fix: use DeepSeekChatConfig instead of OpenAIConfig for deepseek provider (#22971)

* fix: use DeepSeekChatConfig instead of OpenAIConfig for deepseek provider

The deepseek provider was incorrectly using OpenAIConfig().map_openai_params()
instead of DeepSeekChatConfig().map_openai_params(), which meant DeepSeek-specific
parameter mappings were not being applied.

* test: add unit tests for deepseek DeepSeekChatConfig param mapping

Verify that get_optional_params uses DeepSeekChatConfig (not OpenAIConfig)
for the deepseek provider by testing thinking, reasoning_effort, and
budget_tokens stripping behavior.

litellm/utils.py

@@ -4491,7 +4491,7 @@ def get_optional_params(  # noqa: PLR0915
             ),
         )
     elif custom_llm_provider == "deepseek":
-        optional_params = litellm.OpenAIConfig().map_openai_params(
+        optional_params = litellm.DeepSeekChatConfig().map_openai_params(
             non_default_params=non_default_params,
             optional_params=optional_params,
             model=model,

tests/test_litellm/test_utils.py

@@ -3473,6 +3473,53 @@ class TestDropParamsWithPromptCacheKey:
         assert result.get("temperature") == 0.7
 
 
+class TestGetOptionalParamsDeepSeek:
+    """Tests that deepseek provider uses DeepSeekChatConfig for parameter mapping."""
+
+    def test_deepseek_supports_thinking_param(self):
+        """
+        Verify that get_optional_params for deepseek accepts the 'thinking' param,
+        which is only supported by DeepSeekChatConfig, not OpenAIConfig.
+        """
+        from litellm.utils import get_optional_params
+
+        result = get_optional_params(
+            model="deepseek-reasoner",
+            custom_llm_provider="deepseek",
+            thinking={"type": "enabled"},
+        )
+        assert result.get("thinking") == {"type": "enabled"}
+
+    def test_deepseek_supports_reasoning_effort_param(self):
+        """
+        Verify that get_optional_params for deepseek accepts 'reasoning_effort',
+        which is only supported by DeepSeekChatConfig, not OpenAIConfig.
+        """
+        from litellm.utils import get_optional_params
+
+        result = get_optional_params(
+            model="deepseek-reasoner",
+            custom_llm_provider="deepseek",
+            reasoning_effort="high",
+        )
+        assert result.get("thinking") == {"type": "enabled"}
+
+    def test_deepseek_thinking_strips_budget_tokens(self):
+        """
+        DeepSeekChatConfig strips budget_tokens from thinking param.
+        This would not happen with OpenAIConfig.
+        """
+        from litellm.utils import get_optional_params
+
+        result = get_optional_params(
+            model="deepseek-reasoner",
+            custom_llm_provider="deepseek",
+            thinking={"type": "enabled", "budget_tokens": 5000},
+        )
+        assert "budget_tokens" not in result.get("thinking", {})
+        assert result.get("thinking") == {"type": "enabled"}
+
+
 class TestIsStreamingRequest:
     def test_stream_true_in_kwargs(self):
         assert _is_streaming_request(kwargs={"stream": True}, call_type="acompletion") is True