diff --git a/langfuse/_utils/serializer.py b/langfuse/_utils/serializer.py
index 2f25ddd0e..ff98c2a15 100644
--- a/langfuse/_utils/serializer.py
+++ b/langfuse/_utils/serializer.py
@@ -149,6 +149,11 @@ def _default_inner(self, obj: Any) -> Any:
             if isinstance(obj, (str, float, type(None))):
                 return obj
 
+            # Class objects (e.g. a pydantic model passed as a schema) would
+            # otherwise match the instance branches below and dump their namespace
+            if isinstance(obj, type):
+                return f"<class '{obj.__module__}.{obj.__qualname__}'>"
+
             if self._depth >= self._MAX_DEPTH:
                 return f"<{type(obj).__name__}>"
 
@@ -214,7 +219,7 @@ def _default_inner(self, obj: Any) -> Any:
                 type(obj).__name__,
                 exc_info=e,
             )
-            return f'"<not serializable object of type: {type(obj).__name__}>"'
+            return f"<not serializable object of type: {type(obj).__name__}>"
 
     def encode(self, obj: Any) -> str:
         self.seen.clear()  # Clear seen objects before each encode call
diff --git a/tests/unit/test_observe.py b/tests/unit/test_observe.py
index 21ff1418d..60f2f93da 100644
--- a/tests/unit/test_observe.py
+++ b/tests/unit/test_observe.py
@@ -523,3 +523,26 @@ def marked() -> Any:
 
     span = _finished_spans_by_name(memory_exporter, "marked")[0]
     assert span.attributes[LangfuseOtelSpanAttributes.OBSERVATION_OUTPUT] == "done"
+
+
+def test_observe_records_class_argument_as_class_reference(
+    langfuse_memory_client: Any, memory_exporter: Any
+) -> None:
+    class Invoice:
+        total: float = 0.0
+
+    @observe(name="extract")
+    def extract(text: str, schema: type) -> str:
+        return text
+
+    assert extract("hi", schema=Invoice) == "hi"
+
+    langfuse_memory_client.flush()
+
+    span = _finished_spans_by_name(memory_exporter, "extract")[0]
+    observed_input = json.loads(
+        span.attributes[LangfuseOtelSpanAttributes.OBSERVATION_INPUT]
+    )
+    assert observed_input["kwargs"]["schema"] == (
+        f"<class '{__name__}.{Invoice.__qualname__}'>"
+    )
diff --git a/tests/unit/test_serializer.py b/tests/unit/test_serializer.py
index 3e5112f6b..1851de4d0 100644
--- a/tests/unit/test_serializer.py
+++ b/tests/unit/test_serializer.py
@@ -570,3 +570,35 @@ def test_dict_with_non_string_keys_is_serialized(input_obj, expected):
     result = json.loads(EventSerializer().encode(input_obj))
 
     assert result == expected
+
+
+class _PlainClass:
+    x = 1
+
+
+@pytest.mark.parametrize(
+    "input_obj, expected",
+    [
+        (TestBaseModel, f"<class '{__name__}.TestBaseModel'>"),
+        (TestDataclass, f"<class '{__name__}.TestDataclass'>"),
+        (_PlainClass, f"<class '{__name__}._PlainClass'>"),
+        (TestEnum, f"<class '{__name__}.TestEnum'>"),
+        (int, "<class 'builtins.int'>"),
+    ],
+    ids=["pydantic_model", "dataclass", "plain", "enum", "builtin"],
+)
+def test_class_objects_serialize_as_class_reference(input_obj, expected):
+    serializer = EventSerializer()
+
+    assert json.loads(serializer.encode(input_obj)) == expected
+    assert json.loads(serializer.encode({"schema": input_obj})) == {"schema": expected}
+
+
+def test_failed_serialization_falls_back_to_plain_string():
+    class BrokenModel(BaseModel):
+        def model_dump(self, *args, **kwargs):
+            raise RuntimeError("boom")
+
+    result = json.loads(EventSerializer().encode({"value": BrokenModel()}))
+
+    assert result == {"value": "<not serializable object of type: BrokenModel>"}