observability-streaming-tracer-err-error-in-rootlistenerstracer-on-chain-end-callback-6cd6fec3

THEN This is a known bug in LangChain (versions around 0.2.x). It does not affect the actual output but clutters logs. Workaround: either disable LangSmith tracing via LANGCHAIN_TRACING_V2=false or add a custom callback that catches KeyError. For a permanent fix, ensure the inner chain's streaming output always includes the specified output_messages_key, or upgrade to a patched version once released.

from langchain_core.callbacks import BaseCallbackHandler

class SuppressKeyErrorHandler(BaseCallbackHandler):
    def on_chain_end(self, outputs, **kwargs):
        try:
            super().on_chain_end(outputs, **kwargs)
        except KeyError:
            pass

chain_with_handler = conversational_rag_chain.with_config(callbacks=[SuppressKeyErrorHandler()])