> ## 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. # Paymaster addresses and events This page contains information on the addresses of the Circle Paymaster contracts, and the events the contracts emit during operation. ## Paymaster addresses You can access the permissionless paymaster contracts at the following addresses on supported blockchains: ### Mainnet | Blockchain | Symbol | Paymaster contract address | | - | - | - | | Arbitrum | `ARB` | [`0x6C973eBe80dCD8660841D4356bf15c32460271C9`](https://arbiscan.io/address/0x6C973eBe80dCD8660841D4356bf15c32460271C9) | | Base | `BASE` | [`0x6C973eBe80dCD8660841D4356bf15c32460271C9`](https://basescan.org/address/0x6C973eBe80dCD8660841D4356bf15c32460271C9) | ### Testnet | Blockchain | Symbol | Paymaster contract address | | - | - | - | | Arbitrum Sepolia | `ARB-SEPOLIA` | [`0x31BE08D380A21fc740883c0BC434FcFc88740b58`](https://sepolia.arbiscan.io/address/0x31BE08D380A21fc740883c0BC434FcFc88740b58) | | Base Sepolia | `BASE-SEPOLIA` | [`0x31BE08D380A21fc740883c0BC434FcFc88740b58`](https://sepolia.basescan.org/address/0x31BE08D380A21fc740883c0BC434FcFc88740b58) | ### Mainnet | Blockchain | Symbol | Paymaster contract address | | - | - | - | | Arbitrum | `ARB` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://arbiscan.io/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Avalanche | `AVAX` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://snowtrace.io/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Base | `BASE` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://basescan.org/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Ethereum | `ETH` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://etherscan.io/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Optimism | `OP` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://optimistic.etherscan.io/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Polygon | `MATIC` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://polygonscan.com/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | | Unichain | `UNI` | [`0x0578cFB241215b77442a541325d6A4E6dFE700Ec`](https://unichain.blockscout.com/address/0x0578cFB241215b77442a541325d6A4E6dFE700Ec) | ### Testnet | Blockchain | Symbol | Paymaster contract address | | - | - | - | | Arbitrum Sepolia | `ARB-SEPOLIA` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://sepolia.arbiscan.io/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Avalanche Fuji | `AVAX-FUJI` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://testnet.snowtrace.io/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Base Sepolia | `BASE-SEPOLIA` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://sepolia.basescan.org/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Ethereum Sepolia | `ETH-SEPOLIA` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://sepolia.etherscan.io/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Optimism Sepolia | `OP-SEPOLIA` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://sepolia-optimistic.etherscan.io/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Polygon Amoy | `MATIC-AMOY` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://amoy.polygonscan.com/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | | Unichain Sepolia | `UNI-SEPOLIA` | [`0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966`](https://unichain-sepolia.blockscout.com/address/0x3BA9A96eE3eFf3A69E2B18886AcF52027EFF8966) | ## Paymaster events This section explains the events from the Circle Paymaster and when they are emitted. You can use these events to debug or track the state after submitting a user operation (user op) to a bundler. ### TokenPaymasterV07.sol and TokenPaymasterV08.sol These files are the main contract of the Circle Paymaster that support EntryPoint v0.7 and v0.8. The primary functions they integrate are: * `_validatePaymasterUserOp`: validates the user op and charges the prefund token from the sender before execution. * `_postOp`: refunds the token back to the sender when the actual amount of tokens needed is known after execution. #### UserOperationSponsored The contract emits this event after the `_postOp` is executed and the prefund token (if any) is refunded. **Attributes** | Name | Type | Description | | - | - | - | | `token` | `IERC20` | The ERC-20 token paid by the sender | | `sender` | `address` | The sender address | | `userOpHash` | `bytes32` | The hash of the user op | | `nativeTokenPrice` | `uint256` | The price of 1 ether = 1e18 wei, denominated in token | | `actualTokenNeeded` | `uint256` | The final transaction cost to the smart contract account (SCA), denominated in token | | `feeTokenAmount` | `uint256` | The fee spread used to cover the slippage from exchanging USDC for ETH, denominated in token | **Example** Using [this transaction](https://sepolia.etherscan.io/tx/0x44727688a0bc343ce73b422ac46b2b14a352ccc59f9ff0203588d3718458931e#eventlog) as an example, the `userOperationSponsored` event contains the following data: | Name | Type | Value | | - | - | - | | `token` | `IERC20` | `0x0000000000000000000000001c7d4b196cb0c7b01d743fbc6116a902379c7238` | | `sender` | `address` | `0x000000000000000000000000148cab4b1a7e8c23ae62967cfc8df6292ecf27a8` | | `userOpHash` | `bytes32` | `cc6f71a0ba8d9b72e75c45fae7b830c403b46964eb1d3f8daa0b73d14e6c5b0d` | | `nativeTokenPrice` | `uint256` | `3000000000` | | `actualTokenNeeded` | `uint256` | `4551712` | | `feeTokenAmount` | `uint256` | `0` |