Does this example with FastAPI + LangSmith tracing actually work? (FastAPI [standard] 0.119.1+, LangChain 1.0.1+, LangSmith 0.4.37+)

[JulioPeixoto]

I am trying to set up tracing with LangSmith in a FastAPI app and I’d like to check if the following minimal example should work, or whether I am missing something essential. why doesn't it work?

• fastapi[standard] >= 0.119.1
• langchain >= 1.0.1
• langchain-openai >= 1.0.0
• langsmith >= 0.4.37

import os
from fastapi import FastAPI
from fastapi.responses import RedirectResponse
from pydantic import BaseModel
from langchain_openai import ChatOpenAI
from config import settings
from langsmith import traceable

os.environ["LANGSMITH_API_KEY"] = "lsv2_pt_…"
os.environ["LANGSMITH_PROJECT"] = "pr-..."

model = ChatOpenAI(
    model=settings.MODEL_DEPLOYMENT,
    api_key=settings.OPENAI_API_KEY,
)

@traceable
def chat_service(question: str) -> str:
    return model.invoke([HumanMessage(content=question)])

app = FastAPI()

@app.post("/chat")
async def chat(request: ChatRequest):
    return chat_service(request.message)