FinOps teams can tolerate a fuzzy monthly cloud bill for some shared infrastructure. They usually cannot tolerate a fuzzy AI bill. Large language model traffic is bursty, model pricing changes by provider and tier, and one platform team may proxy requests for many internal applications at once. If you do not trace AI cost at the request level, every month ends with the same argument: one team says the central platform overcharged them, another says their costs belong to a shared experiment, and finance sees a growing spend line with no evidence behind it.
Per-request attribution fixes that by turning an AI bill into an evidence trail. Each request gets tied to a tenant, user, workload, model, route, token count, and computed price. That makes it possible to answer concrete questions: which product consumed most of yesterday's GPT spend, whether a new prompt template increased output tokens by 40 percent, or whether a fallback route silently pushed low-margin traffic onto a premium model.
A direct provider integration is already tricky. A multi-tenant AI gateway adds another layer of ambiguity. One shared gateway often sits between many products and many providers. It may rewrite headers, rotate credentials, retry failures, route by latency, and switch models based on policy. All of that helps reliability. All of it also makes billing harder to reconstruct later.
When chargeback numbers look wrong, do not start with the invoice. Start with one disputed request and walk outward. First, identify a single request that both engineering and finance can agree happened. Pull the app request ID, timestamp, tenant, and expected model route. Second, join that request to the gateway trace. Confirm resolved provider/model and check retries or fallbacks. Third, inspect the token record. If provider and gateway disagree, store both and mark one authoritative by written rule.
Per-request AI cost attribution is the control plane for FinOps AI governance in multi-tenant environments. The vendor invoice tells you what left the building. Your gateway and trace data explain why, for whom, and under which routing decision.
Sources: OpenAI organization usage reference, OpenTelemetry GenAI semantic conventions.