# What is a spending limit?

A spending limit caps how much a key, a job or a machine can cost. Runtime
enforces it before the money is spent, not after the bill arrives.

**Runtime has three spending limits, and the outermost one means you can never
owe money.** Credit is prepaid, so a bill cannot exceed your balance. Each key
can have a daily limit. Each sandbox can have a lifetime limit
([security](/docs/security#read-only-keys-and-daily-limits)).
A refused request charges nothing.

## Why it matters for AI agents

An agent that loops can start a machine on each iteration, forget to stop them,
or retry a failing step all night. A person would notice by morning. A budget
alert would notice after the money is gone. A limit checked on each request
stops the next create instead, and gives the agent a clear error to act on.
This matters most for agents that run unattended: overnight evals, CI, and
agents that other people's prompts can steer.

## The three layers on Runtime

| Layer                | Scope                         | What happens at the cap                                                           |
| -------------------- | ----------------------------- | --------------------------------------------------------------------------------- |
| Prepaid balance      | The account                   | Charges stop at zero; you never owe money                                         |
| Daily spending limit | One key's agent, any 24 hours | Create, wake, extension or renewal fails with `spending_limit_reached` (HTTP 402) |
| `maxTotalCostMicros` | One sandbox, its whole life   | The most the sandbox may cost from start to end                                   |

The daily limit counts what is already charged. It also counts what running
sandboxes are about to use, including renewals and parked storage. Room returns
as older spending leaves the window. When the limit is reached, a
running sandbox keeps the time it is already funded for. Set the limit above
what the agent's paused sandboxes cost in a day. Paused storage that cannot be
paid for is treated like storage on an empty balance. Runtime sends a notice,
and deletes the storage seven days later.

Amounts are in microdollars, millionths of a dollar: `5_000_000` is $5.

## Cap one sandbox

```ts check
import { RuntimeError, Sandbox } from "withruntime";

try {
  await using sbx = await Sandbox.create({
    maxTotalCostMicros: 5_000_000, // at most $5 over its life
  });
  console.log((await sbx.exec("echo ok")).stdout);
} catch (error) {
  if (error instanceof RuntimeError && error.code === "spending_limit_reached")
    console.log("over this key's daily limit; stop and ask");
  else throw error;
}
```

```python check
from withruntime import Runtime, RuntimeError

runtime = Runtime()
try:
    with runtime.sandboxes.create(
        max_total_cost_micros=5_000_000
    ) as sbx:
        print(sbx.exec("echo ok").stdout)
except RuntimeError as error:
    if error.code != "spending_limit_reached":
        raise
    print("over this key's daily limit; stop and ask")
```

The SDKs do not retry `spending_limit_reached`. An agent that receives this
error should stop and tell the person it works for.

## What a limit costs you in room

For scale, a 2 vCPU, 4 GiB sandbox costs $0.03125 an hour while it waits
on a model. It costs $0.08 an hour with both CPUs busy. A $25 daily
limit covers about 313 fully busy sandbox-hours a day at
that size ([pricing](/docs/pricing)).

## Who can set one

Only the member who made a key, or an owner or admin, can set, change or remove
that key's daily limit. They do this on the website, on the API keys page. No
key can raise, remove or set a limit. Every change is recorded in the audit log.

## Related

- [What are least-privilege API keys?](/glossary/least-privilege-api-keys)
- [What is a CPU floor?](/glossary/cpu-floor)
- [Pricing](/docs/pricing)
- [Sandbox cost calculator](/calculators/sandbox-cost-calculator)

Facts on this page were checked on 25 October 2026.