diff --git a/litellm/interactions/background_cost_polling.py b/litellm/interactions/background_cost_polling.py
index 0b8c128286a..7894f4099e5 100644
--- a/litellm/interactions/background_cost_polling.py
+++ b/litellm/interactions/background_cost_polling.py
@@ -307,8 +307,13 @@ def _spend_metadata(logging_obj: "LiteLLMLoggingObj") -> Mapping[str, object]:
     rebuilt logging object has no request headers to derive them from.
     """
     litellm_params: Final = _OBJECT_DICT.validate_python(logging_obj.model_call_details.get("litellm_params") or {})
+    metadata: Final = _OBJECT_MAPPING.validate_python(
+        get_litellm_metadata_from_kwargs(kwargs=logging_obj.model_call_details)
+    )
+    if metadata.get("tags") and not isinstance(metadata.get("tags"), list):
+        return metadata
     return {
-        **_OBJECT_MAPPING.validate_python(get_litellm_metadata_from_kwargs(kwargs=logging_obj.model_call_details)),
+        **metadata,
         **_request_tags(litellm_params),
     }
 
diff --git a/litellm/litellm_core_utils/litellm_logging.py b/litellm/litellm_core_utils/litellm_logging.py
index 9c49efc529f..c07392a8a5f 100644
--- a/litellm/litellm_core_utils/litellm_logging.py
+++ b/litellm/litellm_core_utils/litellm_logging.py
@@ -7573,9 +7573,9 @@ class StandardLoggingPayloadSetup:
         )
         request_tags: Final = next(
             (
-                cast(list[str], tags)  # cast-ok: tags are caller-provided
-                for tags in (metadata.get("tags"), litellm_metadata.get("tags"))
-                if isinstance(tags, list) and tags
+                cast(list[str], tags)  # cast-ok: preserve existing caller-provided list elements
+                for tag_metadata in (metadata, litellm_metadata)
+                if isinstance(tags := tag_metadata.get("tags"), list) and tags
             ),
             (),
         )
diff --git a/tests/unit/interactions/test_background_cost_polling.py b/tests/unit/interactions/test_background_cost_polling.py
index d55994dfb19..994aeca83e7 100644
--- a/tests/unit/interactions/test_background_cost_polling.py
+++ b/tests/unit/interactions/test_background_cost_polling.py
@@ -484,13 +484,14 @@ async def test_schedule_registers_an_agent_only_create_that_names_no_model():
 
 
 @pytest.mark.asyncio
-async def test_schedule_registers_a_create_whose_tags_are_a_string_and_keeps_them_as_sent():
+@pytest.mark.parametrize("metadata_field", ["metadata", "litellm_metadata"])
+async def test_schedule_registers_a_create_whose_tags_are_a_string_and_keeps_them_as_sent(metadata_field):
     """
     The regression: storing the create's context read string tags as a list
     and raised, so a create sent with a tags string answered with an error
     after the provider had already started the interaction.
     """
-    logging_obj = _logging_obj(litellm_params={"metadata": _create_metadata(tags="team-a,nightly-batch")})
+    logging_obj = _logging_obj(litellm_params={metadata_field: _create_metadata(tags="team-a,nightly-batch")})
     store = InMemoryBackgroundSettlementStore()
 
     task = await maybe_schedule_background_interaction_cost_polling(
diff --git a/tests/unit/litellm_core_utils/test_litellm_logging.py b/tests/unit/litellm_core_utils/test_litellm_logging.py
index e8ea0c04f6f..cdafa608bf5 100644
--- a/tests/unit/litellm_core_utils/test_litellm_logging.py
+++ b/tests/unit/litellm_core_utils/test_litellm_logging.py
@@ -3675,6 +3675,58 @@ def test_get_request_tags_preserves_non_string_metadata_tags():
     assert 7 in tags
 
 
+@pytest.mark.parametrize("metadata_key", ["metadata", "litellm_metadata"])
+@pytest.mark.parametrize("invalid_tags", ["my-tag", 7, True, {"tag": "value"}, ("my-tag",), None, ""])
+def test_standard_logging_payload_ignores_non_list_tags(
+    logging_obj: LitellmLogging, metadata_key: str, invalid_tags: object
+) -> None:
+    timestamp: Final = datetime.datetime(2026, 1, 1)
+    payload: Final = get_standard_logging_object_payload(
+        kwargs={
+            "litellm_call_id": "invalid-tag-request",
+            "model": "test-model",
+            "messages": [],
+            "litellm_params": {
+                metadata_key: {"tags": invalid_tags},
+                "proxy_server_request": {"headers": {"user-agent": "litellm/1.0.0"}},
+            },
+        },
+        init_response_obj={},
+        start_time=timestamp,
+        end_time=timestamp,
+        logging_obj=logging_obj,
+        status="success",
+    )
+
+    assert payload is not None
+    assert payload["litellm_call_id"] == "invalid-tag-request"
+    assert payload["request_tags"] == ["User-Agent: litellm", "User-Agent: litellm/1.0.0"]
+
+
+@pytest.mark.parametrize("invalid_tags", ["my-tag", 7, {"tag": "value"}, ("my-tag",), [], None])
+def test_get_request_tags_uses_valid_fallback_tags(invalid_tags: object) -> None:
+    fallback_tags: Final = ["fallback-tag"]
+    tags: Final = StandardLoggingPayloadSetup.get_request_tags(
+        litellm_params={
+            "metadata": {"tags": invalid_tags},
+            "litellm_metadata": {"tags": fallback_tags},
+        },
+        proxy_server_request={"headers": {"user-agent": "litellm/1.0.0"}},
+    )
+
+    assert tags == ["fallback-tag", "User-Agent: litellm", "User-Agent: litellm/1.0.0"]
+    assert fallback_tags == ["fallback-tag"]
+
+
+def test_get_request_tags_skips_unused_malformed_metadata() -> None:
+    tags: Final = StandardLoggingPayloadSetup.get_request_tags(
+        litellm_params={"metadata": {"tags": ["primary-tag"]}, "litellm_metadata": "unused"},
+        proxy_server_request={},
+    )
+
+    assert tags == ["primary-tag"]
+
+
 def test_get_request_tags_from_metadata_and_litellm_metadata():
     """
     Test that _get_request_tags correctly picks tags from both 'metadata' and 'litellm_metadata'.