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The Fast Transfer allowance is Circle’s mechanism for providing faster-than-finality USDC transfers. It limits the total value of USDC that can be minted through Fast Transfer before related burns reach hard finality.

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How it works

Circle maintains a Fast Transfer allowance pool that backs all in-process CCTP Fast Transfers. The following steps describe how the allowance works:
  1. Initial state: Circle maintains a Fast Transfer allowance pool (for example, 10 million USDC).
  2. Fast Transfer initiated: When you burn USDC on the source blockchain with Fast Transfer:
    • The burn amount temporarily debits the allowance
    • Circle’s Attestation Service issues an attestation after soft finality
    • You can immediately mint USDC on the destination blockchain
  3. Allowance depleted: If the allowance reaches zero, Fast Transfers are temporarily unavailable until the allowance replenishes.
  4. Allowance replenished: Once burns reach hard finality on source blockchains, the corresponding amounts are credited back to the allowance.
Note: The Fast Transfer allowance is global across all supported blockchains. It’s not specific to a particular source or destination blockchain, but rather tracks the total value of in-process Fast Transfers.

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Check the current allowance

To check the remaining Fast Transfer allowance, call the GET /v2/fastBurn/USDC/allowance endpoint. For a detailed guide on checking the allowance, see Get the Fast Transfer allowance.

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When allowance is insufficient

If the Fast Transfer allowance is insufficient for your transfer, you have two options:

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Option 1: Wait for replenishment

The allowance automatically replenishes as pending Fast Transfers reach hard finality. Monitor the allowance until sufficient capacity is available:
JavaScript
async function waitForAllowance(requiredAmount, timeoutMs = 1200000) {
  const startTime = Date.now();

  while (Date.now() - startTime < timeoutMs) {
    const response = await fetch(
      "https://iris-api-sandbox.circle.com/v2/fastBurn/USDC/allowance",
    );
    const { allowance } = await response.json();
    const allowanceSubunits = BigInt(
      Math.floor(parseFloat(allowance) * 1_000_000),
    );

    if (allowanceSubunits >= requiredAmount) {
      console.log("Sufficient allowance available");
      return true;
    }

    console.log(`Current allowance: ${allowance} USDC, waiting...`);
    await new Promise((resolve) => setTimeout(resolve, 30000)); // Check every 30 seconds
  }

  throw new Error("Timeout waiting for allowance replenishment");
}

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Option 2: Use Standard Transfer

Change minFinalityThreshold to 2000 or higher to use Standard Transfer, which doesn’t consume the Fast Transfer allowance.

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Allowance lifecycle

Understanding what happens during each phase of the allowance lifecycle helps you build more robust applications:
1

Burn initiated

The user calls depositForBurn with minFinalityThreshold ≤ 1000. The transaction confirms on the source blockchain, and the allowance is debited by the burn amount.
2

Soft finality reached

Circle’s Attestation Service issues an attestation, and the attestation becomes available through the API. The user can now mint USDC on the destination blockchain.
3

Hard finality reached

The burn transaction reaches hard finality on the source blockchain. The allowance is credited back by the burn amount, and capacity is restored for new Fast Transfers.