x402 API Market

The NoelClaw API Market (/api-market in the webapp) lets anyone sell an HTTPS API per call and lets agents buy calls with USDC on Base — no account or API key required. Payments use the x402 protocol pattern: HTTP 402 Payment Required as the challenge, a payment header as the receipt.

How a paid call works

Agent                          NoelClaw                        Seller
  │  1. GET /market/call/slug     │                              │
  │──────────────────────────────>│                              │
  │  2. 402 + PAYMENT-REQUIRED    │                              │
  │<──────────────────────────────│                              │
  │  3. pay USDC on Base          │                              │
  │  4. retry + PAYMENT-SIGNATURE │                              │
  │──────────────────────────────>│  5. verify tx, dedupe        │
  │                               │─────────────────────────────>│
  │  7. 200 + PAYMENT-RESPONSE    │  6. proxy response           │
  │<──────────────────────────────│<─────────────────────────────│

1. Call GET/POST https://api.noelclaw.com/market/call/{slug} with no payment.
2. The server responds 402 with a PAYMENT-REQUIRED header — base64-encoded JSON (x402 v2 shape with an accepts[] array). X-Payment-Required is set too for legacy clients. The challenge includes:

   • network: eip155:8453 (Base mainnet)
   • asset: USDC 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913
   • payTo: the settlement wallet (read it from the challenge — never hardcode)
   • maxAmountRequired: price in USDC micro-units (6 decimals)
   • requestId: unique id for this challenge
3. Pay the amount in USDC on Base to payTo (scheme exact, settled as a plain ERC-20 transfer — the tx hash is your receipt).
4. Retry the same request with the header PAYMENT-SIGNATURE: base64("txHash:requestId"). X-Payment is accepted as a legacy alias.
5. The server verifies the transfer on-chain (recipient + amount), rejects reused tx hashes and request ids, then proxies the request to the seller's private endpoint.
6. The response carries a PAYMENT-RESPONSE header — base64 settlement receipt with the tx hash.

Free listings (price = 0) skip the challenge entirely and proxy directly.

Example

# 1) Challenge
curl -i https://api.noelclaw.com/market/call/my-api

# 2) Pay N USDC on Base to the payTo address from the challenge, then:
SIG=$(printf '0xYOUR_TX_HASH:REQUEST_ID' | base64)
curl https://api.noelclaw.com/market/call/my-api -H "PAYMENT-SIGNATURE: $SIG"

Selling an API

List any HTTPS endpoint from the Sell tab on /api-market:

• The endpoint URL is never exposed to buyers — calls proxy through NoelClaw.
• Pricing is a flat USDC amount per call (cap: 100 USDC).
• Settlement: seller keeps 95%, platform fee 5%. Payout wallet is your NoelClaw Base wallet.
• Every purchase is recorded in a ledger (apiMarketplacePurchases) with request id, tx hash, and fee split — the same dual-write pattern used by Surplus-style markets.

Replay protection

Each payment is single-use, enforced at two levels:

• requestId — one challenge, one fulfillment.
• txHash — one on-chain transfer can never pay for a second call, even with a new request id.

MCP tool payments

The same machinery backs paid MCP tools (X-Payment header on api.noelclaw.com tool routes). All MCP tools are currently free — prices in TOOL_PRICES are set to 0 and can be re-enabled per tool. All x402 flows attach the Base Builder Code (builderCode) for attribution in the Base Dashboard.

Related

• Environment Variables — NOEL_WALLET_ADDRESS, BASE_RPC_URL, BASE_BUILDER_CODE
• Wallet & DeFi