Catch token-cost regressions in CI before they ship

Last year a team shipped a "small prompt improvement" on a Friday. By Monday their Claude bill had jumped 40%. Nobody caught it in review — the diff looked fine. The tokens were invisible. We wrote about the extreme end of this in We burned 136 million tokens running an autonomous agent studio — here's how we cut the bill ~90% . That was a runaway agent. But most cost regressions aren't dramatic — they're a prompt that grew by 200 tokens, a context window that stopped being trimmed, a new tool…