Use case
How to prevent an AI agent from exceeding its budget
An agent loop can spend a month of budget in an afternoon. The check has to sit before the call, and it works from an estimate.
Create a strict budget scoped to that agent's x-agent-id. Spendline checks configured budgets before forwarding supported requests. Strict budgets reject requests whose estimated cost would exceed a limit, with HTTP 402. Actual response cost can differ from the estimate.
How it works
Before forwarding a supported call, Spendline adds the agent's recorded spend, any in-flight reservations from concurrent calls, and an estimate for the new call. The estimate uses max_tokens when you send it, and a default output allowance when you do not. If that total would exceed the strict limit, the call is refused with 402 and the provider is not called. Otherwise the estimate is reserved until the real cost is recorded. If Spendline cannot confirm current spend, it refuses with 503 rather than admit an unchecked call.
{
"error": {
"type": "budget_exceeded",
"scope_type": "agent",
"scope_id": "research-agent",
"monthly_limit": 300,
"current_spend": 299.95,
"estimated_request_cost": 0.08
}
}What to know before relying on it
A started response is not cut off part way, so one long call can settle above the cap. Send max_tokens to keep the estimate close to the real cost. The control covers only calls routed through Spendline: a fallback that retries the provider directly after a 402 bypasses it.
Why it matters
An alert after an agent has already spent its budget reports the problem. A check in the request path can refuse the next call.
Frequently asked questions
What does the agent see when it hits the cap?
An HTTP 402 with the scope and the numbers instead of a model response. Treat it as an intentional control response: surface it, queue the work, or stop. Do not retry it directly against the provider.
Can two concurrent calls both slip through right at the cap?
Each admitted call reserves its estimated cost, and the next check counts those reservations, so concurrent calls do not all read the same stale spend figure. Because the reservation is an estimate, the final total can still differ from it.
See what this would look like in your product.
Start with a guided 30-day free pilot on live production traffic. The 30 days start when your traffic flows through Spendline, not at sign-up. We agree the scope, success criteria and paid terms before you start. There is no Spendline subscription fee during the pilot; your model-provider charges still apply.
Prefer email? fida@spendline.ai