fix(deepseek): pass reasoning_effort value through to DeepSeek V4

DeepSeek V4 Pro/Flash support reasoning_effort as a native parameter
with values "high" and "max". The current code collapses all effort
values to {"thinking": {"type": "enabled"}}, discarding the actual
effort level.

Pass reasoning_effort through (with normalization: low/medium→high,
xhigh→max) while preserving backward-compatible thinking enablement.

Fixes #27439

Signed-off-by: Jay <moonandstar99@yahoo.com>

litellm/llms/deepseek/chat/transformation.py

@@ -56,9 +56,17 @@ class DeepSeekChatConfig(OpenAIGPTConfig):
                 # DeepSeek only accepts {"type": "enabled"}, ignore budget_tokens
                 optional_params["thinking"] = {"type": "enabled"}
 
-        # Handle reasoning_effort - map to thinking enabled
+        # Handle reasoning_effort - enable thinking and pass effort level
+        # DeepSeek V4 Pro/Flash support reasoning_effort as a native param
+        # with values "high" and "max". Older models ignore it gracefully.
         elif reasoning_effort is not None and reasoning_effort != "none":
             optional_params["thinking"] = {"type": "enabled"}
+            # Normalize per DeepSeek V4 compatibility mappings
+            if reasoning_effort in ("low", "medium"):
+                reasoning_effort = "high"
+            elif reasoning_effort == "xhigh":
+                reasoning_effort = "max"
+            optional_params["reasoning_effort"] = reasoning_effort
 
         return optional_params
 

tests/litellm/llms/deepseek/chat/test_deepseek_chat_transformation.py

@@ -52,7 +52,7 @@ class TestDeepSeekThinkingParams:
         assert "budget_tokens" not in result.get("thinking", {})
 
     def test_map_reasoning_effort_medium(self):
-        """Test that reasoning_effort='medium' maps to thinking enabled."""
+        """Test that reasoning_effort='medium' enables thinking and normalizes to 'high'."""
         non_default_params = {"reasoning_effort": "medium"}
         optional_params = {}
 
@@ -64,9 +64,10 @@ class TestDeepSeekThinkingParams:
         )
 
         assert result["thinking"] == {"type": "enabled"}
+        assert result["reasoning_effort"] == "high"
 
     def test_map_reasoning_effort_low(self):
-        """Test that reasoning_effort='low' maps to thinking enabled."""
+        """Test that reasoning_effort='low' enables thinking and normalizes to 'high'."""
         non_default_params = {"reasoning_effort": "low"}
         optional_params = {}
 
@@ -78,9 +79,10 @@ class TestDeepSeekThinkingParams:
         )
 
         assert result["thinking"] == {"type": "enabled"}
+        assert result["reasoning_effort"] == "high"
 
     def test_map_reasoning_effort_high(self):
-        """Test that reasoning_effort='high' maps to thinking enabled."""
+        """Test that reasoning_effort='high' is passed through to DeepSeek V4."""
         non_default_params = {"reasoning_effort": "high"}
         optional_params = {}
 
@@ -92,6 +94,37 @@ class TestDeepSeekThinkingParams:
         )
 
         assert result["thinking"] == {"type": "enabled"}
+        assert result["reasoning_effort"] == "high"
+
+    def test_map_reasoning_effort_max(self):
+        """Test that reasoning_effort='max' is passed through to DeepSeek V4."""
+        non_default_params = {"reasoning_effort": "max"}
+        optional_params = {}
+
+        result = self.config.map_openai_params(
+            non_default_params=non_default_params,
+            optional_params=optional_params,
+            model=self.model,
+            drop_params=False,
+        )
+
+        assert result["thinking"] == {"type": "enabled"}
+        assert result["reasoning_effort"] == "max"
+
+    def test_map_reasoning_effort_xhigh_normalizes_to_max(self):
+        """Test that reasoning_effort='xhigh' normalizes to 'max' per DeepSeek V4 compat."""
+        non_default_params = {"reasoning_effort": "xhigh"}
+        optional_params = {}
+
+        result = self.config.map_openai_params(
+            non_default_params=non_default_params,
+            optional_params=optional_params,
+            model=self.model,
+            drop_params=False,
+        )
+
+        assert result["thinking"] == {"type": "enabled"}
+        assert result["reasoning_effort"] == "max"
 
     def test_map_reasoning_effort_none_does_not_enable_thinking(self):
         """Test that reasoning_effort='none' does not enable thinking."""