Fix Langfuse trace_id mapping for failed logs and prioritize session_id

This fix addresses Bug 1 where failed LiteLLM logs were using request_id instead of session_id for Langfuse trace mapping, breaking trace correlation.

Changes:
1. Fix kwargs inconsistency in failure path (litellm_logging.py:2956)
   - Changed from passing self.model_call_details to passing local kwargs variable
   - Matches success path behavior and excludes original_response (potentially a coroutine)

2. Prioritize session_id as trace_id fallback (langfuse.py:607-615)
   - When no explicit trace_id is provided, now uses session_id from metadata
   - This ensures traces with the same session_id are grouped together in Langfuse
   - Maintains backward compatibility: only activates when session_id is set

Testing:
- All 28 existing Langfuse tests pass (excluding pre-existing test_langfuse_e2e_sync which fails due to missing API key)
- Specifically verified trace_id resolution tests still pass

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

litellm/integrations/langfuse/langfuse.py

@@ -604,9 +604,12 @@ class LangFuseLogger:
             session_id = clean_metadata.pop("session_id", None)
             trace_name = cast(Optional[str], clean_metadata.pop("trace_name", None))
             trace_id = clean_metadata.pop("trace_id", None)
+            # If session_id is provided, use it as trace_id for consistent trace mapping
+            if trace_id is None and session_id is not None:
+                trace_id = session_id
             # Use standard_logging_object.trace_id if available (when trace_id from metadata is None)
             # This allows standard trace_id to be used when provided in standard_logging_object
-            if trace_id is None and standard_logging_object is not None:
+            elif trace_id is None and standard_logging_object is not None:
                 trace_id = cast(
                     Optional[str], standard_logging_object.get("trace_id")
                 )

litellm/litellm_core_utils/litellm_logging.py

@@ -2953,7 +2953,7 @@ class Logging(LiteLLMLoggingBaseClass):
                             user_id=kwargs.get("user", None),
                             status_message=str(exception),
                             level="ERROR",
-                            kwargs=self.model_call_details,
+                            kwargs=kwargs,
                         )
                         if _response is not None and isinstance(_response, dict):
                             _trace_id = _response.get("trace_id", None)