diff --git a/tests/llm_translation/test_deepseek_reasoning_effort.py b/tests/llm_translation/test_deepseek_reasoning_effort.py
new file mode 100644
index 00000000000..f9d3f29d2ab
--- /dev/null
+++ b/tests/llm_translation/test_deepseek_reasoning_effort.py
@@ -0,0 +1,41 @@
+"""Verify DeepSeek `reasoning_effort` passes through the transformation.
+
+Covers the fix that preserves the effort value (low/medium/high/xhigh/max) into
+the request body when thinking is on, instead of mapping it to a binary thinking
+toggle and discarding it. See https://api-docs.deepseek.com/guides/thinking_mode
+"""
+from litellm.llms.deepseek.chat.transformation import DeepSeekChatConfig
+
+
+def _map(non_default_params, drop_params=True):
+    cfg = DeepSeekChatConfig()
+    return cfg.map_openai_params(non_default_params, {}, "deepseek/deepseek-v4-flash", drop_params)
+
+
+def test_reasoning_effort_low_passes_through():
+    out = _map({"reasoning_effort": "low"})
+    assert out["reasoning_effort"] == "low"
+    assert out["thinking"] == {"type": "enabled"}
+
+
+def test_reasoning_effort_max_passes_through():
+    out = _map({"reasoning_effort": "max"})
+    assert out["reasoning_effort"] == "max"
+    assert out["thinking"] == {"type": "enabled"}
+
+
+def test_reasoning_effort_none_disables_thinking():
+    out = _map({"reasoning_effort": "none"})
+    assert out["thinking"] == {"type": "disabled"}
+    assert "reasoning_effort" not in out
+
+
+def test_explicit_thinking_disabled_wins_and_drops_effort():
+    out = _map({"thinking": {"type": "disabled"}, "reasoning_effort": "low"})
+    assert out["thinking"] == {"type": "disabled"}
+    assert "reasoning_effort" not in out
+
+
+def test_no_reasoning_effort_leaves_no_key():
+    out = _map({})
+    assert out.get("reasoning_effort") is None