fix(proxy): honor x-litellm-session-id on pass-through spend logs

pass_through_request builds logging kwargs without reading
x-litellm-session-id / x-litellm-trace-id, so every /gemini/... spend
row got a fresh uuid4 while /v1/chat/completions grouped correctly.
Copy the chain id into litellm_params and metadata (Fixes #43540).

litellm/proxy/pass_through_endpoints/pass_through_endpoints.py

@@ -100,6 +100,7 @@ from litellm.proxy.common_utils.sse_keepalive import (
 from litellm.proxy.litellm_pre_call_utils import (
     LiteLLMProxyRequestSetup,
     _get_dynamic_logging_metadata,  # pyright: ignore[reportPrivateUsage]  # shared proxy helper, same import style as _read_request_body above
+    get_chain_id_from_headers,
 )
 from litellm.proxy.route_llm_request import ProxyModelNotFoundError
 from litellm.proxy.utils import normalize_route_for_root_path
@@ -643,17 +644,32 @@ class HttpPassThroughEndpointHelpers(BasePassthroughUtils):
         if isinstance(deployment_model_info, Mapping):
             _metadata["model_info"] = dict(deployment_model_info)
 
-        kwargs: Final = {
-            "litellm_params": {
-                **litellm_params_in_body,
-                "metadata": _metadata,
-                "proxy_server_request": {
-                    "url": str(request.url),
-                    "method": request.method,
-                    "body": copy.copy(_parsed_body),  # use copy instead of deepcopy
-                    "headers": request.headers,
-                },
+        # Match /v1/chat/completions: honor x-litellm-session-id / x-litellm-trace-id
+        # so spend logs group pass-through calls with the rest of the session (#43540).
+        # get_standard_logging_payload_trace_id reads litellm_params["litellm_session_id"].
+        _request_headers = getattr(request, "headers", None) or {}
+        chain_id = get_chain_id_from_headers(dict(_request_headers))
+        if chain_id:
+            # Header wins over any client-supplied body metadata (same as chat).
+            _metadata["session_id"] = chain_id
+            _metadata["trace_id"] = chain_id
+
+        litellm_params: dict = {
+            **litellm_params_in_body,
+            "metadata": _metadata,
+            "proxy_server_request": {
+                "url": str(request.url),
+                "method": request.method,
+                "body": copy.copy(_parsed_body),  # use copy instead of deepcopy
+                "headers": request.headers,
             },
+        }
+        if chain_id:
+            litellm_params["litellm_session_id"] = chain_id
+            litellm_params["litellm_trace_id"] = chain_id
+
+        kwargs: Final = {
+            "litellm_params": litellm_params,
             "call_type": "pass_through_endpoint",
             "litellm_call_id": litellm_call_id,
             "passthrough_logging_payload": passthrough_logging_payload,

tests/test_litellm/proxy/pass_through_endpoints/test_pass_through_endpoints.py

@@ -7305,6 +7305,71 @@ def test_passthrough_client_cannot_forge_session_id_omission(client_metadata_key
     )
 
 
+@pytest.mark.parametrize(
+    "header_name,header_value",
+    [
+        ("x-litellm-session-id", "S1"),
+        ("x-litellm-trace-id", "T1"),
+    ],
+)
+def test_passthrough_honors_session_header_for_spend_logs(header_name: str, header_value: str):
+    """Pass-through routes must pick up x-litellm-session-id / x-litellm-trace-id
+    the same way /v1/chat/completions does (#43540). Without this, every
+    /gemini/... spend row gets a fresh uuid4 session."""
+    from litellm.litellm_core_utils.litellm_logging import StandardLoggingPayloadSetup
+
+    mock_request = MagicMock(spec=Request)
+    mock_request.method = "POST"
+    mock_request.url = "http://0.0.0.0:4000/gemini/v1beta/models/gemini-2.5-flash:generateContent"
+    mock_request.headers = Headers({header_name: header_value})
+    mock_request.scope = {}
+
+    kwargs = HttpPassThroughEndpointHelpers._init_kwargs_for_pass_through_endpoint(
+        request=mock_request,
+        user_api_key_dict=UserAPIKeyAuth(api_key="hashed-key"),
+        passthrough_logging_payload=MagicMock(),
+        logging_obj=MagicMock(),
+        _parsed_body={},
+        litellm_call_id="lit-43540-call-id",
+    )
+
+    litellm_params = kwargs["litellm_params"]
+    assert litellm_params["litellm_session_id"] == header_value
+    assert litellm_params["litellm_trace_id"] == header_value
+    assert litellm_params["metadata"]["session_id"] == header_value
+    assert litellm_params["metadata"]["trace_id"] == header_value
+    assert (
+        StandardLoggingPayloadSetup.get_standard_logging_payload_trace_id(
+            logging_obj=MagicMock(litellm_trace_id="fallback-uuid"),
+            litellm_params=litellm_params,
+        )
+        == header_value
+    )
+
+
+def test_passthrough_without_session_header_does_not_invent_one():
+    """No session header → leave litellm_session_id unset so spend logging
+    keeps its existing uuid4 / omit fallback."""
+    mock_request = MagicMock(spec=Request)
+    mock_request.method = "POST"
+    mock_request.url = "http://0.0.0.0:4000/gemini/v1beta/models/gemini-2.5-flash:generateContent"
+    mock_request.headers = Headers({})
+    mock_request.scope = {}
+
+    kwargs = HttpPassThroughEndpointHelpers._init_kwargs_for_pass_through_endpoint(
+        request=mock_request,
+        user_api_key_dict=UserAPIKeyAuth(api_key="hashed-key"),
+        passthrough_logging_payload=MagicMock(),
+        logging_obj=MagicMock(),
+        _parsed_body={},
+        litellm_call_id="lit-43540-no-header",
+    )
+
+    assert "litellm_session_id" not in kwargs["litellm_params"]
+    assert "litellm_trace_id" not in kwargs["litellm_params"]
+    assert "session_id" not in kwargs["litellm_params"]["metadata"]
+
+
 @pytest.mark.parametrize("client_metadata_key", ["litellm_metadata", "metadata"])
 def test_passthrough_logs_the_resolved_deployment_model_info_over_the_request_body(client_metadata_key: str):
     """A provider route that resolved a router deployment stashes its model_info on request.state. That