A card-funded wallet with nobody reviewing each charge needs a ceiling that isn't just "however much happens to be in the balance." Balance alone caps total exposure; it says nothing about how fast an agent can burn through it. A daily spend limit is the control that does - a cap on how much the wallet can spend in a rolling day, independent of its balance or how auto-reload keeps that balance topped up.
A large balance plus auto-reload plus no daily limit means a bad loop - a retry storm, a scraper that triggers a draft on every page it visits, a bug that resends the same follow-up a thousand times - can spend as much as the balance and the reload amount allow, as fast as the API can process the calls. A daily spend limit puts a ceiling on that independent of how well-funded the wallet is, which matters precisely because an unsupervised agent is the one case where nobody's watching the spend happen in real time.
Because every call is priced in cents - $0.02 to draft, $0.01 to send, $0.03 for priority receive, a one-time $1.00 to claim a custom inbox name - sizing a daily limit is mostly arithmetic. An agent that drafts and sends 200 follow-ups a day is spending roughly $0.03 per message, or $6/day, before any priority receive calls. Set the limit with headroom over your actual expected volume, not at the exact number, so a legitimately busy day doesn't trip the same ceiling meant to catch a runaway loop.
A newly created wallet already has a daily spend cap applied automatically, for its first few days, regardless of what you set yourself - see New-Wallet Risk Controls for the exact figures. If you configure a higher limit on a brand-new wallet, expect the automatic cap to bind first; it loosens as the wallet ages, at which point your own configured limit takes over as the operative ceiling.
Once a wallet has spent up to its daily limit, further calls authenticated with that wallet's secret stop succeeding until the window resets - the same category of failure as a declined card or an exhausted balance, from the agent's point of view. Design for that the way you'd design for any other authentication failure: stop retrying in a tight loop, surface it, and let a human (or a separate monitoring process) decide whether to raise the limit or investigate why it was hit. What Happens When an Autonomous Agent's Card Payment Is Declined covers the adjacent failure mode of a funding charge itself failing, which looks similar from the calling code's perspective.
If you're running more than one agent, a single wallet with one daily limit shared across all of them makes it hard to tell which agent is actually responsible for a given day's spend. Budgeting for a Fleet of Autonomous Agents That Pay Their Own Way covers the case for giving each agent (or each class of agent) its own wallet and its own limit, so a runaway one doesn't exhaust a budget shared with ten well-behaved ones.
A human using a metered dashboard notices a spend spike because they're looking at it. A machine caller making its own decisions about when to call a paid route doesn't have that check unless you build it in explicitly. Designing APIs That Assume the Caller Is a Machine, Not a Human covers this gap more broadly - a daily spend limit is one concrete instance of a control that a human-facing product would leave to the user's own judgment, and a machine-facing one can't.
A daily spend limit and auto-reload solve different problems and work better set together than either does alone: auto-reload keeps the balance from running out mid-task, and the daily limit keeps a single bad day from costing more than you've decided it's allowed to, independent of how much auto-reload would otherwise keep refilling. Set the limit first, from expected call volume, then set auto-reload's threshold and amount to comfortably support spend up to that limit - not the other way around, since a reload amount sized without the limit in mind can end up refilling a balance the limit won't let the agent spend anyway that day.
A limit set from a rough estimate on day one is worth revisiting once you have a few weeks of real call volume to look at. An agent that's consistently running close to its limit on ordinary days has either grown past the original estimate or is spending more per task than expected - either way, that's a prompt to look at what's actually driving the spend, not just to raise the number and move on.