observability-prompt-linking-prompt-linking-fails-when-langfuse-prompt-is-set-i-06134954

THEN Set the langfuse_prompt metadata directly on the LangChain ChatPromptTemplate object, not in the config metadata. Create a ChatPromptTemplate from the Langfuse prompt using get_langchain_prompt(), then assign metadata={'langfuse_prompt': prompt}. Use that template in the chain.

from langfuse import get_client
from langfuse.langchain import CallbackHandler
from langchain.agents import create_agent
from langchain.tools import tool
from langchain_openai import ChatOpenAI

client = get_client()
prompt = client.get_prompt("weather-prompt", type="chat")

@tool
def get_weather(location: str) -> str:
    return f"Weather in {location}: Sunny, 72°F"

model = ChatOpenAI(model="gpt-4.1")
agent = create_agent(model=model, tools=[get_weather])

handler = CallbackHandler()
result = agent.invoke(
    {"messages": prompt.prompt},
    {"callbacks": [handler], "metadata": {"langfuse_prompt": prompt}},
)