fix: #3065 LiteLLM Model Override Not Working with llm_request.model

src/google/adk/models/lite_llm.py

@@ -818,7 +818,7 @@ async def generate_content_async(
       tools = None
 
     completion_args = {
-        "model": self.model,
+        "model": llm_request.model or self.model,
         "messages": messages,
         "tools": tools,
         "response_format": response_format,

tests/unittests/models/test_litellm.py

@@ -548,6 +548,73 @@ async def test_generate_content_async(mock_acompletion, lite_llm_instance):
   )
 
 
+@pytest.mark.asyncio
+async def test_generate_content_async_with_model_override(
+    mock_acompletion, lite_llm_instance
+):
+  llm_request = LlmRequest(
+      model="overridden_model",
+      contents=[
+          types.Content(
+              role="user", parts=[types.Part.from_text(text="Test prompt")]
+          )
+      ],
+      config=types.GenerateContentConfig(
+          tools=[
+              types.Tool(
+                  function_declarations=[
+                      types.FunctionDeclaration(
+                          name="test_function",
+                          description="Test function description",
+                          parameters=types.Schema(
+                              type=types.Type.OBJECT,
+                              properties={
+                                  "test_arg": types.Schema(
+                                      type=types.Type.STRING
+                                  )
+                              },
+                          ),
+                      )
+                  ]
+              )
+          ],
+      ),
+  )
+
+  async for response in lite_llm_instance.generate_content_async(llm_request):
+    assert response.content.role == "model"
+    assert response.content.parts[0].text == "Test response"
+
+  mock_acompletion.assert_called_once()
+
+  _, kwargs = mock_acompletion.call_args
+  assert kwargs["model"] == "overridden_model"
+  assert kwargs["messages"][0]["role"] == "user"
+  assert kwargs["messages"][0]["content"] == "Test prompt"
+
+
+@pytest.mark.asyncio
+async def test_generate_content_async_without_model_override(
+    mock_acompletion, lite_llm_instance
+):
+  llm_request = LlmRequest(
+      model=None,
+      contents=[
+          types.Content(
+              role="user", parts=[types.Part.from_text(text="Test prompt")]
+          )
+      ],
+  )
+
+  async for response in lite_llm_instance.generate_content_async(llm_request):
+    assert response.content.role == "model"
+
+  mock_acompletion.assert_called_once()
+
+  _, kwargs = mock_acompletion.call_args
+  assert kwargs["model"] == "test_model"
+
+
 litellm_append_user_content_test_cases = [
     pytest.param(
         LlmRequest(