Your AI Agent Budget Check Has a Race Condition

When I first started putting budget limits around agent workflows, I thought the solution would be simple. Track the spend. Check what is left. Stop the next call if the budget is gone. That works in a demo. It even works in light testing. Then you run the same workflow with concurrency, retries, or a restart in the middle, and the whole thing gets shaky. The problem is not the math. The problem is where the decision gets made. The naive version A lot of first implementations look roughly like this: def call_model ( prompt : str , estimated_cost : int ) -> str : remaining = get_remaining_budget () if remaining < estimated_cost : raise RuntimeError ( " budget exceeded " ) result = llm_call ( prompt ) actual_cost = calculate_cost ( result ) record_spend ( actual_cost ) return result At first glance, this seems fine. Check the remaining budget Make the call Record the spend For a single worker, single process, no retries, no failures, it mostly works. Production is not that environment. W