Set up x402 payment middleware
Before you can route KYC or AML checks through an x402-enabled API, you need to install the necessary dependencies and configure the payment middleware. This middleware acts as the gatekeeper, intercepting HTTP requests to enforce payment before any sensitive compliance data is exposed. By embedding this logic early, you ensure that every agent interaction is backed by a verified transaction, creating a trust layer that is essential for high-stakes regulatory environments.
Start by adding the official x402 SDK to your project. This package provides the core primitives for handling crypto payments within your application logic. For Node.js environments, you can typically install it via npm or yarn. This step lays the groundwork for all subsequent payment verification logic.
Initialize the middleware with your specific API routes. The middleware should be positioned to intercept requests before they reach your KYC/AML processing endpoints. Configure it to recognize the x-payment header, which signals that a payment has been made. This ensures that unverified requests are blocked at the network level, protecting your backend resources.
Link your middleware to the Coinbase Developer Platform (CDP) to handle actual payment verification. The CDP provides the infrastructure to validate on-chain transactions against your API requests. Follow the official quickstart guide to obtain your API keys and set up the webhooks that confirm payment success. This integration is critical for ensuring that payments are final and irreversible before data is released.
Run a local test to verify that the middleware correctly rejects requests without payment and accepts those with valid x-payment headers. Use the CDP testnet to simulate transactions without spending real assets. Confirm that your KYC/AML endpoints remain inaccessible until the payment is confirmed. This step validates that your security posture is robust before moving to production.
Integrate KYC verification logic
Embedding identity verification into your x402 endpoint transforms a standard payment gateway into a compliance checkpoint. Instead of treating KYC/AML checks as a separate, pre-flight hurdle, you gate access to sensitive data behind successful transactions. This ensures that only verified identities can access the resources they are paying for, satisfying regulatory requirements through code rather than paperwork.
The implementation relies on the x402 protocol's ability to attach metadata to transactions. By validating the payer's identity status within the transaction payload, you create a seamless loop where payment and verification happen simultaneously. This approach reduces friction for legitimate users while maintaining a strict audit trail for compliance officers.
Begin by installing the necessary dependencies for your chosen x402 implementation. Most frameworks require a specific middleware package that intercepts incoming requests. Configure this middleware to recognize headers that indicate a pending KYC state. This setup ensures that the verification logic is ready before the first transaction is processed.
Set up the logic to validate Zero-Knowledge Proofs (ZKPs) or standard identity tokens attached to the transaction. According to Concordium’s integration guidelines, x402 transactions can carry these proofs to satisfy KYC/AML requirements without exposing raw personal data. Your endpoint must verify the cryptographic signature of the identity proof before proceeding.
Once the identity proof is validated, grant access to the specific AML data endpoints. If the proof is invalid or missing, the x402 protocol should reject the transaction and return a specific error code. This rejection acts as a natural barrier, preventing unauthorized access to sensitive financial information.
Every successful verification and transaction must be logged immutably. Store the transaction hash, the timestamp, and the result of the KYC check. These logs are critical for compliance audits. Ensure that your logging mechanism is secure and tamper-proof, providing a clear history of who accessed what data and when.
This integration creates a robust security layer. By tying identity verification directly to the payment flow, you eliminate the need for separate, disjointed authentication steps. The result is a smoother user experience that still meets the highest standards of regulatory compliance.
Structure payment schemes for access
When building x402 endpoints for KYC and AML checks, your billing model dictates how the agent handles transactions. The protocol supports two distinct approaches: exact pricing for single, predictable checks, and volume-based billing for high-throughput workflows. Choosing the right structure ensures your API remains cost-effective for agents while covering the operational costs of verification providers.
Exact pricing for single checks
This model is ideal for low-volume scenarios or one-off verification requests. The endpoint returns a fixed x-payment-required header with a precise amount. The agent pays exactly that amount to access the response. This simplicity reduces friction for clients who prefer predictable costs without complex tier calculations.
For example, a standard identity verification might cost exactly $0.50. The agent sends the payment, and the API immediately returns the KYC status. This approach works well for standalone services where usage is sporadic and the cost per transaction is stable.
Volume-based billing for high-throughput workflows
High-volume agents often require dynamic pricing based on usage tiers. Instead of a fixed fee, the endpoint calculates costs based on the number of checks performed within a billing period. This model aligns costs with actual usage, offering discounts for bulk processing.
To implement this, your x402 endpoint must track transaction counts and apply tiered rates. The agent sends payments that accumulate against a balance or credit limit. Once the limit is reached, the endpoint requests a new payment to continue service. This structure supports scalable agent commerce where costs fluctuate with demand.
Comparing billing models
The table below contrasts the two primary billing structures for x402 KYC/AML endpoints. Use this comparison to decide which model fits your agent's operational needs.
| Feature | Exact Pricing | Volume-Based |
|---|---|---|
| Cost Predictability | Fixed per transaction | Variable based on usage |
| Best For | Low-volume, one-off checks | High-throughput, scalable agents |
| Implementation Complexity | Simple header calculation | Requires usage tracking and tier logic |
| Agent Cash Flow | Pay-as-you-go | Prepaid or credit-based limits |
Choose exact for fixed-price verification
When building agent commerce workflows for KYC and AML, you need to know the cost before the transaction starts. This is where fixed-price verification via x402 shines. Unlike dynamic pricing models that fluctuate with network congestion or token volatility, fixed pricing gives you a single, predictable number for every identity check.
For compliance officers, predictability is not just a convenience—it is a requirement. You cannot budget for operational costs if the price of verifying a user’s identity changes with every block. Fixed pricing allows you to calculate your exact cost per user, making it easier to integrate these costs into your broader business model without unexpected margin erosion.
Implementation with Exact Pricing
To implement this, you configure your x402 endpoint to accept a static payment amount. This means your API client sends a specific value (e.g., 0.001 ETH) along with the KYC payload. If the payment matches the expected amount exactly, the verification proceeds. If it does not, the request is rejected with a 402 status code.
This approach simplifies the codebase significantly. You do not need complex logic to calculate dynamic fees or handle partial payments. The x402 specification handles the validation, ensuring that only paid requests trigger the sensitive identity checks. This reduces the attack surface and ensures that your compliance checks are only performed for verified, paid transactions.
Why Fixed Pricing Wins for One-Off Checks
KYC and AML checks are often one-off events. A user submits their documents once to gain access to a platform. They do not need a subscription or a recurring fee structure. Fixed pricing aligns perfectly with this use case. It is simple for the user to understand and easy for the developer to implement.
By using fixed-price verification, you remove the friction of dynamic pricing calculations from the user experience. The agent knows the cost upfront, the user pays a clear fee, and your system receives the payment before processing sensitive data. This creates a transparent, auditable trail that satisfies both technical and regulatory requirements.
Use upto for dynamic billing
High-volume agent commerce requires billing that scales with actual usage rather than fixed subscriptions. The upto scheme in x402 enables this by capping the maximum charge per transaction or session. This approach protects users from unexpected costs while ensuring agents are compensated fairly for high-intensity tasks like complex KYC verifications.
Implementing upto involves setting a max_charge parameter in your x402 payment headers. The agent’s endpoint calculates the actual cost based on the work performed, but the transaction will never exceed the upto limit. If the computed cost is lower, the user pays the lower amount; if it is higher, the transaction is rejected or truncated, preventing overbilling.
This mechanism is particularly useful for tiered access models. For example, a basic identity check might cost $0.01, while a full AML screening could cost $0.50. By setting upto to $0.50, you allow the agent to charge dynamically within that range based on the depth of the check required. This flexibility builds trust with users who need to manage their operational budgets.
Final implementation checklist
Before routing KYC or AML checks through an x402-enabled API, run through this verification list to ensure your integration is secure and compliant.
Ensure all required packages are installed and aligned with the latest protocol standards. Mismatched versions can break the payment handshake.
Set up your payment gateway to handle HTTP 402 responses correctly. This ensures that compliance checks trigger the necessary payment flows without blocking legitimate users.
Run end-to-end tests with mock data to verify that identity checks trigger correctly. Confirm that the system rejects non-compliant requests and accepts verified ones.
Ensure your API enforces strict security headers, including rate limiting and input validation. This prevents abuse and protects sensitive user data during the verification process.
Review your logging mechanism to ensure it captures all necessary audit trails. Regulatory bodies require detailed records of KYC/AML decisions and payment transactions.
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