fix: resolve duplicate traces for sessions

Author: jake-marsden

src/api/routers/chat.py

@@ -64,6 +64,12 @@ def extract_langfuse_metadata(chat_request: ChatRequest, headers: dict) -> dict:
             if meta_key not in metadata:
                 metadata[meta_key] = headers_lower[header_key]
 
+    # ALWAYS use chat_id as session_id for consistency
+    # This ensures all messages in the same chat conversation are grouped
+    # in the same Langfuse session, preventing duplicate sessions
+    if "chat_id" in metadata:
+        metadata["session_id"] = metadata["chat_id"]
+    
     return metadata
 
 
@@ -94,12 +100,20 @@ async def chat_completions(
     # Create trace name using chat_id if available
     trace_name = f"chat:{metadata.get('chat_id', 'unknown')}"
 
+    # Extract user_id and session_id separately for Langfuse trace attributes
+    user_id = metadata.get("user_id")
+    session_id = metadata.get("session_id")
+    
+    # Create clean metadata dict for additional context (without duplicating trace attributes)
+    # Remove user_id and session_id from metadata since they're passed as separate parameters
+    clean_metadata = {k: v for k, v in metadata.items() if k not in ["user_id", "session_id"]}
+    
     # Update trace with metadata, user_id, and session_id
     langfuse_context.update_current_trace(
         name=trace_name,
-        user_id=metadata.get("user_id"),
-        session_id=metadata.get("session_id"),
-        metadata=metadata,
+        user_id=user_id,
+        session_id=session_id,
+        metadata=clean_metadata,
         input={
             "model": chat_request.model,
             "messages": [msg.model_dump() for msg in chat_request.messages],