Tracking LLM costs across an entire app is easy. Finding out which customer is actually burning through your OpenAI bill? That's a nightmare.

For a while, we were just eating the cost. You look at the Stripe dashboard, look at the OpenAI invoice, and pray the margins make sense. But when a single power user decides to process 10k documents through Claude on a Saturday night, averages stop mattering real fast.

tbh, most billing dashboards are useless for this. They tell you you spent $400 yesterday, but not who spent it.

Here is what actually works in production, without over-engineering your entire stack.

The Metadata trick

If you're using OpenAI or Anthropic, you can pass metadata with every request. Don't just log it in your db. Pass the user_id or tenant_id directly to the provider.

OpenAI supports this natively. Anthropic has custom headers. OpenRouter makes it trivial.

const response = await openai . chat . completions .…