How to actually track your AI / LLM API spend before the bill surprises you

You wire up the OpenAI SDK, ship the feature, and it works. Three weeks later someone in finance forwards a screenshot of a bill that tripled and asks what happened. You open the provider dashboard, see one big number, and… that's it. No per-feature breakdown, no idea which change caused it, no way to tell whether it's a bug or just growth. I've watched this happen at enough teams that I now treat "we can't explain our AI bill" as a predictable stage every company hits about two months after their first LLM feature ships. Here's how to get ahead of it — starting with plain code, then the tradeoffs, then where a dedicated tool actually earns its keep. Disclosure up front: I work on StackSpend, which does the full version of this. I've kept the first 80% of this post vendor-neutral because most of it you can and should build yourself before you buy anything. The core problem: the bill is a single number, your costs are not Provider dashboards give you total spend over time. What you actually need to make decisions is spend broken down by the dimensions you care about: Per feature — is it the summarizer or the chat assistant that's expensive? Per customer / tenant — which accounts cost more to serve than they pay? Per model — how much are you spending on GPT-4-class vs cheaper models? Per environment — is a runaway staging job quietly burning money? None of those dimensions exist in the raw bill. You have to attach them yourself, at call time, because after the request is gone the context is gone with it. Step 1: capture usage at the call site Every major provider returns token usage in the response. The trick is to log it with your own business context attached — the feature name, the tenant, the environment. Here's the pattern in TypeScript with the OpenAI SDK: import OpenAI from " openai " ; const openai = new OpenAI (); // Prices per 1M tokens — keep these in config, they change often. const PRICING : Record < string , { input : number ; output : number } > = { " g