> ## 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 screening Transaction Screening is a real-time tool that automatically checks blockchain transactions and associated addresses for risks related to sanctioned entities, terrorist financing, or other suspicious behaviors. This is a core component of the [Compliance Engine](/wallets/compliance-engine) and it allows you to configure screening rules to meet the specific compliance requirements of your jurisdiction or industry. You can trigger screening evaluations using two methods: * Embedded: screening occurs implicitly during regular programmable wallet transaction flow. * Standalone: screening is triggered explicitly through the Compliance API. Compliance Engine on both testnet and mainnet is only available for eligible customers. Contact Circle through the [Compliance Engine request form](https://www.circle.com/wallets/compliance-engine/#get-in-touch) to get access. ## Features * **Custom rules**: Tailor your own screening rules to match your specific use and regulatory requirements. * **Blocklist/Allowlist**: You can specify blocklisted or allowlisted addresses, enhancing control over transactions and minimizing the chance of interacting with known bad actors. * **Real-time alerts**: The system provides immediate notifications when a transaction violates pre-set screening rules. Alerts can trigger actions such as blocking a transaction, freezing a wallet, or sending it for further manual review. * **Detailed logs and API integration**: You can access detailed logs and results of screened transactions, enabling easier auditing and review. The system also offers a robust API for integrating screening capabilities into existing workflows. ## Embedded screening For Wallets using embedded screening, the existing [Get a transaction](/api-reference/wallets/developer-controlled-wallets/get-transaction) API endpoint includes a response field, `transactionScreeningEvaluation`, for transaction screening result, detailing: * **Rule** found during screening * **Actions** taken for the decision * **Reasons** for the decision, related risk signals found during the screening: source, risk score, risk category, etc. ## Standalone screening If you are using Wallets with standalone screening, Compliance Engine exposes a new [Screen a blockchain address](/api-reference/wallets/compliance/screen-address) API endpoint, enabling you to screen a specific blockchain address on a given [supported blockchain](/wallets/supported-blockchains) independently of any transaction flow. The endpoint response returns the following: * **Screening result**: High level of the screening result, either `APPROVED` or `DENIED`. * **Screening decision**: Same information as the `transactionScreeningEvaluation` field above. * **Detailed vendor response**: Findings from vendors to provide a deeper understanding of the screening results and their source. Note: values will only be available on testnet for the test cases provided on the [Testing guide](/wallets/compliance-engine/tx-screening-testing). ## Workflow Compliance Engine is available for eligible users. Reach out to Circle through the [Compliance Engine request form](https://www.circle.com/wallets/compliance-engine/#get-in-touch) to get access. Once approved, you can: 1. Customize your screening rules via the console to match your own risk and compliance requirements:\ a. Check the [Compliance Engine whitepaper](https://6778953.fs1.hubspotusercontent-na1.net/hubfs/6778953/PDFs/Whitepapers/ComplianceEngine_whitepaper.pdf) (Part 3) for some helpful questions to consider when establishing your requirements.\ b. Check the [Rule management](/wallets/compliance-engine/tx-screening-rule-management) guide for possible configurations. 2. Trigger a screening evaluation:\ a. Through the [Screen a blockchain address](/api-reference/wallets/compliance/screen-address) API endpoint.\ b. Through generating an onchain transaction via the [Transactions API](/api-reference/wallets/developer-controlled-wallets/list-transactions). You can review detailed results for all rule matches and alerts on the [Circle Console](https://console.circle.com/).