fix(proxy): honor caller litellm_session_id in spend log session_id

The default path of _get_session_id_for_spend_log never consulted
kwargs['litellm_session_id'], so a caller-supplied session was
overwritten by the per-call trace id and LiteLLM_SpendLogs.session_id
could never group by litellm_session_id. Prefer it after the batch
lifecycle's deterministic session: when the caller sent no session the
value is only the derived trace-id copy, so sessionless requests are
unchanged.

Fixes #40851

litellm/proxy/spend_tracking/spend_tracking_utils.py

@@ -682,9 +682,12 @@ def _get_session_id_for_spend_log(
     omit_when_missing: bool,
     batch_trace_session_id: str | None = None,
 ) -> str | None:
-    """Under `omit` only `metadata.session_id`, the key Langfuse reads, counts as a session; `litellm_session_id` may
-    be a copied trace id. Batch call types carry a deterministic session derived from the batch id, which outranks
-    the per-request trace ids because those differ between the create call and the cost poller's row."""
+    """Under `omit` only `metadata.session_id`, the key Langfuse reads, counts as a session. Batch call types
+    carry a deterministic session derived from the batch id, which outranks everything else because the
+    per-request trace ids differ between the create call and the cost poller's row. Otherwise the caller's
+    `litellm_session_id` wins: it is only absent or a copy of the trace id when the caller did not send one,
+    so preferring it changes nothing for sessionless requests while letting a real session id reach the
+    spend log instead of being overwritten by the per-call trace id."""
     if omit_when_missing:
         session_id: Final = metadata.get("session_id") if metadata else None
         return str(session_id) if session_id else None
@@ -693,6 +696,9 @@ def _get_session_id_for_spend_log(
 
     if batch_trace_session_id is not None:
         return batch_trace_session_id
+    caller_session_id: Final = kwargs.get("litellm_session_id")
+    if caller_session_id is not None:
+        return str(caller_session_id)
     if standard_logging_payload is not None and standard_logging_payload.get("trace_id") is not None:
         return str(standard_logging_payload.get("trace_id"))
     if kwargs.get("litellm_trace_id") is not None:

tests/test_litellm/proxy/spend_tracking/test_spend_tracking_utils.py

@@ -233,6 +233,31 @@ def test_batch_session_outranks_the_per_request_trace_id():
     assert session_id == "batch-uid-1"
 
 
+def test_caller_litellm_session_id_wins_over_the_per_request_trace_id():
+    """A caller-supplied litellm_session_id is a real session, not a copied trace id; the
+    spend log must keep it instead of overwriting it with the per-call trace id."""
+    session_id: Final = _get_session_id_for_spend_log(
+        kwargs={"litellm_session_id": "sess-1", "litellm_trace_id": "trace-abc"},
+        metadata={"trace_id": "trace-abc"},
+        standard_logging_payload=_TRACE_ONLY_STANDARD_LOGGING,
+        omit_when_missing=False,
+    )
+    assert session_id == "sess-1"
+
+
+def test_batch_session_outranks_a_caller_litellm_session_id():
+    """The batch lifecycle grouping must survive even when the create call carries a
+    caller session id, or the poller's cost row would land in a different session."""
+    session_id: Final = _get_session_id_for_spend_log(
+        kwargs={"litellm_session_id": "sess-1", "litellm_trace_id": "trace-abc"},
+        metadata={"trace_id": "trace-abc"},
+        standard_logging_payload=_TRACE_ONLY_STANDARD_LOGGING,
+        omit_when_missing=False,
+        batch_trace_session_id="batch-uid-1",
+    )
+    assert session_id == "batch-uid-1"
+
+
 def test_omit_policy_still_suppresses_batch_sessions():
     session_id: Final = _get_session_id_for_spend_log(
         kwargs={},