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Use the x402 HTTP payment protocol. Your API answers an unpaid request with HTTP 402 Payment Required, describing a USDC price, Base network, and recipient. The AI agent signs the payment authorization and retries with an encoded X-PAYMENT header. An x402 facilitator verifies and settles the payment, then your server returns the protected response.
This pattern lets an API charge per request without creating user accounts, issuing API keys, or maintaining subscriptions. For Base mainnet, Coinbase’s implementation reference identifies the network with CAIP-2 ID eip155:8453 and uses the ExactEvmScheme middleware with a payTo Ethereum address.
What an x402 payment endpoint does
x402 uses the HTTP status that was reserved for payment-required responses and makes it machine-readable for software agents. A normal browser may simply display a payment error; an AI agent can inspect the response, authorize USDC, and retry automatically.
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GET /v1/lookup. - Your server returns
402 Payment Requiredwith the amount, USDC token, Base network, recipient address, and any other requirements. - The agent’s wallet signs an authorization for that exact payment.
- The agent retries the request with the encoded
X-PAYMENTheader. - The facilitator checks the authorization and settles the transfer onchain.
- Only after verification and settlement does your server return the API response. It may also include an optional payment-response header.
The important trust boundary is step five: a request parameter or unsigned client claim is not proof of payment. Your protected handler should rely on the middleware and facilitator result, not on an amount supplied by the caller.
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What you need before writing code
- An HTTPS API endpoint that you control and can modify to return HTTP 402.
- A Base mainnet recipient address for the USDC revenue. Use an address whose operational and accounting controls you understand; a payment sent to the wrong address is generally not recoverable by your API.
- A facilitator configuration. A facilitator performs signature verification and settlement for the protocol. Coinbase’s reference includes a production facilitator option through
@coinbase/x402; authentication, limits, pricing, and endpoint availability should be checked against the current provider terms before launch. - A wallet strategy for the agents that will call you. The wallet must be able to sign USDC authorizations and enforce a spending policy for unattended calls.
- A decision about mainnet versus a test environment. Keep test and production recipient addresses and environment variables separate, and confirm the current test-network details in the SDK documentation rather than assuming that a mainnet identifier applies to a test network.
- At least one route whose cost and response value are clear enough for an agent to evaluate before paying.
Install the server components
Coinbase’s reference implementation uses Express and these packages:
npm install express @x402/express @x402/core @x402/evm @x402/extensionsKeep the packages on compatible versions. x402 SDK signatures and facilitator integrations can change, so check the versioned documentation for the release you install before treating an example as copy-and-paste production code.
Configure Base mainnet and USDC
Your route policy needs four pieces of information:
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eip155:8453, the CAIP-2 identifier used for Base mainnet in the Coinbase implementation reference. - Scheme:
exact, handled by the EVM ExactEvmScheme middleware. - Price: a fixed US-dollar-denominated amount for the request, such as
$0.01. The implementation converts that requirement into the USDC authorization the agent signs. - payTo: the Ethereum address that receives the settled USDC.
The following is an intentionally explicit configuration shape. Treat it as a map of the values your current SDK middleware must receive, then use the installed package’s current function signature when wiring it into Express:
const paymentPolicy = {
"/v1/lookup": {
accepts: [{
scheme: "exact",
network: "eip155:8453",
price: "$0.01",
payTo: process.env.PAY_TO
}],
description: "One lookup request",
mimeType: "application/json"
}
};Do not put a private key in this policy. The server needs the recipient address and facilitator credentials, if required; an agent wallet’s signing key belongs in the agent’s isolated wallet environment.
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Add payment middleware before the protected route
Register the ExactEvmScheme for Base and place the x402 middleware ahead of the handler that returns billable data. The middleware should inspect the request, produce the payment challenge when no valid payment is present, and call the facilitator for verification and settlement when an X-PAYMENT header arrives.
import express from "express";
import { ExactEvmScheme } from "@x402/evm";
import { paymentMiddleware } from "@x402/express";
const app = express();
const payTo = process.env.PAY_TO;
// Pass the route policy, facilitator configuration, and the
// ExactEvmScheme registration using the API shape required by
// the versions installed in your project.
const scheme = new ExactEvmScheme();
app.use(/* x402 paymentMiddleware configured with:
- paymentPolicy
- scheme for eip155:8453
- facilitator settings
*/);
app.get("/v1/lookup", async (req, res) => {
const result = await runLookup(req.query);
res.json(result);
});
app.listen(process.env.PORT || 3000);The middleware call is shown as a configuration boundary rather than a frozen signature because the reference packages evolve. In a real deployment, confirm the constructor and registration names in the documentation for the exact versions in your lockfile. The invariant is the ordering: x402 must run before the paid handler, and the handler must execute only after the middleware has accepted the payment.
Understand the 402 response an agent receives
An unpaid request should be answered with status 402 and payment requirements that a compatible agent can parse. Those requirements should identify the USDC amount, the exact network, the recipient, the accepted scheme, and enough route information for the agent to know what it will receive.
A useful challenge also gives the agent a stable description of the operation and the response media type. Keep the paid operation deterministic where possible: if the same request can be charged twice because of a timeout, the client needs a way to distinguish a retry from a new purchase.
The agent signs an authorization for the challenge it received. It then sends the encoded authorization in X-PAYMENT on the retry. Your server should not accept a price in a query string, a JSON field, or a custom header as a substitute for the signed requirement.
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Choose and secure the agent wallet
Micropayments become useful when an agent can pay without a human approving every request, but unattended signing introduces a larger blast radius. Set controls before allowing the wallet to call arbitrary endpoints.
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Circle Wallets authorization flow
Circle’s September 12, 2025 guide demonstrates an AI agent using Circle Wallets to sign a USDC authorization for an x402 request. The agent receives the 402 challenge, signs the requested payment, retries, and receives the API response after middleware verification. The guide describes pricing down to cents or less, which is suitable for small, high-volume operations when the transaction and facilitator costs remain economical.
Agent-specific wallet controls
Coinbase’s Agentic Wallets material describes spend caps, gasless Base transactions, and private keys held in secure Coinbase infrastructure rather than exposed to the prompt or model. Those controls address three common failure modes: an agent paying more than intended, an agent needing native gas for every call, and a model or prompt log exposing signing material.
Minimum guardrails
- Set a per-request maximum and a daily or monthly aggregate cap.
- Allowlist the Base network, USDC contract, and recipient addresses that the agent may pay.
- Keep signing credentials outside prompts, chat history, application logs, and model-visible tool output.
- Expire authorizations quickly enough that a captured header cannot be reused indefinitely.
- Require human review for a policy change, recipient change, or unusually large payment.
- Separate development wallets from production wallets and monitor each independently.
Make discovery part of the payment design
A paid endpoint is difficult for agents to use if they cannot find it or cannot understand its input and output. Base describes an x402 ecosystem in which developers can list agent-facing products in agentic.market and charge per request without API keys or subscriptions.
Coinbase’s implementation reference also includes a Bazaar discovery extension. It publishes endpoint output examples and schemas so another agent can discover the service, determine the request shape, and estimate the value before initiating payment.
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Publish, at minimum:
- The endpoint URL and HTTP method.
- Required query parameters or JSON fields, including types and allowed ranges.
- The price and currency for one successful request.
- The Base network identifier and the kind of USDC payment accepted.
- An example response with sensitive data removed.
- Latency, rate limits, freshness, and known failure conditions.
- A statement of whether a failed or empty result is billable.
Keep the schema and the middleware policy synchronized. An agent that discovers a $0.01 route but receives a 402 challenge for a different amount will either stop or risk an unintended payment.
Set a price that covers the complete request
Coinbase’s 2026 implementation reference gives these starting ranges. They are guidance, not guaranteed market rates or a promise that every request in a category has the same cost.
| Work type | Suggested price range | What to check before choosing a value |
|---|---|---|
| Simple data lookup | $0.001–$0.01 | Data licensing, cache hit rate, facilitator cost, and the value of a single answer. |
| API proxy or enrichment | $0.01–$0.10 | Upstream API charges, transformation work, retries, and response size. |
| Compute-heavy query | $0.10–$0.50 | Compute time, concurrency, queueing, and the cost of abandoned or repeated jobs. |
| AI inference | $0.05–$1.00 | Model usage, prompt and output tokens, context length, and the value of a completed result. |
Model the full unit economics rather than multiplying a nominal price by request volume. Include facilitator fees, infrastructure, upstream services, failed attempts, abuse controls, refunds, and retries. A one-cent endpoint can lose money if an attacker can force expensive work before payment settlement or if a timeout causes duplicate processing.
Use separate route prices when the resource costs differ. A cheap metadata lookup should not subsidize a compute-heavy export, and a single flat price makes it harder for an agent to select the most economical operation.
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Retries and idempotency
Network failures can occur after settlement but before the client receives your response. Give each billable operation an idempotency key or another request identifier, and persist the relationship between that identifier, the payment, and the result. On a safe retry, return the previously generated result instead of charging and running the work again.
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Replay and expiry
Accept a payment only for the route, amount, recipient, network, and validity window represented by the signed authorization. Reject an expired authorization, a reused authorization, or one whose nonce has already been consumed. The facilitator should perform cryptographic and settlement checks, while your application enforces business-level replay rules.
Refunds and partial failures
Decide what happens when payment settles but the upstream data source fails. Options include returning a documented error with a refund process, retrying the upstream work within a bounded window, or offering a replacement result. State the policy in the endpoint metadata; agents need to know whether an error is a lost payment, an automatic retry, or a credit.
Run the request lifecycle safely
- Receive: parse the request and reject malformed input before doing expensive work.
- Challenge: when no valid payment is present, return 402 with the exact USDC requirement.
- Verify: pass the encoded
X-PAYMENTvalue to the x402 middleware and facilitator. - Authorize work: start costly processing only after the middleware reports an accepted payment state.
- Execute: run the lookup, proxy, computation, or inference with its own timeout and resource limit.
- Respond: return the promised media type and result, plus the optional payment response header when your integration supports it.
- Record: store a non-sensitive payment reference, route, amount, status, latency, and idempotency key for reconciliation.
Do not log the complete X-PAYMENT header or wallet secrets. Redact authorization material while retaining enough identifiers to investigate a failed settlement.
Facilitator choices and operating costs
| Decision axis | Managed facilitator | Self-managed or alternate facilitator |
|---|---|---|
| Custody and trust | You rely on a provider to verify and settle, simplifying operations but adding provider dependency. | You control more of the infrastructure, while assuming more reliability, security, and chain-integration work. |
| Authentication | May require provider credentials, quotas, and account limits; confirm the current requirements. | Depends on the selected implementation and its operational controls. |
| Network coverage | Check that Base mainnet and the scheme you use are supported in production. | Verify support, settlement reliability, and test-network behavior yourself. |
| Framework fit | The reference packages target Express and EVM middleware. | Confirm adapters or build an integration for your framework. |
| Failure handling | Provider outages or rate limits can block paid requests; add timeouts and a clear degraded mode. | You must operate monitoring, retries, upgrades, and incident response. |
The production Coinbase facilitator shown in the reference is one possible deployment path, not a guarantee of availability or a substitute for checking current terms. Select a facilitator only after confirming its Base mainnet support, authentication method, limits, settlement timing, fee model, logging policy, and incident process.
Test before switching to mainnet
- Run the API with a test recipient and test-network configuration isolated from production secrets.
- Confirm that an unpaid request returns 402 rather than executing the handler.
- Have a compatible agent parse the challenge, sign USDC, and retry with
X-PAYMENT. - Verify that an invalid signature, wrong network, wrong recipient, expired authorization, and reused authorization are rejected.
- Simulate a facilitator timeout after the client sends payment and verify that your idempotency logic prevents duplicate work.
- Check that the successful response contains the expected data and any payment-response metadata.
- Reconcile your application record with the facilitator’s settlement result before funding a production wallet.
- Replace every test address, credential, endpoint, and discovery record before enabling Base mainnet.
Test the smallest realistic price and a high-concurrency burst. Micropayments can expose edge cases that a single manual request will not: rounding, nonce contention, duplicate retries, facilitator throttling, and an agent that keeps paying after your service has begun returning errors.
Troubleshoot common failures
| Symptom | Likely cause | Action |
|---|---|---|
| Every request returns 402 | The agent did not parse the challenge, the header is missing, or the middleware is not registered on the expected route. | Inspect the response requirements, confirm the agent sends encoded X-PAYMENT, and verify middleware order. |
| Payment is rejected for the wrong network | The challenge, wallet, and facilitator disagree about Base mainnet versus a test network. | Check that all three use the same network identifier; Base mainnet is eip155:8453 in the reference configuration. |
| Signature verification fails | The authorization was altered, expired, signed by an unsupported wallet, or does not match the amount or recipient. | Generate a fresh authorization from the exact 402 requirements and inspect clock and nonce handling. |
| Settlement succeeds but no data arrives | A response timeout, upstream failure, or application crash occurred after payment. | Use the stored idempotency key and payment reference to retry safely or apply the documented refund policy. |
| Agents cannot find the API | No discovery listing, incomplete schema, or stale price metadata. | Publish the endpoint and examples through the discovery mechanism you support, then keep metadata synchronized with route policy. |
| Costs exceed revenue | Price does not include compute, facilitator fees, retries, abuse, or upstream calls. | Measure cost per completed request, cache appropriate work, tighten limits, and revise route pricing. |
Production checklist
- Base mainnet is explicitly configured as
eip155:8453. - The recipient address is controlled, verified, and stored outside source code.
- The ExactEvmScheme middleware protects every billable route.
- Handlers cannot perform expensive work before payment verification.
- Wallets have per-request and aggregate spend caps.
- Private keys and payment headers are excluded from prompts and logs.
- Authorizations are checked for route, amount, recipient, expiry, nonce, and replay.
- Idempotency records cover facilitator timeouts and client retries.
- Pricing includes infrastructure, upstream, facilitator, and abuse costs.
- Discovery metadata includes schemas, examples, prices, limits, and failure behavior.
- Monitoring covers 402 rates, verification failures, settlement latency, handler latency, duplicate requests, and revenue reconciliation.
- Facilitator authentication, limits, endpoint availability, and fees have been rechecked for the production launch.
When x402 is the right payment model
x402 is a strong fit when an agent needs a small, independently priced operation and can decide whether the result is worth the quoted amount. It removes account creation and recurring subscription management from the request path, which is particularly useful for machine-to-machine data, search, enrichment, inference, and compute services.
A conventional account or contract may still be preferable when you need negotiated volume pricing, long-running jobs, seats and permissions, chargebacks, human support workflows, or a guaranteed monthly budget shared across many users. You can also expose both models: use x402 for public, metered operations and an authenticated plan for customers with predictable high volume.
The complete request in one sentence
Publish a priced Base route, return a machine-readable 402 challenge, require the agent to sign the exact USDC authorization, verify and settle it through a facilitator, then execute and return the resource with wallet, replay, idempotency, pricing, and discovery controls in place.
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