> ## 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. # How signing APIs work > Learn how Circle's signing APIs work for developer-controlled wallets when you manage broadcasting yourself. Circle provides signing APIs that allow developer-controlled wallets to sign transactions, messages, or typed data. Use these APIs when Circle doesn't provide full [blockchain infrastructure](/wallets/blockchain-infrastructure) for your blockchain, or when you prefer to manage broadcasting yourself. For step-by-step instructions, see [Sign Transactions on EVM Chains](/wallets/sign-tx-evm), [Solana](/wallets/sign-tx-solana), or [NEAR](/wallets/sign-tx-near). ## Signing endpoints Circle provides four signing endpoints for developer-controlled wallets. You call one endpoint per request, depending on what you are signing: * [Sign transaction](/api-reference/wallets/developer-controlled-wallets/sign-transaction): Sign a raw transaction for broadcasting to a blockchain. * [Sign message](/api-reference/wallets/developer-controlled-wallets/sign-message): Sign an arbitrary message using EIP-191 (EVM) or a plain message (NEAR). * [Sign typed data](/api-reference/wallets/developer-controlled-wallets/sign-typed-data): Sign structured data using EIP-712 (EVM only). * [Sign delegate action](/api-reference/wallets/developer-controlled-wallets/sign-delegate-action): Sign a delegate action to authorize another account to act on your behalf (NEAR only). ## The signing flow To sign and broadcast a transaction: 1. Build the raw transaction in your application, including nonce, gas parameters, and any blockchain-specific encoding. For example, NEAR requires Borsh serialization before signing. 2. Call the [Sign transaction](/api-reference/wallets/developer-controlled-wallets/sign-transaction) endpoint. Circle signs the transaction and returns the signed payload. 3. Broadcast the signed transaction to a node provider of your choice. The following diagram illustrates this flow. ```mermaid theme={null} sequenceDiagram participant App as Your application participant Circle as Circle Signing API participant Node as Your node provider App->>Circle: Send raw transaction to signing endpoint Circle-->>App: Return signed transaction App->>Node: Broadcast signed transaction Node-->>App: Transaction hash ``` When you manage broadcasting, you are also responsible for monitoring transaction status, deposit detection, and balance retrieval. ## Signing for EVM wallets For blockchains in the [Supported blockchains](/wallets/supported-blockchains) Other EVM category, create a wallet using `blockchain: EVM` for mainnet or `blockchain: EVM-TESTNET` for testing. These are virtual blockchain parameters not tied to a specific blockchain. You specify the target blockchain using `chainId` at signing time. For a list of supported testnet chain IDs, see [EVM testnet chain IDs for signing](/wallets/sign-tx-chain-id). An EVM wallet shares the [same address](/wallets/unified-wallet-addressing-evm) as other wallets in the same wallet set. If you sign a transaction targeting a blockchain by [chain ID](/wallets/sign-tx-chain-id), the transaction executes from the wallet on that blockchain. For example, signing a transaction with chain ID `42161` executes from your Arbitrum wallet, even if you intended to sign for a different blockchain. Verify the chain ID before signing to avoid unintended transactions and stuck funds. ## SCA wallets and lazy deployment [Smart contract account (SCA) wallets](/wallets/account-types#smart-contract-accounts-sca-and-msca) use lazy deployment. The wallet contract is not deployed onchain until the first transaction. Calls to [Sign message](/api-reference/wallets/developer-controlled-wallets/sign-message) or [Sign typed data](/api-reference/wallets/developer-controlled-wallets/sign-typed-data) before the wallet is deployed will fail. The wallet must complete at least one onchain transaction before signing arbitrary messages or typed data.