Update app.py

app.py

@@ -75,8 +75,11 @@ async def init():
 
 @cl.on_message
 async def main(message):
-    chain = cl.user_session.get("chain")
-    res = await chain.acall(message)
+    cb = cl.AsyncLangchainCallbackHandler(
+        stream_final_answer=True, answer_prefix_tokens=["FINAL", "ANSWER"]
+    )
+    cb.answer_reached = True
+    res = await chain.acall(message, callbacks=[cb])
 
     answer = res["result"]
     source_elements = []