I have a simple example about langchain runnables. From https://python.langchain.com/v0.1/docs/expression_language/interface/

from langchain_core.prompts import ChatPromptTemplate
from langchain_openai import ChatOpenAI

model = ChatOpenAI(model="gpt-4")
prompt = ChatPromptTemplate.from_template("tell me a joke about {topic}")
chain = prompt | model
print(chain.invoke({"topic": "chickens"}))

It should return like following in web site:

AIMessage(content="Why don't bears wear shoes? nnBecause they have bear feet!")

But it returns unstructured response:

content="Why don't bears wear shoes? nnBecause they have bear feet!" response_metadata={'token_usage': {'completion_tokens': 19, 'prompt_tokens': 13, 'total_tokens': 32}, 'model_name': 'gpt-4-0613', 'system_fingerprint': None, 'finish_reason': 'stop', 'logprobs': None} id='run-bd7cda7e-dee2-4107-af3f-97282faa9fa4-0' usage_metadata={'input_tokens': 13, 'output_tokens': 19, 'total_tokens': 32}

how can I fix this issue?