fix(volcengine): raise on unknown reasoning_effort when drop_params=False

Addresses greptile review concern: unknown reasoning_effort values
(future or misspelled) were silently dropped, which violates LiteLLM's
drop_params semantic where False should surface unsupported params as
errors instead of no-ops.

- Raise UnsupportedParamsError when reasoning_effort is unrecognized
  AND drop_params=False AND litellm.drop_params=False.
- Continue silently dropping when drop_params=True (or global flag set).
- Add test_reasoning_effort_unknown_value covering both branches.

litellm/llms/volcengine/chat/transformation.py

@@ -1,5 +1,7 @@
 from typing import Optional, Union
 
+import litellm
+from litellm.exceptions import UnsupportedParamsError
 from litellm.llms.openai_like.chat.transformation import OpenAILikeChatConfig
 
 
@@ -103,7 +105,20 @@ class VolcEngineChatConfig(OpenAILikeChatConfig):
                 optional_params["thinking"] = {"type": "enabled"}
             elif reasoning_effort == "auto":
                 optional_params["thinking"] = {"type": "auto"}
-            # Unknown values: silently drop (super() already handles drop_params semantics)
+            else:
+                # Unknown reasoning_effort value: respect drop_params semantics.
+                # When drop_params=True (or litellm.drop_params=True): silently drop.
+                # Otherwise: raise so callers don't silently get default thinking behavior.
+                if not (drop_params or litellm.drop_params):
+                    raise UnsupportedParamsError(
+                        status_code=400,
+                        message=(
+                            f"VolcEngine does not support reasoning_effort="
+                            f"{reasoning_effort!r}. Supported values: 'none', "
+                            f"'minimal', 'low', 'medium', 'high', 'xhigh', 'auto'. "
+                            f"Pass drop_params=True to silently drop unknown values."
+                        ),
+                    )
 
         if "thinking" in optional_params:
             """

tests/test_litellm/llms/volcengine/test_volcengine.py

@@ -176,6 +176,34 @@ class TestVolcEngineConfig:
         )
         assert e2e["extra_body"]["thinking"] == {"type": "disabled"}
 
+    def test_reasoning_effort_unknown_value(self):
+        """Unknown reasoning_effort values must respect drop_params semantics."""
+        import pytest
+
+        from litellm.exceptions import UnsupportedParamsError
+
+        config = VolcEngineConfig()
+
+        # drop_params=False: raise on unknown value (not silently drop)
+        with pytest.raises(UnsupportedParamsError) as exc_info:
+            config.map_openai_params(
+                non_default_params={"reasoning_effort": "ultra"},
+                optional_params={},
+                model="doubao-seed-1.6",
+                drop_params=False,
+            )
+        assert "reasoning_effort" in str(exc_info.value)
+        assert "ultra" in str(exc_info.value)
+
+        # drop_params=True: silently drop unknown value, no thinking written
+        out = config.map_openai_params(
+            non_default_params={"reasoning_effort": "ultra"},
+            optional_params={},
+            model="doubao-seed-1.6",
+            drop_params=True,
+        )
+        assert out == {}
+
     def test_e2e_completion(self):
         from openai import OpenAI