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You can repoint LangChain through Butter using its ChatOpenAI class, which accepts the necessary base URL and headers. See it in the example below. 
import os
from langchain_openai import ChatOpenAI
from langgraph.graph import StateGraph, START, MessagesState

# Create an OpenAI model pointing to Butter
model = ChatOpenAI(
    base_url="https://proxy.butter.dev/v1",
    default_headers={"Butter-Auth": f"Bearer {os.getenv('BUTTER_API_KEY')}"},
    api_key=os.getenv('OPENAI_API_KEY'),
    model="gpt-4"
)

# Build graph
graph = StateGraph(MessagesState)
graph.add_node("chatbot", lambda state: {"messages": [model.invoke(state["messages"])]})
graph.add_edge(START, "chatbot")
graph = graph.compile()

# Run
for event in graph.stream({"messages": [("user", "What is the English word for mantequilla?")]}, stream_mode="values"):
    event["messages"][-1].pretty_print()