> ## 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. # Transaction throughput > How per-wallet queue limits and Circle's queue shape blockchain transaction throughput, how to increase it, and how to resolve stuck transactions. For API request rate limits (`GET`/`POST` calls per second on Wallets endpoints), see [Wallets API rate limits](/api-reference/wallets/rate-limits). ## Per-wallet queue limits EVM blockchains use a mempool that queues pending transactions per sender address, up to a limit set by full nodes. Theoretical throughput is `queueLimit / blockTime`, though priority fees, congestion, and network liveness affect actual inclusion. Solana doesn't use a mempool and has no per-account queue limit, which is one reason its throughput is significantly higher. | Blockchain | EOA wallet | SCA wallet | | - | - | - | | Ethereum | 32 transactions | 1 transaction | | Polygon PoS | 16 transactions | 1 transaction | | Avalanche | 32 transactions | 1 transaction | | Solana (no mempool) | Unlimited | Not supported | ## Circle Wallets queue To smooth over short bursts that exceed the blockchain limit, Circle Wallets queues additional transactions for a bounded duration. The service caps how many transactions a single wallet can hold in this queue. When you hit the cap, the API returns [error code `155264`](/api-reference/wallets/error-codes): > Wait for pending transactions to be included on the blockchain before > submitting new requests. EVM chains restrict the number of queued transactions > per sender address. ## Monitor pending transactions Use webhooks or the transactions API to track state changes on pending transactions. * **Webhooks** (recommended): subscribe to [webhook notifications](/api-reference/webhooks) for real-time state updates. More efficient than polling. * **Transactions API**: call [get transaction](/api-reference/wallets/developer-controlled-wallets/get-transaction) by ID, or [list transactions](/api-reference/wallets/developer-controlled-wallets/list-transactions) filtered by state (`INITIATED`, `CLEARED`, `QUEUED`, `SENT`, or `STUCK`). ## Increase throughput Three strategies scale throughput past the per-wallet ceiling: * **Raise the fee level**: setting `feeLevel: MEDIUM` or `HIGH` prioritizes inclusion, which shortens confirmation time and reduces backpressure on subsequent transactions from the same wallet. See [Gas fees](/wallets/gas-fees) for how fee levels work. * **Shard across multiple sender wallets**: splitting sends across multiple wallets bypasses the per-wallet queue limit and increases parallelism. Works on every supported blockchain. * **Batch with smart contracts**: for repetitive workloads, batch operations in a single contract call instead of sending individual transactions. For example, an airdrop contract that processes 500 transfers per call reduces 1,000,000 individual transfers to 2,000 batched calls. A batched call costs more gas per call than a single transfer, but the total gas and time saved make batching more efficient at scale. See the [Airdrop template for Circle Contracts](/contracts/airdrop) for a working example. ## Resolve stuck transactions A `STUCK` transaction has been sent to the blockchain but can't be included in a block because the network fee is below the current floor. This usually happens when gas prices spike during congestion, or when the `feeLevel` (or custom `priorityFee`/`maxFee`) is set too low. Resolve a stuck transaction two ways: * **Accelerate**: call the [accelerate transaction API](/api-reference/wallets/developer-controlled-wallets/create-developer-transaction-accelerate) to resubmit at the `HIGH` fee level. * **Cancel**: call the [cancel transaction API](/api-reference/wallets/developer-controlled-wallets/create-developer-transaction-cancel) before the transaction is mined. Cancellation also incurs gas fees. Monitor the new state using [webhook notifications](/api-reference/webhooks) (recommended) or by polling the list transactions API. If a transaction stays `STUCK` and you take no action, it can block subsequent transactions from the same wallet. `STUCK` is a signal for intervention, not a terminal failure. For the full state reference, see [Transaction states and errors](/api-reference/wallets/transaction-states-and-errors).