> ## 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. # Use EURC and cirBTC with Gateway > Differences between USDC and the EURC and cirBTC assets in Gateway, including supported blockchains, the API token field, decimals, fees, and Nanopayments. EURC and cirBTC use the same unified balance, transfer flow, and API as USDC. Each asset keeps its own balance, and a transfer must use the same asset on the source and the destination. Existing USDC integrations are unchanged. The following sections cover only what differs from USDC. ## Balances and transfers by asset Gateway tracks a separate balance for each asset. When you deposit EURC, Gateway credits a EURC balance, and a cirBTC deposit credits a cirBTC balance. Gateway never converts between assets. A transfer burns an asset on the source blockchain and mints the same asset on the destination blockchain, so both blockchains must support the asset. ## Supported blockchains by asset Each asset is available on a subset of the Gateway blockchains. cirBTC is not available on Solana. | Asset | Mainnet | Testnet | | - | - | - | | EURC | Arc, Ethereum, Avalanche, Base, Solana, World Chain | Arc Testnet, Ethereum Sepolia, Avalanche Fuji, Base Sepolia, Solana Devnet, World Chain Sepolia | | cirBTC | Arc, Ethereum | Arc Testnet, Ethereum Sepolia | EURC is also issued on Cronos and Stellar, but Gateway is not offered on those blockchains. For token addresses, see [EURC contract addresses](https://developers.circle.com/stablecoins/eurc-contract-addresses.md) and [cirBTC contract addresses](https://developers.circle.com/assets/cirbtc-contract-addresses.md). For the USDC matrix, see [Gateway supported blockchains](https://developers.circle.com/gateway/references/supported-blockchains.md). The Gateway Wallet and Gateway Minter contracts are the same as for USDC. For their addresses, see [Gateway contract addresses](https://developers.circle.com/gateway/references/contract-addresses.md). ## Asset selection in API requests The `/v1/balances` and `/v1/deposits` requests select the asset with a `token` field. The accepted values are `USDC`, `EURC`, and `cirBTC`. The casing must match exactly, so the API rejects `CIRBTC`. When `token` is omitted, Gateway uses USDC. The following example queries a EURC balance on Arc Testnet (domain `26`): ```bash theme={null} curl -X POST https://gateway-api-testnet.circle.com/v1/balances \ -H "Content-Type: application/json" \ -d '{ "token": "EURC", "sources": [ { "domain": 26, "depositor": "" } ] }' ``` ```json theme={null} { "token": "EURC", "balances": [ { "domain": 26, "depositor": "", "balance": "10.000000" } ] } ``` The `/v1/transfer` and `/v1/estimate` requests identify the asset through token contract addresses instead. Each burn intent carries a `sourceToken` and a `destinationToken`, which hold the address of the asset on the source and destination blockchains. On Solana, `destinationRecipient` must be the token account for the asset you transfer. ## Decimals Each asset uses its own number of decimal places. Amounts, fees, and gas values are all expressed in the asset's atomic units. | Asset | Decimals | | - | - | | USDC | 6 | | EURC | 6 | | cirBTC | 8 | > [!WARNING] > > Treating cirBTC as a 6-decimal asset makes amounts 100 times too large or too > small. Decimals are specific to each asset, so the USDC value doesn't carry > over. ## Fees Gateway charges fees and gas in the token you transfer. A cirBTC transfer pays its fees in cirBTC, and `maxFee` is in that token's atomic units. The `/v1/estimate` response is the authoritative fee for a transfer. For the fee model, see [EURC and cirBTC fees](https://developers.circle.com/gateway/references/fees.md#eurc-and-cirbtc-fees). ## Forwarding Service The Forwarding Service supports USDC, EURC, and cirBTC. For details, see [Forwarding Service](https://developers.circle.com/gateway/references/forwarding-service.md). ## Nanopayments Nanopayments extends to EURC and cirBTC on EVM blockchains only. Non-USDC assets require EIP-3009 version 2 authorizations. | Asset | Nanopayments blockchains | | - | - | | EURC | Arc, Avalanche, Base, Ethereum, World Chain | | cirBTC | Arc, Ethereum | For the version 2 signing details, see [EIP-3009 signing](https://developers.circle.com/gateway-nanopayments/howtos/eip-3009-signing.md). The `/x402/supported` endpoint returns the live list of supported assets for each network. ## Deposit and withdrawal safety The safety rules for USDC apply to every asset. A direct ERC-20 transfer to the Gateway Wallet contract loses the funds, so deposits must go through the Gateway deposit methods. Trustless withdrawal and its delay also apply to every asset.