How to put a hard daily cap on your LangChain.js API costs

If an agent decides how many API calls to make, your cost ceiling is whatever the agent feels like today. Most days that's fine. The day a tool errors out and the chain retries its way around it, it isn't, and nothing in your logs will look wrong while it happens.

I maintain budget-guard, a small open source circuit breaker for LLM API bills. It has a LangChain.js adapter, and the whole integration is one callback handler:

import { ChatOpenAI } from '@langchain/openai';

import { BudgetGuardHandler } from 'budget-guard/langchain';

const handler = new BudgetGuardHandler({