LangChain

pip install -U langchain langchain-openai

import os
from langchain_openai import ChatOpenAI

llm = ChatOpenAI(
    model="openai/gpt-5-nano",
    api_key=os.environ["PHASEO_API_KEY"],
    base_url="https://api.phaseo.app/v1",
)

result = llm.invoke("Write one sentence about routing reliability.")
print(result.content)

def call_model(state):
    response = llm.invoke(state["messages"])
    return {"messages": [response]}