diff --git a/litellm/integrations/opentelemetry.py b/litellm/integrations/opentelemetry.py
index 6c8510380a8..519b90efc70 100644
--- a/litellm/integrations/opentelemetry.py
+++ b/litellm/integrations/opentelemetry.py
@@ -955,7 +955,14 @@ def _emit_once(self, kwargs: dict, *scope: object) -> bool:
             spans_logged = {}
             _otel_internal["spans_logged"] = spans_logged
 
-        dedupe_key = (self.__class__.__name__, id(self), *scope)
+        # Normalise scope parts so they are always hashable. `guardrail_mode`
+        # can be a list (e.g. ["pre_call", "post_call"]) and a tuple containing
+        # a list is itself unhashable, causing a TypeError when used as a dict key.
+        normalized_scope = tuple(
+            tuple(part) if isinstance(part, list) else part
+            for part in scope
+        )
+        dedupe_key = (self.__class__.__name__, id(self), *normalized_scope)
         if spans_logged.get(dedupe_key) is True:
             return False
 
diff --git a/tests/test_litellm/integrations/test_opentelemetry.py b/tests/test_litellm/integrations/test_opentelemetry.py
index b65e629c890..4fdc17aaad9 100644
--- a/tests/test_litellm/integrations/test_opentelemetry.py
+++ b/tests/test_litellm/integrations/test_opentelemetry.py
@@ -87,6 +87,38 @@ def test_create_guardrail_span_with_no_info(self):
         # Verify that start_span was never called
         otel.tracer.start_span.assert_not_called()
 
+    @patch("litellm.integrations.opentelemetry.datetime")
+    def test_guardrail_mode_as_list_does_not_crash(self, mock_datetime):
+        """Regression test for https://github.com/BerriAI/litellm/issues/28486.
+
+        When a guardrail is configured with a list ``mode`` (e.g.
+        ``["pre_call", "post_call"]``), ``_create_guardrail_span`` previously
+        crashed with ``TypeError: unhashable type: 'list'`` inside
+        ``_emit_once``, because the list value was unpacked directly into the
+        tuple used as a ``dict`` key.  Lists are not hashable; only tuples are.
+
+        The fix normalises each scope part in ``_emit_once`` to a tuple when
+        it is a list, so the dedupe key is always hashable.
+        """
+        otel = OpenTelemetry()
+        otel.tracer = MagicMock()
+        mock_span = MagicMock()
+        otel.tracer.start_span.return_value = mock_span
+
+        guardrail_info = {
+            "guardrail_name": "test_guardrail",
+            "guardrail_mode": ["pre_call", "post_call"],  # list-valued mode
+            "start_time": 1609459200.0,
+            "end_time": 1609459201.0,
+        }
+        kwargs = {
+            "standard_logging_object": {"guardrail_information": [guardrail_info]}
+        }
+
+        # Must not raise TypeError: unhashable type: 'list'
+        otel._create_guardrail_span(kwargs=kwargs, context=None)
+        otel.tracer.start_span.assert_called_once()
+
     @patch("litellm.integrations.opentelemetry.datetime")
     def test_guardrail_response_dict_is_json_serialized(self, mock_datetime):
         """Dict guardrail_response (e.g. OpenAI moderation result) must reach