fix(proxy): an empty trace field in litellm_metadata shadows the promoted one

Addresses review. Promotion copies a trace control field from `metadata` into
`litellm_metadata` only when the key is absent, so a key that is present but
empty in `litellm_metadata` wins and the `metadata` value is never used. The
check still looked at `metadata` in that case, which let
`missing_session_id: reject` accept a request that ends up with an empty
session id, and let an empty trace_id or session_id block the headers

The check now follows the same rule as promotion. If the active container has
the key, its value decides. Only when the key is absent does the promoted
`metadata` value count. This makes the conflicting-bucket cases behave exactly
as on main

litellm/proxy/litellm_pre_call_utils.py

@@ -151,12 +151,13 @@ def _session_id_from_baggage(baggage: str) -> str | None:
 
 
 def _caller_set_trace_field(data: Mapping[str, object], metadata_variable_name: str, field: str) -> bool:
+    active: Final = data.get(metadata_variable_name)
+    active_mapping: Final = cast(Mapping[str, object], active) if isinstance(active, Mapping) else None
+    if active_mapping is not None and field in active_mapping:
+        return bool(active_mapping.get(field))
     promoted: Final = metadata_variable_name == "litellm_metadata" and field in LITELLM_TRACE_CONTROL_METADATA_FIELDS
-    sources: Final = (metadata_variable_name, "metadata") if promoted else (metadata_variable_name,)
-    return any(
-        isinstance(container := data.get(name), Mapping) and bool(cast(Mapping[str, object], container).get(field))
-        for name in sources
-    )
+    requester: Final = data.get("metadata")
+    return promoted and isinstance(requester, Mapping) and bool(cast(Mapping[str, object], requester).get(field))
 
 
 def _stampable_key_hash(user_api_key_dict: UserAPIKeyAuth) -> str | None:

tests/test_litellm/proxy/test_litellm_pre_call_utils.py

@@ -8245,6 +8245,40 @@ async def test_promoted_caller_trace_ids_beat_traceparent_and_baggage(path: str,
     assert updated["litellm_metadata"]["session_id"] == "caller-session"
 
 
+@pytest.mark.asyncio
+async def test_missing_session_id_reject_ignores_requester_session_id_shadowed_by_empty_litellm_metadata():
+    with pytest.raises(ProxyException) as exc_info:
+        await add_litellm_data_to_request(
+            data={
+                "model": "gpt-4o",
+                "input": "hi",
+                "metadata": {"session_id": "caller-session"},
+                "litellm_metadata": {"session_id": ""},
+            },
+            request=_request_for("/v1/responses"),
+            user_api_key_dict=UserAPIKeyAuth(api_key="hashed-key"),
+            proxy_config=MagicMock(),
+            general_settings={"missing_session_id": "reject"},
+        )
+    assert exc_info.value.code == "400"
+
+
+def test_add_litellm_metadata_from_request_headers_empty_litellm_metadata_field_falls_back_to_headers():
+    headers = {
+        "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
+        "baggage": "session.id=baggage-session-42",
+    }
+    data = {
+        "metadata": {"trace_id": "caller-trace", "session_id": "caller-session"},
+        "litellm_metadata": {"trace_id": "", "session_id": ""},
+    }
+    LiteLLMProxyRequestSetup.add_litellm_metadata_from_request_headers(
+        headers=headers, data=data, _metadata_variable_name="litellm_metadata"
+    )
+    assert data["litellm_trace_id"] == "4bf92f3577b34da6a3ce929d0e0e4736"
+    assert data["litellm_session_id"] == "baggage-session-42"
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "client_body",