diff --git a/litellm/integrations/otel/plumbing/providers.py b/litellm/integrations/otel/plumbing/providers.py
index d81ef89f0fb..aec4b780324 100644
--- a/litellm/integrations/otel/plumbing/providers.py
+++ b/litellm/integrations/otel/plumbing/providers.py
@@ -500,13 +500,14 @@ def _is_litellm_span(span: ReadableSpan) -> bool:
 def with_request_user_id(span: ReadableSpan) -> ReadableSpan:
     """``span`` carrying the id the request's caller named, for a backend that drops a span without one.
 
-    Called as the span ends, on the request's own task. A held span must already have
-    been through here, since a tree can be released on another request's task.
+    That id replaces a ``user.id`` already on the span, such as what a mapper wrote for another backend,
+    so the whole request names one caller. Called as the span ends, on the request's own task. A held span
+    must already have been through here, since a tree can be released on another request's task.
     """
     user_id: Final = request_user_id()
-    attributes: Final = span.attributes or _NO_ATTRIBUTES
-    if user_id is None or User.ID in attributes:
+    if user_id is None:
         return span
+    attributes: Final = span.attributes or _NO_ATTRIBUTES
     return _CountedSpanView(span, MappingProxyType({**attributes, User.ID: user_id}))
 
 
diff --git a/litellm/proxy/common_utils/callback_config_validation.py b/litellm/proxy/common_utils/callback_config_validation.py
index f4197433a0b..9cc449a6cb4 100644
--- a/litellm/proxy/common_utils/callback_config_validation.py
+++ b/litellm/proxy/common_utils/callback_config_validation.py
@@ -504,8 +504,8 @@ def _mixpanel_config_error(callback_vars: Mapping[str, str]) -> str | None:
             f"Supported regions: {', '.join(sorted(MIXPANEL_OTLP_ENDPOINT_BY_REGION))}."
         )
 
-    token: Final = callback_vars.get("mixpanel_project_token")
-    if not token or not token.strip() or token in _UNSENDABLE_MIXPANEL_TOKENS:
+    token: Final = (callback_vars.get("mixpanel_project_token") or "").strip()
+    if not token or token in _UNSENDABLE_MIXPANEL_TOKENS:
         return "mixpanel_project_token must be the project's own token, sent in full on every create or update."
     unknown: Final = tuple(
         key for key in callback_vars if key.startswith(_MIXPANEL_VAR_PREFIX) and key not in _MIXPANEL_VARS
diff --git a/tests/integration/observability/test_mixpanel_user_id.py b/tests/integration/observability/test_mixpanel_user_id.py
index eb256f3e4a7..e04cb57f663 100644
--- a/tests/integration/observability/test_mixpanel_user_id.py
+++ b/tests/integration/observability/test_mixpanel_user_id.py
@@ -525,7 +525,9 @@ def test_every_request_on_a_long_lived_key_names_its_own_caller(
         )
         settled: Final = tuple((caller, _settled_trace(mixpanel.control_url, trace_id)) for caller, trace_id in traces)
 
-    assert [(caller, _without_the_id(_reached(mixpanel, spans, host), caller)) for caller, spans in settled] == [
+    at_project: Final = tuple((caller, _reached(mixpanel, spans, host)) for caller, spans in settled)
+    assert [caller for caller, spans in at_project if not spans] == []
+    assert [(caller, _without_the_id(spans, caller)) for caller, spans in at_project] == [
         (caller, []) for caller in callers
     ]
 
@@ -685,8 +687,8 @@ def test_a_collector_never_gets_the_id_only_a_team_project_asked_for(
 
 @pytest.mark.parametrize(
     "token",
-    ["", "   ", None, "None", "***REDACTED***"],
-    ids=["empty", "whitespace", "null", "none-string", "masked"],
+    ["", "   ", None, "None", " None ", "***REDACTED***", " ***REDACTED*** "],
+    ids=["empty", "whitespace", "null", "none-string", "padded-none-string", "masked", "padded-masked"],
 )
 def test_a_team_or_key_callback_refuses_a_token_it_could_never_send(
     token: JsonValue, operator_setup: OwnedProxy, mixpanel: ConnectSink
@@ -766,6 +768,55 @@ def test_mixpanel_listed_with_another_callback_gets_one_labeled_copy_of_the_mode
     assert _sent_to(_exports_of(mixpanel, at_mixpanel)) == {(OPERATOR_HOST, _authorization(OPERATOR_TOKEN))}
 
 
+def test_the_operators_project_names_one_caller_when_a_request_also_sends_a_trace_user_id(
+    combined_setup: tuple[OwnedProxy, tuple[str, ...]], provider: Wire, mixpanel: ConnectSink, audit_sinks: SpanSinks
+) -> None:
+    owned, callbacks = combined_setup
+    proxy: Final = owned.gateway
+    caller: Final = f"caller-{uuid.uuid4().hex}"
+    traced_as: Final = f"traced-as-{uuid.uuid4().hex}"
+    with proxy.scenario() as scenario:
+        model: Final = scenario.model(api_base=provider.url + "/v1")
+        key: Final = scenario.key(key_alias=f"mixpanel-two-ids-{uuid.uuid4().hex}")
+        trace_id: Final = _traced_call(
+            proxy,
+            key,
+            {"model": model, **_chat_call("hi"), "metadata": {"user.id": caller, "trace_user_id": traced_as}},
+        )
+        at_mixpanel: Final = _settled_trace(mixpanel.control_url, trace_id, _has_model_call)
+        at_other: Final = _settled_trace(audit_sinks.operator, trace_id, _has_model_call)
+
+    assert _without_the_id(_outside_auth(at_mixpanel), caller) == []
+    assert {span["attributes"].get(USER_ID) for span in at_other if OPERATION in span["attributes"]} == (
+        {traced_as} if "langfuse_otel" in callbacks else {None}
+    )
+
+
+def test_a_team_project_names_one_caller_when_a_request_also_sends_a_trace_user_id(
+    combined_setup: tuple[OwnedProxy, tuple[str, ...]], provider: Wire, mixpanel: ConnectSink
+) -> None:
+    owned, _ = combined_setup
+    proxy: Final = owned.gateway
+    caller: Final = f"caller-{uuid.uuid4().hex}"
+    with proxy.scenario() as scenario:
+        model: Final = scenario.model(api_base=provider.url + "/v1")
+        key, host, _token = _team_or_key(scenario, "team")
+        trace_id: Final = _traced_call(
+            proxy,
+            key,
+            {
+                "model": model,
+                **_chat_call("hi"),
+                "metadata": {"user.id": caller, "trace_user_id": f"traced-as-{uuid.uuid4().hex}"},
+            },
+        )
+        spans: Final = _settled_trace(mixpanel.control_url, trace_id, _has_model_call)
+
+    at_project: Final = _reached(mixpanel, spans, host)
+    assert [span for span in at_project if OPERATION in span["attributes"]], at_project
+    assert _without_the_id(at_project, caller) == []
+
+
 @pytest.mark.parametrize("entry", ["mcp", "rest"])
 def test_a_team_project_gets_the_callers_id_on_an_mcp_tool_call(
     entry: EntryPoint, tenant_setup: OwnedProxy, mixpanel: ConnectSink
diff --git a/tests/unit/integrations/otel/test_otel_v2_destinations.py b/tests/unit/integrations/otel/test_otel_v2_destinations.py
index c14fff5afbd..202e359abe5 100644
--- a/tests/unit/integrations/otel/test_otel_v2_destinations.py
+++ b/tests/unit/integrations/otel/test_otel_v2_destinations.py
@@ -5977,7 +5977,7 @@ class TestMixpanelGetsTheCallersId:
         assert "session.id" not in by_name(mixpanel)["chat gpt-4o"].attributes
         assert "user.id" not in by_name(other)["chat gpt-4o"].attributes
 
-    def test_the_id_the_mapper_wrote_wins_over_the_id_read_at_auth(self) -> None:
+    def test_the_id_read_at_auth_replaces_the_id_a_mapper_wrote(self) -> None:
         provider, mixpanel, _ = self._fan_out()
 
         self._request(
@@ -5986,9 +5986,22 @@ class TestMixpanelGetsTheCallersId:
             model_call_attributes={"gen_ai.operation.name": "chat", "user.id": "from-mapper"},
         )
 
-        assert by_name(mixpanel)["chat gpt-4o"].attributes.get("user.id") == "from-mapper"
+        assert by_name(mixpanel)["chat gpt-4o"].attributes.get("user.id") == "caller-1"
         assert by_name(mixpanel)["POST /v1/chat/completions"].attributes.get("user.id") == "caller-1"
 
+    def test_a_span_from_other_code_names_the_caller_only_for_mixpanel(self) -> None:
+        provider, mixpanel, other = self._fan_out()
+
+        def run() -> None:
+            set_request_destinations((self.MIXPANEL, self.ARIZE))
+            set_request_user_id("caller-1")
+            get_tracer(provider, "application").start_span("checkout", attributes={"user.id": "its-own"}).end()
+
+        in_fresh_context(run)
+
+        assert by_name(mixpanel)["checkout"].attributes.get("user.id") == "caller-1"
+        assert by_name(other)["checkout"].attributes.get("user.id") == "its-own"
+
     def test_a_request_without_an_id_adds_nothing(self) -> None:
         provider, mixpanel, _ = self._fan_out()
 
diff --git a/tests/unit/proxy/management_endpoints/test_callback_management_endpoints.py b/tests/unit/proxy/management_endpoints/test_callback_management_endpoints.py
index 84712f1b81c..34347519eee 100644
--- a/tests/unit/proxy/management_endpoints/test_callback_management_endpoints.py
+++ b/tests/unit/proxy/management_endpoints/test_callback_management_endpoints.py
@@ -630,7 +630,7 @@ class TestMixpanelCallbackValidation:
             with pytest.raises(HTTPException) as exc:
                 raise_on_invalid_key_logging_config(self._key_metadata({"mixpanel_region": "eu"}))
             assert "mixpanel_project_token must be" in str(exc.value.detail)
-            for unsendable in ("", "   ", "None", "***REDACTED***"):
+            for unsendable in ("", "   ", "None", " None ", "***REDACTED***", " ***REDACTED*** "):
                 with pytest.raises(HTTPException) as exc:
                     _validate_team_callback(self._team_data({"mixpanel_project_token": unsendable}))
                 assert "mixpanel_project_token must be" in str(exc.value.detail)