The question every engineering leader asks before turning an agent loose is the same: what stops it from spending forever? Alerts are the wrong answer - an alert fires after the money moved. The right answer is architectural, and it has four parts.
Reserve, then settle
Every paid operation reserves its estimated cost against a ledger before executing, then settles to the measured cost after. If the reservation would cross the ceiling, the call fails with a structured 402 before anything is spent. The ledger is the authority; dashboards merely read it.
Ceilings at every layer
One global cap is too blunt: a runaway loop can eat a month in an hour while staying under it. Stack the ceilings - per tool call (estimates bound payloads), per run (an agent gets a hard cost cap), per turn (interactive sessions carry their own limit), per month (the workspace envelope). Each layer catches what the one above cannot see.
Degrade in a designed order
Between full service and full stop there should be a ladder, chosen on purpose: shed freshness first (optional enrichment, sampling frequency), fidelity second (cheaper models for classification), coverage last. Users prefer a leaner daily service to a rich weekly one; encode that preference and pin the ordering with a test.
Idempotency is a billing feature
Retries are how distributed systems live, and a retried run that executes twice is a double charge. Idempotency keys on run creation, unique reservation ids that make duplicate execution trip a constraint, and success-only settlement for lookups that miss - these turn "safe to retry" from a hope into a property. Budget safety is not one feature; it is the sum of these four.
Every playbook on this blog ships as a runnable mission.
Open a free workspace and the playbook library is waiting - describe the outcome and the agents carry it end to end, with 1,000 trial credits on us.