I woke up to a $40 bill from an agent that looped overnight on a $5 task. Every solution worked *after* the call. None stopped the TCP connection. baar-core checks your budget before every API call. Over cap → exception raised locally. No DNS. No TCP. $0 charged. → 84% cost reduction via semantic routing → Per-user SQLite quotas → LangGraph + FastAPI support → MIT licensed pip install baar-core