> ## 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. # Agent marketplace > A curated, compliance-first catalog of x402 services that accept USDC, built for AI agents to discover and pay per request. The Agent Marketplace is a curated catalog of [x402](/x402-facilitators/x402)-compatible services that accept USDC. Every listing is screened (each seller's payout wallet is continuously sanctions-checked) and continuously health-checked, so your agent finds services that are compliant and live. Buyers and sellers meet here. Buyers (AI agents) find services and pay per request, with no subscriptions, no accounts, and no API keys. Sellers (API owners) list a service once and get paid in USDC for each call. Browse the catalog at [agents.circle.com/services](https://agents.circle.com/services), or query it programmatically with the [Discovery API](/agent-stack/agent-marketplace/discovery-api). ## Explore Query the public catalog over one HTTP endpoint. Filter by network, price, payment rail, and protocol type. No API key. Monetize your API by charging agents per request in USDC. Scaffold it in minutes with a Circle Skill. Submit your service for review. Once approved, it appears in the catalog and the Discovery API. Explore 600+ live services across 15+ blockchain networks in your browser. ## How a paid request works Every service in the catalog follows the same [x402](/x402-facilitators/x402) handshake: 1. An agent calls the service with no payment. The service responds `402 Payment Required`, plus a machine-readable price list (the `accepts` array). Each entry states which token (`asset`), on which blockchain (`network`), how much (`amount`), and to which address (`payTo`). 2. The agent's wallet picks an option it can pay, signs a USDC payment, and retries the request with the signed payment attached. 3. The service settles the payment and returns the real response. No accounts, no API keys: the payment is the authentication. ## Why the Agent Marketplace * **One integration, many blockchains**: A single query returns services priced across 15+ networks. No per-chain registry hunting. * **Curated, not scraped**: Filter 600+ services by query, network, price, payment rail, and protocol type, instead of parsing thousands of raw endpoints. * **Compliance built in**: Each seller's payout wallet is continuously sanctions-screened. Blocked sellers drop out, so you inherit a watchlist-clean catalog. * **Agent-native responses**: Each listing ships structured payment requirements plus input and output JSON Schema, so any wallet can construct and execute payment without a human reading docs. * **Live, not stale**: Endpoints are continuously health-checked, and unreachable ones are auto-excluded. * **Standards-based**: CAIP-2 networks, the x402 protocol, the [x402 Bazaar](https://docs.x402.org/extensions/bazaar) discovery schema, and OpenAPI and A2A discovery surfaces. No API key, no account.