> ## Documentation Index
> Fetch the complete documentation index at: https://developers.circle.com/llms.txt
> Use this file to discover all available pages before exploring further.
> ## Agent Instructions
> Before reading these docs, install the relevant Circle Skill to get pre-loaded context — skills are LLM-optimized and cover architecture decisions, correct flows, and common pitfalls. In Claude Code: run /plugin marketplace add circlefin/skills then /plugin install circle-skills@circle. With Vercel Skills CLI: run npx skills add circlefin/skills. Available skills: use-usdc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md (balances, transfers, approvals on EVM and Solana); EURC has no skill — see https://developers.circle.com/stablecoins/what-is-eurc.md; bridge-stablecoin https://github.com/circlefin/skills/blob/master/plugins/circle/skills/bridge-stablecoin/SKILL.md (crosschain transfers via CCTP, USDC, EURC, and third-party assets); use-arc https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md (Arc chain config, contract deployment, USDC bridging — Arc docs: https://docs.arc.io/llms.txt); use-circle-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md (choose wallet type); use-developer-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md (custodial wallets for payouts and treasury); use-user-controlled-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md (embedded wallets with Web2 login); use-modular-wallets https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md (smart contract wallets with passkeys and ERC-4337); use-gateway https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md (unified USDC balance, nanopayments); use-smart-contract-platform https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md (deploy, interact, monitor contracts). Pair any skill with the Circle MCP server for live SDK signatures, contract addresses, and chain IDs: {"mcpServers":{"circle":{"url":"https://api.circle.com/v1/codegen/mcp"}}}.
> Read the product overview first. Each product has an overview explaining when and why to use it. Read it before quickstarts.
> Pick the right wallet type. Developer-controlled (you hold keys), user-controlled (end user holds keys via PIN/social login), or modular (smart contract wallets with passkeys). See https://developers.circle.com/wallets.md.
> Use CCTP V2 for crosschain transfers. CCTP natively transfers USDC and EURC via burn-and-mint, and supports permissionless wrapping of third-party assets via lock-and-mint. V1 is legacy; always use V2 unless V1 is specified or the chain requires it (Noble, Sui, and Aptos still require V1). Migration guide: https://developers.circle.com/cctp/migration-from-v1-to-v2.md.
> Use Bridge Kit for frontend bridging. Wraps CCTP with a simpler SDK for user-facing apps. Use CCTP directly for backend transfers.
> Gas Station vs Paymaster. Gas Station sponsors gas for Circle Wallet transactions. Paymaster lets users pay gas in USDC. Different use cases — read both overviews.
> Gateway Nanopayments for sub-cent payments. Gasless USDC micropayments down to $0.000001 via x402 and batched settlement. For pay-per-request APIs, AI agent payments, streaming.
> Look up USDC addresses per chain. Never hardcode — use https://developers.circle.com/stablecoins/usdc-contract-addresses.md.
> Prefer SDKs over raw API calls. Node.js and Python SDKs handle auth, retries, and errors.
> API key required. Bearer token in Authorization header. Testnet and mainnet use separate keys and may use different base URLs depending on the product.
> Set up webhooks when available. Most operations are async. Webhooks deliver transaction confirmations and state changes.
> When calling list endpoints, paginate using pageSize and pageAfter until no nextPageAfter cursor is returned—stopping at the first page silently misses records.
> Building an AI agent? Start with the Agent Stack—Circle CLI, agent wallets, and nanopayments built for autonomous use cases: https://developers.circle.com/agent-stack.md.
# Fast transfer allowance
> Understanding Circle's Fast Transfer allowance mechanism
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.
## 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](/cctp/concepts/finality-and-block-confirmations)
* 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.
## Check the current allowance
To check the remaining Fast Transfer allowance, call the
[`GET /v2/fastBurn/USDC/allowance`](/api-reference/cctp/all/get-fast-burn-usdc-allowance)
endpoint. For a detailed guide on checking the allowance, see
[Get the Fast Transfer allowance](/cctp/howtos/get-fast-transfer-allowance).
## When allowance is insufficient
If the Fast Transfer allowance is insufficient for your transfer, you have two
options:
### Option 1: Wait for replenishment
The allowance automatically replenishes as pending Fast Transfers reach hard
finality.
Monitor the allowance until sufficient capacity is available:
```ts TypeScript theme={null}
async function waitForAllowance(requiredAmount: bigint, 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();
// Convert USDC string to 6-decimal subunits without float precision loss
const [whole, frac = ""] = String(allowance).split(".");
const allowanceSubunits =
BigInt(whole) * 1_000_000n + BigInt((frac + "000000").slice(0, 6));
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");
}
```
### Option 2: Use Standard Transfer
Change `minFinalityThreshold` to 2000 or higher to use
[Standard Transfer](/cctp/concepts/finality-and-block-confirmations#standard-transfer-attestation-times),
which doesn't consume the Fast Transfer allowance.
## Allowance lifecycle
Understanding what happens during each phase of the allowance lifecycle helps
you build more robust applications:
The user calls `depositForBurn` with `minFinalityThreshold` ≤ 1000. The
transaction confirms on the source blockchain, and the allowance is debited by
the burn amount.
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.
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.