Token Ledger Blocks AI Calls Once a Free Daily Budget Runs Out

A developer describes a zero-cost AI feature that stops making model calls once its daily token allowance is used up. The main risk, they argue, is not repeat traffic but estimation error, where one long prompt or a retry loop can exhaust the whole day's budget in a single call. The approach uses a small standard-library Python ledger that reserves an estimated token count before each request and then settles it against actual usage. When the budget is exhausted, the code returns a zero-token fallback rather than calling the model. Character counts divided by four serve as a rough estimate for English text, with the guard calibrating against real usage because code, CJK text, and JSON tend to run higher.