test(test_anthropic_experimental_pass_through_messages_handler.py): add unit test confirming custom llm respected

tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py

@@ -34,3 +34,33 @@ def test_anthropic_experimental_pass_through_messages_handler():
             print(f"Error: {e}")
         mock_completion.assert_called_once()
         mock_completion.call_args.kwargs["api_key"] == "test-api-key"
+
+
+def test_anthropic_experimental_pass_through_messages_handler_custom_llm_provider():
+    """
+    Test that litellm.completion is called when a custom LLM provider is given
+    """
+    from litellm.llms.anthropic.experimental_pass_through.messages.handler import (
+        anthropic_messages_handler,
+    )
+
+    with patch("litellm.completion", return_value="test-response") as mock_completion:
+        try:
+            anthropic_messages_handler(
+                max_tokens=100,
+                messages=[{"role": "user", "content": "Hello, how are you?"}],
+                model="my-custom-model",
+                custom_llm_provider="my-custom-llm",
+                api_key="test-api-key",
+            )
+        except Exception as e:
+            print(f"Error: {e}")
+
+        # Assert that litellm.completion was called when using a custom LLM provider
+        mock_completion.assert_called_once()
+
+        # Verify that the custom provider was passed through
+        call_kwargs = mock_completion.call_args.kwargs
+        assert call_kwargs["custom_llm_provider"] == "my-custom-llm"
+        assert call_kwargs["model"] == "my-custom-llm/my-custom-model"
+        assert call_kwargs["api_key"] == "test-api-key"