Charging LLM spend back to the team that spent it

Most companies start with one API key, because one key is easy, and the bill that comes back is one number. Finance still has to split it, so it gets split by the only thing anyone can measure, which is usually headcount and occasionally an argument. Both are guesses, and AI usage is the wrong shape for a guess: it concentrates. A six-person team running a document pipeline can outspend a twelve-person team using a chat assistant several times over, and a headcount split will move money in exactly the wrong direction while looking scrupulously fair. The damage isn't the accounting, it's the incentive. A team charged for someone else's usage has no reason to reduce its own, and the team actually driving the bill sees a discount on its own consumption. You can only charge back what you attributed, and the cleanest attribution is recorded at call time rather than reconstructed afterwards.

• One API key serves every team and finance splits the bill by headcount.
• A team disputes its allocation and nobody can produce evidence either way.
• Nobody knows which internal team drove last month's increase.
• The teams told to cut usage aren't the ones generating it.
• Your allocation method hasn't changed since the first month you used it.

1. Ask how last month's AI spend was split, and what evidence backed the split.
2. Name the team you believe spent the most, then check whether you can prove it.
3. Find out whether one key serves several teams, and how many.
4. Work out what each team would owe on measured usage and compare it to what it was charged.

Illustrative example

A modelled month at $24,000 of shared spend across four teams of 12, 8, 6 and 4 people, priced on Claude Haiku 4.5 at real rates of $1.00 and $5.00 per million from the price book effective 2026-07-02. Team C runs a document pipeline and the others run assistants. Token volumes, team sizes and the usage split are modelled.

Team C is a fifth of the company and two thirds of the bill, and the fair-looking split hands it a $11,400 monthly discount paid for mostly by team A. Nobody chose that. It fell out of the only method available when the only number available is the total.

A provider knows which internal team owns a call only if you built your key or project structure around it, and that knowledge stops at whatever boundary you drew. Below it, your organisation chart lives in your company and the invoice knows about keys and models, so the join has to be made by you, on the call, at the time. Culpa records the team alongside the customer, the feature, the release and the run, prices each call from a versioned price book in exact decimal, and keeps the ledger on infrastructure you control, which matters more than usual here because an allocation that decides internal budgets is going to be challenged and the detail behind it has to survive the challenge. Showback becomes a query, chargeback becomes defensible, and the team that can reduce the bill is the one that gets the invoice.

• LLM Cost Forecast Calculator: A Range, Not a Number
• An LLM Tokenizer Change Is a Price Rise With No Price Change
• Per-Customer LLM Cost Forecast: Forecasting the Total Hides the Risk
• Power User AI Cost: Why Your Heaviest Account Is Heavy