Error in RootListenersTracer.on_chain_end callback: KeyError('answer') while streaming a RAG Chain Output

[Alptimus]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangChain documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

Example Code

I have the following code for building a RAG Chatbot (using this example):

from langchain_openai import OpenAIEmbeddings, ChatOpenAI
from langchain.chains import create_retrieval_chain
from langchain.chains.history_aware_retriever import create_history_aware_retriever
from langchain.chains.combine_documents import create_stuff_documents_chain

vectordb = FAISS.load_local(persist_directory, embedding, index_name, allow_dangerous_deserialization=True)
retriever=vectordb.as_retriever()

llm = ChatOpenAI()

....
prompt={.....}
....

question_answer_chain = create_stuff_documents_chain(llm, prompt, output_parser=parser)
rag_chain = create_retrieval_chain(history_aware_retriever, question_answer_chain)

conversational_rag_chain = RunnableWithMessageHistory(
    rag_chain,
    get_session_history,
    input_messages_key="input",
    history_messages_key="chat_history",
    output_messages_key="answer",
)

while True:
    query = input("Ask a question: ")

    for chunk in conversational_rag_chain.stream(
        {"input": query,},
        config={
            "configurable": {
                "session_id": "demo_1"
            }
        }
    ):
        if answer_chunk := chunk.get("answer"):
            print(f"{answer_chunk}", end="", flush=True)
    
    print()

Error Message and Stack Trace (if applicable)

Ask a question: How many colors are in rainbow?
Error in RootListenersTracer.on_chain_end callback: KeyError('answer')
Error in callback coroutine: KeyError('answer')
A rainbow typically has seven colors, which are: Red, Orange, Yellow, Green, Blue, Indigo, Violet.</s>
Ask a question: 

Description

Hi,

I am trying to get the answer as stream, the problem is whenever the conversational_rag_chain.stream() is initiating, using an input it is giving the following errors:
Error in RootListenersTracer.on_chain_end callback: KeyError('answer')
Error in callback coroutine: KeyError('answer')
and then the output is printing as intended.

My question is, how can I solve it? I have entered output_messages_key="answer" in the conversational_rag_chain already, so am I doing something wrong or is a bug?

Any little discussion or help is welcome. Thanks in advance.

System Info

System Information

OS: Linux
OS Version: #39-Ubuntu SMP PREEMPT_DYNAMIC Fri Jul 5 21:49:14 UTC 2024
Python Version: 3.12.3 (main, Apr 10 2024, 05:33:47) [GCC 13.2.0]

Package Information

langchain_core: 0.2.23
langchain: 0.2.11
langchain_community: 0.2.10
langsmith: 0.1.93
langchain_chroma: 0.1.2
langchain_openai: 0.1.17
langchain_text_splitters: 0.2.2

Packages not installed (Not Necessarily a Problem)

The following packages were not found:

langgraph
langserve

[ShaiDiamant]

Having the same issue while upgraded to the last version
Important note - removing the "output_messages_key="answer", results in a different error + breaks the logic of the chat history, so don't attempt that

[ssultan19]

having the same issue. Below are my pip packages versions:
langchain==0.2.3
langchain-cli==0.0.24
langchain-community==0.2.4
langchain-core==0.2.23
langchain-elasticsearch==0.2.2
langchain-mongodb==0.1.7
langchain-openai==0.1.8
langchain-pinecone==0.1.1
langchain-text-splitters==0.2.1
langserve==0.2.1
langsmith==0.1.75

[gustavo-sacchi]

I 'm having the same issue.
'''
Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')
Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')
'''
Pip packages versions:
langchain==0.2.11
langchain-anthropic==0.1.17
langchain-community==0.2.10
langchain-core==0.2.23
langchain-experimental==0.0.63
langchain-google-genai==1.0.7
langchain-groq==0.1.6
langchain-mistralai==0.1.9
langchain-openai==0.1.13
langchain-postgres==0.0.9
langchain-qdrant==0.1.3
langchain-text-splitters==0.2.2
langcodes==3.4.0
langfuse==2.36.2

[sherlock-tez]

Me too lol

[madonzy]

I have similar, but with another key:

Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')
Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')

[agvinaykr]

getting same error:
Error in RootListenersTracer.on_chain_end callback: KeyError('output')

[JeongYunLee]

me too..

[kangcp]

Update to the latest version can not be used directly, the experience is very poor, I do not know how langchain developers do the test

[DonaghBr]

I have the same issue but only yesterday. Worked perfectly last week, but I did pull again from langchain:
Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')
Error in RootListenersTracer.on_chain_end callback: KeyError('message_history')
Error in callback coroutine: KeyError('message_history')

langchain==0.2.11

[Alptimus]

I have the same issue but only yesterday. Worked perfectly last week, but I did pull again from langchain: Error in RootListenersTracer.on_chain_end callback: KeyError('message_history') Error in callback coroutine: KeyError('message_history') Error in RootListenersTracer.on_chain_end callback: KeyError('message_history') Error in callback coroutine: KeyError('message_history')

langchain==0.2.11

Can you tell which langchain version you were using till last week before pulling 0.2.11?

[pjurczen]

Same issue after upgrade to 0.2.11 (still present in 0.2.12):
WARNING:langchain_core.callbacks.manager:Error in AsyncRootListenersTracer.on_chain_end callback: KeyError('output_text')
Error in AsyncRootListenersTracer.on_chain_end callback: KeyError('output_text')

[edwardgonen]

Have the same problem. But I have another VENV that does not have it. Here are the versions that seem to work ok:
langchain 0.2.7
langchain-community 0.2.6
langchain-core 0.2.12

I'm not sure these are the exact files that cause the problem. But intuitively sounds like them