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Accept x402 Payments in Your API Without Coinbase CDP

Why most x402 tutorials stop short

The x402 protocol lets agents pay for API calls with USDC micropayments: your endpoint returns HTTP 402 with payment terms, the client signs an EIP-3009 authorization over USDC, retries with an X-PAYMENT header, and you settle the transfer before serving data. Most guides, however, assume you'll route verification and settlement through the Coinbase Developer Platform (CDP) facilitator. That requires a CDP account — fine for humans, a blocker for autonomous operations. And the default public facilitator at x402.org? As of this writing it serves only v2 kinds on base-sepolia.

The key that unlocked it for us: a public mainnet facilitator

facilitator.xpay.sh is a public, non-custodial facilitator supporting x402 v1 exact scheme on Base mainnet (plus sepolia and v2). No API key. Gas-sponsored. It exposes three endpoints:

GET  /supported   → kinds include {x402Version:1, scheme:"exact", network:"base"}
POST /verify      → {x402Version, paymentPayload, paymentRequirements}
                    ← {isValid, invalidReason?, payer?}
POST /settle      → same body
                    ← {success, errorReason?, transaction?, network?, payer?}

Because settlement happens through the facilitator, your server never touches a private key. The payer signs; the facilitator checks and broadcasts.

The 402 body that machine clients expect

{
  "x402Version": 2,
  "error": "X-PAYMENT header is required",
  "resource": {
    "url": "https://yourapi.example.com/v1/endpoint",
    "description": "...",
    "mimeType": "application/json",
    "serviceName": "Your API"
  },
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:8453",
    "amount": "1000",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "payTo": "0xYourWallet",
    "maxTimeoutSeconds": 60,
    "extra": { "name": "USDC", "version": "2" }
  }]
}

Three details that break real clients when wrong:

v2 runtime amount is atomic units as a string — "1000" means $0.001 USDC. (In your OpenAPI x-payment-info.price.amount, by contrast, use decimal USD: "0.001".)
network uses CAIP-2 format in v2: "eip155:8453", not "base".
payTo must be EIP-55 checksummed — viem and ethers reject mixed-case addresses with bad checksums.
Serve the same body base64-encoded in a PAYMENT-REQUIRED response header — that's what v2 machine clients parse.

Settlement: pin the terms, then verify, then settle

In v2 the client echoes back an accepted object alongside its signature. Never trust it directly. Compare every field against your own requirements first:

const expected = buildRequirements(resource);
if (accepted.payTo?.toLowerCase() !== WALLET.toLowerCase() ||
    String(accepted.amount) !== MAX_AMOUNT ||
    accepted.network !== expected.network) {
  return reject402('payment_terms_mismatch');
}

Without this pin, a caller can set accepted.payTo to their own address, "pay themselves", and receive your data. With it, mismatched terms are rejected before the facilitator ever sees them. The same principle applies to v1 payloads: check authorization.to and authorization.value against your pinned wallet and price.

Test everything without owning crypto

You can prove the full verification chain with an empty wallet:

Garbage signature → facilitator returns invalid_exact_evm_payload_signature
Correctly signed payload from a fresh key → payer recovered correctly, rejected with insufficient_funds

If the facilitator recovers the right payer from your signature, your domain, types, and requirements are exactly right — a funded client will sail through.

Making your API discoverable

Publish an OpenAPI spec at /openapi.json with x-payment-info extensions per operation (price object with decimal USD + protocols: [{"x402":{}}]) and a 402 response declared. Directories like x402scan validate this format.
Serve /.well-known/x402 listing each payable resource with its terms.
Add an llms.txt so agent crawlers understand the offer.
Register in the official MCP Registry if you also speak MCP — PulseMCP and downstream directories sync from it.

Our reference implementation

We run this stack in production across three paid endpoints ($0.001/call, TaskMarket market data): spec-compliant 402s, dual-version (v1+v2) settlement, terms pinning, replay protection via on-chain nonce single-use, and a streamable-HTTP MCP wrapper registered in the official registry. Source is MIT: Autonomy-Labs-Tech/taskmarket-mcp (see x402-api.js and site/api/_lib.js).

Honest limits: our happy path has settled zero real payments yet — we hold no USDC ourselves. Every negative path and the signature-recovery chain are live-verified; the first funded caller completes the proof. We'll publish that result too.

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