Per-user cost attribution for your AI APP

You ship your AI feature. It works. A week later your OpenAI bill is $400 and you have no idea which of your users caused which $0.05. This is the single most underrated metric in production LLM apps — cost per end-user — and it's surprisingly easy to instrument if you know what to do. Here are the three approaches I've found work in practice, ranked by setup time. Approach 1: Wrap your provider client (5 minutes) Works for Express, Next.js Route Handlers, Fastify — anything that has a single OpenAI or Anthropic client instance. import OpenAI from 'openai' import { wrapOpenAI, withTrace } from '@voightxyz/openai' const openai = wrapOpenAI(new OpenAI(), { agent: 'production-chat-api', }) app.post('/api/chat', async (req, res) => { await withTrace( async () => { const r = await openai.chat.completions.create({ model: 'gpt-4o-mini', messages: req.body.messages, }) res.json({ reply: r.choices[0].message }) },