> ## 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-to: Sign a seller proof

> Construct the Facilitator-Seller-Proof header for keyless Facilitator Service requests

Construct a `Facilitator-Seller-Proof` header for keyless Facilitator Service
requests. The proof carries an EIP-712 signature that proves you control `payTo`
and binds the request to a specific purpose and body.

## Prerequisites

Before you begin, ensure that you've:

* Read [The keyless trial](/facilitator-service/keyless-trial.md) so you understand
  the trial allowance
* Obtained the private key controlling `payTo`, or a deployed ERC-1271 smart
  contract account at `payTo`
* Installed [viem](https://viem.sh/), or a comparable EIP-712 signing library

## Steps

**1. Construct the EIP-712 domain**

The domain anchors the signature to Facilitator Service.

```typescript sign-proof.ts theme={null}
const domain = {
  name: "Circle Facilitator Seller Request",
  version: "1",
  chainId: 5042002, // EIP-155 chain ID for the payment's network
};
```

Set `chainId` to the numeric EIP-155 chain ID for the `network` you're settling
on.

**2. Define the typed data**

The `SellerRequest` struct is what Facilitator Service reconstructs and verifies
against the signature.

```typescript sign-proof.ts theme={null}
const types = {
  SellerRequest: [
    { name: "purpose", type: "string" },
    { name: "method", type: "string" },
    { name: "bodyHash", type: "bytes32" },
    { name: "network", type: "string" },
    { name: "payTo", type: "address" },
    { name: "nonce", type: "bytes32" },
    { name: "issuedAt", type: "uint64" },
    { name: "expiresAt", type: "uint64" },
  ],
};
```

**3. Populate the message**

```typescript sign-proof.ts theme={null}
import { keccak256, toBytes, toHex } from "viem";

const nonce = crypto.getRandomValues(new Uint8Array(32));
const issuedAt = Math.floor(Date.now() / 1000);

const message = {
  purpose: "settle", // "verify", "settle", or "status"
  method: "POST", // uppercase HTTP method
  bodyHash: keccak256(toBytes(body)), // keccak256 of raw HTTP body, GET hashes empty bytes
  network: "eip155:5042002",
  payTo: "0x7c3ea945Fc4253255d8260fc18c2dee3D8C5dD3A",
  nonce: toHex(nonce),
  issuedAt: BigInt(issuedAt),
  expiresAt: BigInt(issuedAt + 300),
};
```

The signed message must meet these rules:

* `purpose` must match the route you're calling.
* On [`/verify`](/api-reference/facilitator-service/all/verify-payment) and
  [`/settle`](/api-reference/facilitator-service/all/settle-payment), `network`
  and `payTo` must equal the request body's `paymentRequirements.network` and
  `paymentRequirements.payTo`. A mismatch returns HTTP 401.
* `issuedAt` may be at most 30 seconds ahead of the current time.
* `expiresAt` must be later than the current time and no more than 5 minutes
  after `issuedAt`.
* A nonce is unique across purposes in one `(network, payTo)`. Exact retries
  reuse the nonce. Reuse with a different digest returns HTTP 401.

**4. Sign the typed data**

```typescript sign-proof.ts theme={null}
import { privateKeyToAccount } from "viem/accounts";

const account = privateKeyToAccount(
  process.env.SELLER_PRIVATE_KEY as `0x${string}`,
);

const signature = await account.signTypedData({
  domain,
  types,
  primaryType: "SellerRequest",
  message,
});
```

EOA signatures are recovered to `payTo`. Deployed smart contract accounts at
`payTo` are validated through ERC-1271.

**5. Base64url-encode the envelope**

The envelope carries the signature and the fields Facilitator Service needs to
reconstruct what was signed.

```typescript sign-proof.ts theme={null}
const envelope = {
  version: 1,
  signature,
  network: message.network,
  payTo: message.payTo,
  nonce: message.nonce,
  issuedAt: Number(message.issuedAt),
  expiresAt: Number(message.expiresAt),
};

const header = Buffer.from(JSON.stringify(envelope)).toString("base64url");
```

Send `header` as the `Facilitator-Seller-Proof` header on your Facilitator
Service request.