Digital Health

ClawHub x402 Payments (USDC via PayAI + DHM via EVVM)

This skill documents the two x402 payment flows in the NHS EVVM / ClawHub app: USDC via PayAI Echo and DHM via EVVM native. Reference implementation lives in this repo.

Reference paths

Chain: Base Sepolia (chainId 84532).

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Flow 1: USDC x402 via PayAI Echo

PayAI returns 402 with an accepts array (not options). Client picks a USDC option, builds EIP-3009 TransferWithAuthorization, signs EIP-712, sends signature in PAYMENT-SIGNATURE header, retries the same URL; server returns 200 and may set PAYMENT-RESPONSE header with result (e.g. transaction hash).

Client steps

1. Request resource
   GET (e.g. https://x402.payai.network/api/base-sepolia/paid-content).

2. Parse 402

   • Prefer PAYMENT-REQUIRED response header (base64-encoded JSON).
   • Fallback: response body may be JSON with accepts array.
   • Type: { x402Version?, error?, resource?, accepts: Array<{ scheme, network, amount, asset, payTo, maxTimeoutSeconds?, extra? }> }.

3. Pick USDC option

   • From accepts, choose entry where asset matches USDC on Base Sepolia or extra.name === "USDC".
   • Use amount, asset, payTo, extra.name / extra.version for EIP-712.

4. Build EIP-3009 authorization

   • Domain: name = extra?.name ?? "USDC", version = extra?.version ?? "2", chainId = 84532, verifyingContract = asset.
   • Type: TransferWithAuthorization: from, to, value, validAfter (0), validBefore (e.g. now + 300s), nonce (32 random bytes as hex).
   • Sign with signTypedData (EIP-712).

5. Send payment and retry

   • Build payload: { x402Version: 2, scheme, network, accepted: { scheme, network, amount, asset, payTo, maxTimeoutSeconds, extra? }, payload: { signature, authorization: message }, extensions: {} }.
   • PAYMENT-SIGNATURE = base64(JSON.stringify(payload)).
   • Same URL: GET with header PAYMENT-SIGNATURE: .

6. Read result

   • On 200: body is content. Optional PAYMENT-RESPONSE or X-PAYMENT-RESPONSE header (base64 JSON) may contain transaction (tx hash) etc.

Config

• VITE_X402_USDC_ECHO_URL: PayAI Echo endpoint (default: https://x402.payai.network/api/base-sepolia/paid-content).
• USDC on Base Sepolia: 0x036CbD53842c5426634e7929541eC2318f3dCF7e.

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Flow 2: DHM x402 via EVVM native

Server returns 402 with PAYMENT-REQUIRED: 1 and a JSON body containing options (EVVM pay options with to, suggestedNonce, etc.). Client signs an EVVM pay message (personal_sign), POSTs to server’s payment endpoint; server executes pay() on EVVM Core and returns content + txHash.

Server (402 + payment endpoint)

1. Protected resource
   GET /clinical/mri-slot (or similar): if not paid, respond with 402, PAYMENT-REQUIRED: 1, and body:

   • resource, description, to (recipient address), suggestedNonce
   • options: array with at least one option: id, type: "evvm_pay", chainId, evvmId, coreAddress, token (DHM), to, suggestedNonce, amount, priorityFee, executor (or null), isAsyncExec.

2. Payment execution
   POST /payments/evvm/dhm body: from, to, toIdentity, token, amount, priorityFee, executor, nonce, isAsyncExec, signature.
   Server calls EVVM Core pay(...) with executor key, waits for receipt, returns { status, txHash, content }.

Client steps

1. Request resource
   GET /clinical/mri-slot.

2. Detect 402
   res.status === 402 or res.headers.get("PAYMENT-REQUIRED") === "1". Parse body as JSON: { resource, description?, to, suggestedNonce?, options }.

3. Pick option
   options.find(o => o.type === "evvm_pay" || o.id === "dhm-evvm") ?? options[0]. Ensure to and suggestedNonce are present.

4. Build EVVM pay message

   • Hash payload for Core: keccak256(encodeAbiParameters("string, address, string, address, uint256, uint256", ["pay", to, toIdentity, token, amount, priorityFee])).
   • Message string: evvmId, coreAddress, hashPayload, executor, nonce, isAsyncExec (comma-separated).
   • Use buildEvvmPayMessageCoreDoc from frontend/src/lib/evvmSign.ts with: evvmId, coreAddress, to, "", token, amount, priorityFee, executor, nonce, isAsyncExec.

5. Sign and submit

   • signMessage (personal_sign) the message string.
   • POST to POST /payments/evvm/dhm with JSON body: from, to, toIdentity: "", token, amount, priorityFee, executor, nonce, isAsyncExec, signature.
   • Response 200: content (unlocked resource), txHash.

Config

• VITE_X402_SERVER_URL: DHM x402 server (e.g. https://evvm-x402-dhm.fly.dev or localhost).
• Server env: EXECUTOR_PRIVATE_KEY, RPC_URL, RECIPIENT_ADDRESS, EVVM_ID, EVVM_CORE_ADDRESS, DHM_TOKEN_ADDRESS (see server/.env.example).

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Checklist for adding or debugging

USDC (PayAI)

• 402 parsed from header or body; accepts used (not options).
• EIP-712 domain and TransferWithAuthorization match USDC contract (name/version from extra or "USDC"/"2").
• PAYMENT-SIGNATURE is base64 JSON; same URL retried with GET + header.
• PAYMENT-RESPONSE decoded when present for tx hash / receipt.

DHM (EVVM)

• 402 body has options[].to and suggestedNonce; client uses them in the signed message.
• Message built with hashDataForPayCore + buildEvvmMessageV3 (see evvmSign.ts).
• POST body matches server expectation (from, to, token, amount, nonce, executor, isAsyncExec, signature).
• Server has EXECUTOR_PRIVATE_KEY and RPC to submit pay().

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Quick copy-paste (types)

PayAI 402 (accepts):

type PaymentRequirement = {
  scheme: string;
  network: string;
  amount: string;
  asset: string;
  payTo: string;
  maxTimeoutSeconds?: number;
  extra?: { name?: string; version?: string; [k: string]: unknown };
};
// 402 body: { x402Version?, error?, resource?, accepts: PaymentRequirement[] }

EVVM 402 (options):

type PaymentOption = {
  id: string;
  type: string;
  chainId: number;
  evvmId: string;
  coreAddress: string;
  token: string;
  to?: string;
  suggestedNonce?: string;
  amount: string;
  priorityFee: string;
  executor: string | null;
  isAsyncExec: boolean;
};
// 402 body: { resource, description?, to?, suggestedNonce?, options: PaymentOption[] }

For full code, see the reference paths at the top of this skill.

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Homework for hackathon: agent-to-agent with Privy

The flows above use a browser wallet (human-in-the-loop). Participants can extend the app so an agent can pay autonomously using the Privy Agentic Wallets skill.

Leverage the Privy skill

• Skill: privy-io/privy-agentic-wallets-skill — create server wallets that AI agents control with policy guardrails; sign and send transactions via the Privy API (no user click).
• Install in project:
  git clone https://github.com/privy-io/privy-agentic-wallets-skill.git .cursor/skills/privy
  (or into ~/.openclaw/workspace/skills/privy for OpenClaw). Add PRIVY_APP_ID and PRIVY_APP_SECRET from dashboard.privy.io.

Homework tasks

1. Same protocol, different signer
   Keep the x402 protocol (402 → build payload → sign → POST) unchanged. The only change is who signs: instead of signMessageAsync / signTypedDataAsync in the browser, the agent path uses the Privy API to sign with a Privy server wallet (same message / typed data).

2. DHM agent payer

   • Create a Privy server wallet on Base Sepolia (via Privy skill) with a policy that limits spending (e.g. max amount, or only EVVM Core + your x402 server).
   • Implement an agent path: GET 402 from /clinical/mri-slot → build EVVM pay message (reuse buildEvvmPayMessageCoreDoc) → sign the message via Privy’s sign API (see Privy skill references) → POST to /payments/evvm/dhm with the same body.
   • Expose this as a small backend route or script that the agent calls (e.g. “pay for MRI slot as agent”), so the same resource can be unlocked without a connected browser wallet.

3. USDC agent payer (optional)

   • Same idea for PayAI Echo: GET 402 → pick USDC option → build EIP-3009 TransferWithAuthorization → sign via Privy’s sign typed data API (EIP-712) → send PAYMENT-SIGNATURE and retry.
   • Use a Privy server wallet with a policy that restricts to the PayAI/USDC flow if desired.

4. Dual mode (stretch)

   • In the UI or API, support both “Pay as me” (current wallet) and “Pay as agent” (Privy server wallet). Shared: 402 parsing and payload building; only the signer (browser vs Privy) differs.

Why this fits the skill

• The protocol (x402, EVVM pay, EIP-3009) stays the same; the skill above is the single source of truth for payloads and endpoints.
• The Privy skill adds how to get an agent-owned wallet and how to sign with it. Combining both skills gives hackathon participants a clear path: learn x402 from this skill, add autonomous payers using the Privy skill.