2127,6 +2127,38 @@ async def test_content_to_message_param_assistant_thought_and_content_message():
   assert message["reasoning_content"] == "internal reasoning"
 
 
+@pytest.mark.asyncio
+async def test_content_to_message_param_preserves_chunked_reasoning_deltas():
+    thought_part_1 = types.Part.from_text(text="Hel")
+    thought_part_1.thought = True
+    thought_part_2 = types.Part.from_text(text="lo")
+    thought_part_2.thought = True
+    content = types.Content(
+            role="assistant", parts=[thought_part_1, thought_part_2]
+    )
+
+    message = await _content_to_message_param(content)
+
+    assert message["role"] == "assistant"
+    assert message["content"] is None
+    assert message["reasoning_content"] == "Hello"
+
+
+@pytest.mark.asyncio
+async def test_content_to_message_param_preserves_reasoning_newlines():
+    thought_part_1 = types.Part.from_text(text="line 1\n")
+    thought_part_1.thought = True
+    thought_part_2 = types.Part.from_text(text="line 2")
+    thought_part_2.thought = True
+    content = types.Content(
+            role="assistant", parts=[thought_part_1, thought_part_2]
+    )
+
+    message = await _content_to_message_param(content)
+
+    assert message["reasoning_content"] == "line 1\nline 2"
+
+
 @pytest.mark.asyncio
 async def test_content_to_message_param_function_call():
   content = types.Content(