diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
index 5be84a29b77..e916de34402 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_anthropic_experimental_pass_through_messages_handler.py
@@ -149,3 +149,76 @@ async def test_bedrock_converse_budget_tokens_preserved():
         assert "thinking" in additional_fields, "thinking parameter should be in additionalModelRequestFields"
         assert thinking_config.get("type") == "enabled", "thinking.type should be 'enabled'"
         assert thinking_config.get("budget_tokens") == 1024, f"thinking.budget_tokens should be 1024, but got {thinking_config.get('budget_tokens')}"
+
+
+def test_openai_model_with_thinking_converts_to_reasoning_effort():
+    """
+    Test that when using a non-Anthropic model (like OpenAI gpt-5.2) with thinking parameter,
+    the thinking is converted to reasoning_effort and NOT passed as thinking.
+    
+    This ensures we don't regress on issue #16052 where non-Anthropic models would fail
+    with UnsupportedParamsError when thinking was passed directly.
+    """
+    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=1024,
+                messages=[{"role": "user", "content": "What is 2+2?"}],
+                model="openai/gpt-5.2",
+                api_key="test-api-key",
+                thinking={
+                    "type": "enabled",
+                    "budget_tokens": 1024
+                },
+            )
+        except Exception as e:
+            print(f"Error: {e}")
+
+        mock_completion.assert_called_once()
+        
+        call_kwargs = mock_completion.call_args.kwargs
+        
+        # Verify reasoning_effort is set (converted from thinking)
+        assert "reasoning_effort" in call_kwargs, "reasoning_effort should be passed to completion"
+        assert call_kwargs["reasoning_effort"] == "minimal", f"reasoning_effort should be 'minimal' for budget_tokens=1024, got {call_kwargs.get('reasoning_effort')}"
+        
+        # Verify thinking is NOT passed (non-Claude model)
+        assert "thinking" not in call_kwargs, "thinking should NOT be passed for non-Claude models"
+
+
+class TestThinkingParameterTransformation:
+    """Core tests for thinking parameter transformation logic."""
+
+    def test_claude_model_preserves_thinking_with_budget_tokens(self):
+        """Test that Claude models get thinking parameter passed through with exact budget_tokens."""
+        from litellm.llms.anthropic.experimental_pass_through.adapters.transformation import (
+            LiteLLMAnthropicMessagesAdapter,
+        )
+        
+        thinking = {"type": "enabled", "budget_tokens": 5000}
+        result = LiteLLMAnthropicMessagesAdapter.translate_thinking_for_model(
+            thinking=thinking,
+            model="bedrock/converse/us.anthropic.claude-sonnet-4-20250514-v1:0",
+        )
+        
+        assert result == {"thinking": thinking}
+        assert result["thinking"]["budget_tokens"] == 5000
+
+    def test_non_claude_model_converts_thinking_to_reasoning_effort(self):
+        """Test that non-Claude models convert thinking to reasoning_effort."""
+        from litellm.llms.anthropic.experimental_pass_through.adapters.transformation import (
+            LiteLLMAnthropicMessagesAdapter,
+        )
+        
+        thinking = {"type": "enabled", "budget_tokens": 1024}
+        result = LiteLLMAnthropicMessagesAdapter.translate_thinking_for_model(
+            thinking=thinking,
+            model="openai/gpt-5.2",
+        )
+        
+        assert result == {"reasoning_effort": "minimal"}
+        assert "thinking" not in result