diff --git a/langfuse/openai.py b/langfuse/openai.py
index 057d90cab..dbff11d37 100644
--- a/langfuse/openai.py
+++ b/langfuse/openai.py
@@ -539,8 +539,8 @@ def _parse_usage(usage: Optional[Any] = None) -> Any:
     for tokens_details in [
         "prompt_tokens_details",
         "completion_tokens_details",
-        "input_token_details",
-        "output_token_details",
+        "input_tokens_details",
+        "output_tokens_details",
     ]:
         if tokens_details in usage_dict and usage_dict[tokens_details] is not None:
             tokens_details_dict = (
diff --git a/tests/test_parse_usage.py b/tests/test_parse_usage.py
new file mode 100644
index 000000000..cfd6e4da4
--- /dev/null
+++ b/tests/test_parse_usage.py
@@ -0,0 +1,100 @@
+from langfuse.openai import _parse_usage
+
+
+class TestParseUsageNone:
+    def test_returns_none_for_none(self):
+        assert _parse_usage(None) is None
+
+
+class TestParseUsageEmbedding:
+    def test_embedding_usage_returns_input_only(self):
+        usage = {"prompt_tokens": 5, "total_tokens": 5}
+        result = _parse_usage(usage)
+        assert result == {"input": 5}
+
+
+class TestParseUsageChatCompletions:
+    def test_prompt_tokens_details_flattened(self):
+        usage = {
+            "prompt_tokens": 100,
+            "completion_tokens": 50,
+            "total_tokens": 150,
+            "prompt_tokens_details": {"cached_tokens": 20, "audio_tokens": None},
+            "completion_tokens_details": {"reasoning_tokens": 10},
+        }
+        result = _parse_usage(usage)
+        assert result["prompt_tokens"] == 100
+        assert result["completion_tokens"] == 50
+        assert result["total_tokens"] == 150
+        # None values are stripped, non-None kept
+        assert result["prompt_tokens_details"] == {"cached_tokens": 20}
+        assert result["completion_tokens_details"] == {"reasoning_tokens": 10}
+
+    def test_details_as_object(self):
+        """Token details may arrive as an object with __dict__ instead of a dict."""
+
+        class Details:
+            def __init__(self):
+                self.cached_tokens = 30
+                self.audio_tokens = None
+
+        usage = {
+            "prompt_tokens": 100,
+            "completion_tokens": 50,
+            "total_tokens": 150,
+            "prompt_tokens_details": Details(),
+            "completion_tokens_details": None,
+        }
+        result = _parse_usage(usage)
+        assert result["prompt_tokens_details"] == {"cached_tokens": 30}
+        assert result["completion_tokens_details"] is None
+
+
+class TestParseUsageResponseApi:
+    """Tests for OpenAI Response API usage format (input_tokens_details / output_tokens_details)."""
+
+    def test_input_and_output_tokens_details_flattened(self):
+        usage = {
+            "input_tokens": 200,
+            "output_tokens": 80,
+            "total_tokens": 280,
+            "input_tokens_details": {"cached_tokens": 50},
+            "output_tokens_details": {"reasoning_tokens": 30},
+        }
+        result = _parse_usage(usage)
+        assert result["input_tokens"] == 200
+        assert result["output_tokens"] == 80
+        assert result["input_tokens_details"] == {"cached_tokens": 50}
+        assert result["output_tokens_details"] == {"reasoning_tokens": 30}
+
+    def test_none_values_stripped_from_details(self):
+        usage = {
+            "input_tokens": 200,
+            "output_tokens": 80,
+            "total_tokens": 280,
+            "input_tokens_details": {"cached_tokens": 50, "audio_tokens": None},
+            "output_tokens_details": {"reasoning_tokens": None},
+        }
+        result = _parse_usage(usage)
+        assert result["input_tokens_details"] == {"cached_tokens": 50}
+        assert result["output_tokens_details"] == {}
+
+    def test_details_as_object(self):
+        class InputDetails:
+            def __init__(self):
+                self.cached_tokens = 40
+
+        class OutputDetails:
+            def __init__(self):
+                self.reasoning_tokens = 15
+
+        usage = {
+            "input_tokens": 100,
+            "output_tokens": 50,
+            "total_tokens": 150,
+            "input_tokens_details": InputDetails(),
+            "output_tokens_details": OutputDetails(),
+        }
+        result = _parse_usage(usage)
+        assert result["input_tokens_details"] == {"cached_tokens": 40}
+        assert result["output_tokens_details"] == {"reasoning_tokens": 15}