A 2-Token Prompt and a 39,966-Token Bill: Measuring What My Agent Actually Costs

There is a small cluster of posts going around right now about auditing your LLM invoice, and about how cost calculators get the numbers wrong. I went to check mine and hit a problem before I got to the arithmetic: my pipeline doesn't produce an invoice, and the plumbing I built two months ago is the reason why. This project has a script, git_commit.py , that turns a staged git diff into a Conventional Commit message. It shells out to the Claude CLI. There is no ANTHROPIC_API_KEY anywhere in the project, on purpose — an early version used urllib against the API directly and broke immediately for anyone running on an OAuth session instead of a raw key, so every AI call in the repo goes through a claude -p subprocess instead. That decision is still right. It also means there is no API key, so there is no per-key usage dashboard, so there is no line item to audit. For several months this script has been making a model call on essentially every commit, and I have never once known what any of them cost. The call site throws the numbers away Here is the actual invocation, trimmed: raw = subprocess . check_output ( [ " claude " , " -p " , " --safe-mode " , SYSTEM + " \n\n " + diff ], text = True , timeout = 20 , env = _claude_subprocess_env (), ) subprocess.check_output returns stdout. With the CLI's default output format, stdout is the commit message string and nothing else. Every number I would want — tokens in, tokens out, dollars — is computed on the other side of that call and then discarded, because I asked for a string and a string is what I got. This is the part I want to flag for anyone wiring up a headless model call the same way. It isn't that the metering is missing. It's that the default output format is lossy in exactly the dimension you'd later want to audit, and you won't discover that by reading your own code, because your own code looks fine. It asks for text, it gets text. The fix is one flag: raw = subprocess . check_output ( [ " claude " , " -p " , " -