Update app.py

app.py

@@ -91,6 +91,12 @@ async def main(message):
         stream_final_answer=True, 
         answer_prefix_tokens=["FINAL", "ANSWER"]
     )
+
+    cb = cl.AsyncLangchainCallbackHandler(
+        stream_final_answer=True,
+        answer_prefix_tokens=["FINAL", "ANSWER"]
+    )
+    
     cb.answer_reached = True
     res = await chain.acall(message, callbacks=[cb])
     return