fix(deepseek): preserve native reasoning effort for V4 models

litellm/llms/deepseek/chat/transformation.py

@@ -59,6 +59,14 @@ class DeepSeekChatConfig(OpenAIGPTConfig):
         elif reasoning_effort is not None:
             optional_params["thinking"] = {"type": "disabled" if reasoning_effort == "none" else "enabled"}
 
+        if (
+            model.removeprefix("deepseek/").startswith(("deepseek-v4-", "deepseek-flash"))
+            and optional_params.get("thinking") == {"type": "enabled"}
+            and isinstance(reasoning_effort, str)
+            and reasoning_effort != "none"
+        ):
+            return {**optional_params, "reasoning_effort": reasoning_effort}
+
         return optional_params
 
     def _fill_reasoning_content(self, messages: list[AllMessageValues]) -> list[AllMessageValues]:

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

@@ -1,7 +1,47 @@
+from typing import Final
+
+import pytest
+
 import litellm
 from litellm.llms.deepseek.chat.transformation import DeepSeekChatConfig
 
 
+@pytest.mark.parametrize(
+    "model", ("deepseek-flash", "deepseek-v4-pro", "deepseek-v4-flash-vision-exp", "deepseek/deepseek-v4-flash")
+)
+@pytest.mark.parametrize("effort", ("minimal", "low", "medium", "high", "xhigh", "max", "ultra"))
+@pytest.mark.parametrize("thinking", (None, "enabled"))
+def test_v4_keeps_requested_effort_with_enabled_thinking(model: str, effort: str, thinking: str | None) -> None:
+    params: Final = {"reasoning_effort": effort, **({"thinking": {"type": thinking}} if thinking else {})}
+    result: Final = DeepSeekChatConfig().map_openai_params(params, {}, model, False)
+    assert result == {"thinking": {"type": "enabled"}, "reasoning_effort": effort}
+
+
+@pytest.mark.parametrize(
+    "thinking,effort,expected",
+    (
+        (None, None, {}),
+        (None, "none", {"thinking": {"type": "disabled"}}),
+        ("disabled", "low", {"thinking": {"type": "disabled"}}),
+        ("disabled", "max", {"thinking": {"type": "disabled"}}),
+        ("enabled", "none", {"thinking": {"type": "enabled"}}),
+        ("enabled", None, {"thinking": {"type": "enabled"}}),
+    ),
+)
+def test_v4_retains_explicit_thinking_precedence(
+    thinking: str | None, effort: str | None, expected: dict[str, dict[str, str]]
+) -> None:
+    params: Final = {"reasoning_effort": effort, **({"thinking": {"type": thinking}} if thinking else {})}
+    result: Final = DeepSeekChatConfig().map_openai_params(params, {}, "deepseek-flash", False)
+    assert result == expected
+
+
+@pytest.mark.parametrize("model", ("deepseek-chat", "deepseek-reasoner", "deepseek/deepseek-reasoner"))
+def test_legacy_models_retain_toggle_only_mapping(model: str) -> None:
+    result: Final = DeepSeekChatConfig().map_openai_params({"reasoning_effort": "low"}, {}, model, False)
+    assert result == {"thinking": {"type": "enabled"}}
+
+
 def _function_tool(name: str) -> dict:
     return {
         "type": "function",