diff --git a/litellm/completion_extras/litellm_responses_transformation/transformation.py b/litellm/completion_extras/litellm_responses_transformation/transformation.py
index 579cf83bffa..0fc4aaab7ac 100644
--- a/litellm/completion_extras/litellm_responses_transformation/transformation.py
+++ b/litellm/completion_extras/litellm_responses_transformation/transformation.py
@@ -63,6 +63,9 @@ if TYPE_CHECKING:
     from litellm.types.utils import Choices
 
 
+_REASONING_EFFORT_LEVELS: Final = frozenset({"none", "minimal", "low", "medium", "high", "xhigh", "max"})
+
+
 class _ReasoningSummaryText(TypedDict):
     type: str
     text: str
@@ -1001,23 +1004,11 @@ class LiteLLMResponsesTransformationHandler(CompletionTransformationBridge):
         )
 
         # If string is passed, map with optional summary based on flag/env var
-        if reasoning_effort == "none":
-            return Reasoning(effort="none", summary="detailed") if auto_summary_enabled else Reasoning(effort="none")
-        elif reasoning_effort == "high":
-            return Reasoning(effort="high", summary="detailed") if auto_summary_enabled else Reasoning(effort="high")
-        elif reasoning_effort == "xhigh":
-            return Reasoning(effort="xhigh", summary="detailed") if auto_summary_enabled else Reasoning(effort="xhigh")
-        elif reasoning_effort == "medium":
-            return (
-                Reasoning(effort="medium", summary="detailed") if auto_summary_enabled else Reasoning(effort="medium")
-            )
-        elif reasoning_effort == "low":
-            return Reasoning(effort="low", summary="detailed") if auto_summary_enabled else Reasoning(effort="low")
-        elif reasoning_effort == "minimal":
-            return (
-                Reasoning(effort="minimal", summary="detailed") if auto_summary_enabled else Reasoning(effort="minimal")
-            )
-        return None
+        if reasoning_effort not in _REASONING_EFFORT_LEVELS:
+            return None
+        if auto_summary_enabled:
+            return Reasoning(effort=reasoning_effort, summary="detailed")
+        return Reasoning(effort=reasoning_effort)
 
     def _add_web_search_tool(
         self,
diff --git a/tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py b/tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
index b8bd5c951ee..6cf2be98f81 100644
--- a/tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
+++ b/tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
@@ -1521,7 +1521,7 @@ def test_map_reasoning_effort_adds_summary_detailed():
     handler = LiteLLMResponsesTransformationHandler()
 
     # Test all string effort levels - DEFAULT BEHAVIOR (no summary)
-    effort_levels = ["none", "low", "medium", "high", "xhigh", "minimal"]
+    effort_levels = ["none", "minimal", "low", "medium", "high", "xhigh", "max"]
 
     # Save original flag value
     original_flag = litellm.reasoning_auto_summary
@@ -3400,3 +3400,14 @@ def test_output_item_done_with_stream_map_keeps_empty_delta():
     )
     assert chunk.choices[0].delta.tool_calls is None
     assert chunk.choices[0].finish_reason is None
+
+
+def test_map_reasoning_effort_rejects_unknown_level():
+    """An unrecognized effort must map to None so it is not forwarded upstream."""
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        LiteLLMResponsesTransformationHandler,
+    )
+
+    handler = LiteLLMResponsesTransformationHandler()
+
+    assert handler._map_reasoning_effort("banana") is None