Spend Limits and Budgets

Brain's policy engine provides granular spend controls from global account budgets to per-agent daily limits, asset restrictions, and per-merchant caps. All limits are enforced at the smart account level, not just in the backend.

Budget Hierarchy

Budgets are enforced at two levels:

• Account level — a global cap that applies across all agents under a BrainAccount

• Agent level — per-agent limits that must also satisfy the account-level constraint

An agent can never spend more than the lesser of its own limit and the account's global limit.

Setting a Daily Spend Limit

const policy = new PolicyBuilder()
  .setSpendLimit('500 USDC/day')    // Daily rolling window
  .setSpendLimit('2000 USDC/week')  // Weekly cap (additive constraint)
  .setAssetCap('ETH', '1 ETH/day') // Per-asset cap
  .build();

Per-Merchant Limits

const policy = new PolicyBuilder()
  .setMerchantCap('0xMerchantA', '100 USDC/day')
  .setMerchantCap('0xMerchantB', '50 USDC/day')
  .setMerchantBlocklist(['0xUntrustedMerchant'])
  .build();

Checking Remaining Budget

const budget = await client.policies.getBudgetStatus(agent.agentId);
console.log(budget);
// {
//   dailyLimit: '500 USDC',
//   spent: '213.50 USDC',
//   remaining: '286.50 USDC',
//   resetsAt: '2025-09-02T00:00:00Z'
// }

Budget Reset Windows

Period	Reset Behaviour
day	Resets at 00:00 UTC each day
week	Resets at 00:00 UTC each Monday
month	Resets at 00:00 UTC on the 1st of each month

Approval Thresholds for Large Transactions

For actions above a certain value, you can require multi-party approval:

const policy = new PolicyBuilder()
  .setSpendLimit('10000 USDC/day')
  .setApprovalThreshold('1-of-1')            // Standard: policy engine only
  .setHighValueThreshold('5000 USDC',        // Above this amount...
    { approvalThreshold: '2-of-3' })         // ...require multi-sig
  .build();

On-Chain Enforcement

Spend limits are enforced at the BrainAccount smart contract level inside validateUserOp. Even if the backend policy engine were unavailable or compromised, the smart account would reject any UserOperation that exceeds configured limits.

Limits are not advisory. They are hard constraints enforced on-chain before any funds can move.