Set up the x402 payment middleware

Before you can process KYC/AML checks via x402, you need a functional payment layer. This middleware acts as the bridge between your API and the blockchain, handling the heavy lifting of transaction verification. Think of it as the security guard at the door: it ensures the entry fee is paid before letting the request through to your compliance logic.

We will use the official Coinbase Developer Platform (CDP) SDK, as it provides the most reliable integration path for x402 endpoints. The setup is straightforward but requires precise configuration to avoid runtime errors during compliance checks.

1
Install the CDP SDK

Start by adding the Coinbase CDP SDK to your project. This package contains the core libraries needed to construct and sign x402 transactions. Run the following command in your terminal to install the latest stable version:

Shell
Shell
npm install @coinbase/developer-sdk

If you are using TypeScript, ensure you also install the type definitions to get proper autocomplete and error checking during development.

2
Initialize the CDP Client

Next, initialize the CDP client with your API credentials. You will need your API key and secret from the Coinbase Developer Console. These credentials authenticate your server to the x402 network.

JavaScript
JavaScript
import { CdpClient } from '@coinbase/developer-sdk';

const cdpClient = new CdpClient({
  apiKeyId: process.env.CDP_API_KEY_ID,
  apiKeySecret: process.env.CDP_API_KEY_SECRET,
});

Store these credentials in environment variables, never in your source code. This step establishes the authenticated session required for all subsequent middleware operations.

3
Configure the x402 Middleware

Finally, wire up the x402 middleware in your application router. This middleware intercepts incoming requests, verifies the payment transaction on-chain, and only proceeds to your KYC/AML logic if the payment is confirmed.

JavaScript
JavaScript
import { x402Middleware } from '@coinbase/developer-sdk';

app.use('/api/kyc-check', x402Middleware({
  network: 'base-sepolia', // Use 'base-mainnet' for production
  requiredAmount: '0.01',
  currency: 'ETH',
}));

The middleware will automatically handle transaction verification, reducing the risk of unpaid or fraudulent compliance requests reaching your backend systems.

With the middleware configured, your API is ready to accept x402 payments. The next step is to define the specific KYC/AML checks that will be triggered upon successful payment.

Integrate KYC verification logic

x402 Endpoints for KYC/AML Checks works best as a clear sequence: define the constraint, compare the realistic options, test the tradeoff, and choose the path with the fewest hidden costs. That order keeps the advice usable instead of decorative. After each step, pause long enough to check whether the recommendation still fits the reader's actual situation. If it depends on perfect timing, unusual access, or a best-case budget, include a simpler fallback.

1
Define the constraint
Name the space, budget, timing, or skill limit that shapes the x402 Endpoints for KYC/AML Checks decision.
2
Compare realistic options
Use the same criteria for each option so the tradeoff is visible.
3
Choose the practical path
Pick the option that still works after cost, maintenance, and fallback needs are included.

Choose a payment scheme for KYC settlement

x402 endpoints support three distinct settlement patterns. Your choice dictates how funds move from the buyer to your KYC/AML service provider. Selecting the wrong scheme can lead to stranded funds or excessive gas fees for small checks.

Exact settlement

Exact settlement charges the precise amount specified in the PaymentPayload. This is the standard for most API calls. It works best when the KYC fee is fixed, such as a flat rate for a standard identity verification.

Upto settlement

Upto settlement allows the buyer to pay up to a maximum limit. The actual charged amount is determined by the service provider during the settlement phase. Use this for tiered KYC services where the final cost depends on the depth of the background check performed.

Batch settlement

Batch settlement aggregates multiple payment requests into a single on-chain transaction. This reduces gas costs significantly. It is ideal for high-volume KYC workflows, such as onboarding thousands of users simultaneously.

SchemeBest ForFlexibility
ExactFixed feesLow
UptoVariable depth checksHigh
BatchHigh-volume onboardingMedium
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Test endpoints with simulated transactions

Before routing real user data, you need to prove your integration handles the x402 payment flow and KYC checks correctly. This phase isolates your logic from live market volatility and regulatory penalties. We will walk through simulating the payment payload, triggering the KYC status response, and verifying the final state.

Simulate the payment payload

The x402 specification relies on a signed PaymentPayload to authorize access. You do not need real funds to test the endpoint logic; you only need a valid signature structure. Use a test wallet to sign a payload that mimics the expected transaction parameters.

  1. Generate a test PaymentPayload with a mock transaction hash.
  2. Sign the payload using your test private key.
  3. Send the signed payload to your endpoint as if it were a live request.

This step confirms your server can parse the signature and extract the necessary user context. According to the x402 documentation, every payload is signed by the buyer and settled directly onchain, so your integration must validate this signature before proceeding [src-serp-2].

Verify KYC status responses

Once the payment payload is accepted, your endpoint should trigger or check the KYC status. This is where you validate that the user meets the regulatory requirements for your service. Use a simulated KYC provider or a mock database to return different statuses (e.g., approved, pending, rejected).

Test these scenarios:

  • Approved: The user gains immediate access to the protected resource.
  • Pending: The endpoint returns a 402 Payment Required or a specific 403 Forbidden with a retry-after header, prompting the user to wait.
  • Rejected: The endpoint denies access and logs the reason for compliance auditing.

Run the pre-launch checklist

Before moving to production, ensure your integration passes these critical checks. This checklist ensures you are not just testing code, but testing compliance logic.

  • Verify signature validation rejects malformed payloads
  • Confirm KYC status updates trigger correct HTTP responses
  • Test timeout handling for slow KYC provider responses
  • Log all failed KYC checks for audit trails
  • Ensure no PII is stored in plain text during testing

By following this sequence, you ensure your x402 endpoints are robust, compliant, and ready for real-world transactions.

Frequently asked: what to check next

Can I use x402 to satisfy KYC and AML requirements?

Yes. x402 integration allows you to satisfy Know Your Customer (KYC) and Anti-Money Laundering (AML) requirements by embedding verification status directly into the transaction flow. This ensures that only compliant users can initiate or receive payments, keeping your service within regulatory bounds from the start.

How do zero-knowledge proofs (ZKPs) work in x402?

For x402 transactions, users can prove a specific attribute—such as being over 18 or located in a permitted jurisdiction—without revealing their actual identity or personal data. This allows you to verify compliance without storing sensitive PII, reducing your liability and privacy risks.

What prevents a malicious facilitator from stealing funds?

Every x402 PaymentPayload is signed by the buyer and settled directly onchain. This cryptographic signature ensures that the buyer authorizes the specific amount and recipient, preventing intermediaries from altering the settlement terms or diverting funds.