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You can pay an AI agent or any x402-enabled API in USDC without creating an account, obtaining an API key, or starting a subscription. The client makes a request, receives an HTTP 402 Payment Required response containing the price and payment instructions, signs a USDC authorization in a funded wallet, then retries with a payment header. After verification and settlement, the service returns its result and settlement confirmation. Coinbase’s documented command-line client packages this flow as npx [email protected] x402 pay <url>, with spending caps, POST data, custom headers, and JSON output available as options.
This guide explains the five useful invocations, what happens on the wire, how to discover services on a bounty board, how to protect a wallet, and what providers must do to accept payments. Amounts, networks, supported assets, catalogs, and business availability can change, so always follow the live payment requirement returned by the endpoint.
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What is x402, and why can it pay an AI agent without an account?
x402 is an open HTTP payment protocol built around status code 402 and signed stablecoin payments. A paid service does not authenticate you with a profile or API credential. Instead, payment itself becomes the credential for one request: the server states what it requires, and a client with a funded signing wallet proves that it paid.
Cloudflare’s x402 documentation describes the model as services returning a 402 response with payment instructions while clients pay programmatically without accounts, sessions, or API keys. Coinbase’s x402 whitepaper similarly describes pay-per-use access for AI agents and APIs without accounts, subscriptions, or API keys. You still need a wallet containing enough USDC and the ability to sign locally; “no account” does not mean “no identity-bearing key.”
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What you need before the first payment
- An endpoint that implements x402 and is reachable from your runtime.
- A programmatic wallet whose private key is held by the buyer-side runtime and which contains USDC for the required network.
- The x402 command-line client or another compatible client.
- A way to inspect the returned 402 requirements before authorizing anything.
- A spending policy, such as a maximum amount per request, appropriate to the wallet balance.
Base USDC is the primary example in the cited x402 materials, but the service’s live requirement controls the exact network and asset. Do not hard-code a chain, token contract, recipient, facilitator, or amount from an old example.
How does an x402 payment work on the wire?
The interaction is a challenge-and-retry exchange. The first request is normally unpaid; the 402 response tells the client exactly what authorization is needed. The client signs locally, sends proof in a payment header, and receives the response only after the service or its facilitator verifies and settles it.
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- Challenge: The server replies with HTTP 402 and machine-readable payment requirements.
- Local authorization: The wallet signs a USDC payment authorization. The requirement supplies the amount, token, network, payee, nonce, and facilitator details.
- Retry: The client encodes the signed proof in the payment header and repeats the request.
- Verification and settlement: The server or facilitator checks the signature and settles the payment on-chain, then returns the service data and a settlement response header.
Because the signed authorization is tied to a particular requirement, changing the recipient, network, amount, or nonce after signing invalidates the intended payment. Save the payment response and any transaction hash as a receipt for reconciliation.
Why the payment header matters
The header is the machine-to-machine replacement for a checkout session. It carries the encoded authorization that lets the provider verify that this request is paid. A normal API key identifies a customer across many calls; an x402 proof is generally attached to the individual call and can be evaluated together with the server’s current requirement.
What are the five practical x402 commands?
The Coinbase agentic-wallet reference documents one x402 pay command with options rather than five unrelated binaries. These five invocations cover the common cases: a basic request, a cap, a POST body, custom request metadata, and machine-readable output.
| Use case | Command | What it changes |
|---|---|---|
| Basic paid request | npx [email protected] x402 pay <url> |
Fetches the URL, handles the 402 challenge, signs, retries, and prints the result. |
| Maximum-price guard | npx [email protected] x402 pay <url> --max-amount 10000 |
Refuses authorization above 10,000 USDC atomic units, documented as $0.01. |
| POST JSON | npx [email protected] x402 pay <url> -X POST -d '{"prompt":"hello"}' |
Sends a POST request with a JSON body. |
| Query and headers | npx [email protected] x402 pay <url> -q '{"id":"42"}' -h '{"Accept":"application/json"}' |
Adds query parameters and an explicit request header. |
| Automation output | npx [email protected] x402 pay <url> --json |
Requests JSON-formatted command output for a script or agent. |
Replace <url> with the actual paid endpoint. The angle brackets indicate a value to substitute; they are not typed literally.
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How should I set --max-amount?
--max-amount is denominated in USDC atomic units, not decimal dollars. The documented conversions are:
| Atomic units | USDC value | Typical interpretation |
|---|---|---|
| 1,000,000 | $1.00 | One dollar maximum |
| 100,000 | $0.10 | Ten-cent maximum |
| 50,000 | $0.05 | Five-cent maximum |
| 10,000 | $0.01 | One-cent maximum |
Set the cap before the signing step. A request whose live quote exceeds the cap should be rejected by the client rather than silently authorized. Quote first, then approve: the cap protects against an unexpectedly expensive endpoint but does not tell you whether the endpoint’s output is useful.
How do shell quoting and JSON bodies affect a command?
Use single quotes around shell arguments containing a dollar sign, braces, or nested JSON so the shell does not expand or reinterpret them. For example, keep '{"prompt":"hello"}' as one argument. In a script, validate JSON before invoking the client and avoid interpolating untrusted text directly into a shell command; pass arguments through the process API when possible.
How do I pay an endpoint step by step?
Start with a low-risk request and inspect the quote. The sequence below keeps the authorization decision separate from service discovery and execution.
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- Make the initial request. Use the basic command or the method and payload options. The expected response for a paid route is HTTP 402, not a generic authentication failure.
- Read every payment field. Verify the requested amount and unit, network, token contract, recipient, nonce, and facilitator. Also check any expiry or request-binding information the implementation supplies.
- Apply a cap. Choose a
--max-amountvalue in atomic units that is high enough for the intended quote but low enough to limit loss. - Confirm wallet readiness. The wallet must hold the required USDC on the required network and be able to sign. Keep the private key in the buyer runtime; never send it to the provider.
- Run the paid invocation. The client signs locally, adds the payment proof header, and retries with the original method, query, and body.
- Record the receipt. Retain the service response, payment response header, amount, recipient, and transaction hash when supplied.
- Validate the result. Check that the response matches the requested operation before allowing an agent to trigger follow-up spending.
What if the service returns a different network or token?
Stop and evaluate the live requirement. x402 implementations and supported networks can evolve; a wallet funded on one chain cannot automatically satisfy a requirement on another. If the client does not support the required asset or network, use a compatible wallet or choose another service rather than editing the payment proof manually.
How can I find an AI bounty or paid service?
If you already know the endpoint, inspect its live 402 response. If you need discovery, a machine-readable catalog such as the402 can provide service metadata, provider trust verdicts, purchase records, and owner-defined purchasing rules.
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Use a catalog as a lead, not as authorization
A listing describes a possible service; it does not replace the endpoint’s current payment challenge. Open the listed endpoint, verify that the method and payload fit your agent, and inspect the fresh amount, network, recipient, and facilitator before signing.
Compare services on the fields that affect risk
| Comparison field | Why it matters | What to verify |
|---|---|---|
| Amount and unit | Determines the actual USDC cost and whether your cap applies. | Decimal price, atomic-unit representation, and whether the charge is per request. |
| Network and token | Determines wallet funding and transaction costs. | Live chain identifier and token contract, not a copied listing description. |
| Facilitator | Handles verification or settlement for the implementation. | Facilitator identity and whether the client supports it. |
| Trust evidence | Helps an agent avoid unknown or unreliable providers. | Catalog verdicts, purchase records, ownership rules, and recency. |
| Refund or dispute policy | x402 payment is not automatically reversible. | Provider’s written remedy for bad output, downtime, or duplicate requests. |
| Response quality | A cheap endpoint can still waste an agent’s time or tokens. | Schema, latency expectations, error behavior, and representative output. |
| Method and payload support | A valid payment cannot fix an incompatible request. | GET versus POST, required fields, content type, and size limits. |
There is no universal success-rate statistic or independent market-size figure established in the cited materials. Treat trust records as decision evidence, not a guarantee.
What wallet and security practices prevent expensive mistakes?
The safest pattern is local signing with a narrowly funded wallet. The provider verifies a signature; it should never need the buyer’s private key. Separate the wallet used for experiments from treasury funds and enforce policy in the agent runtime before a signature request is created.
- Cap every call: Set
--max-amountbefore signing, and impose a daily or session budget outside the CLI. - Inspect the recipient: Compare the payee in the live requirement with the provider you intended to use.
- Check the chain: Confirm that the wallet is connected to the required network and that its USDC balance is there.
- Preserve receipts: Store the quote, signed-request metadata, response header, and transaction hash where your accounting system can reconcile them.
- Use least privilege: Give an agent only the wallet permissions and balance its task requires.
- Guard retries: A timeout does not prove that no settlement occurred. Check the response or transaction status before retrying to avoid paying twice.
- Review provider policy: Understand what happens when output is malformed, unavailable, or materially different from the request.
Are x402 payments reversible?
No automatic chargeback is implied. The exact push payment is irreversible once executed; a refund requires a separate transfer or a provider’s business-logic process. For valuable work, ask how the provider handles failures before allowing autonomous retries.
What errors should I troubleshoot first?
Most failures fall into four categories: an unusable quote, a wallet or network mismatch, a request-shape error, or a settlement uncertainty.
The response is not 402
A 401 or 403 may indicate ordinary authentication or authorization, while a 404 may mean the route is wrong. Confirm the exact paid URL and method. An ordinary endpoint cannot be made x402-compatible by adding a payment header.
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The cap is rejected
Convert the quoted atomic units to USDC and raise the cap only if the price is expected and acceptable. If the amount is unexpectedly high, stop and inspect the endpoint or choose another service.
The wallet cannot pay
Check that the wallet is funded with the required USDC on the requirement’s network, that the signing key is available to the runtime, and that the client supports the stated token and facilitator. Do not move funds based solely on a catalog entry.
The provider rejects the retry
Ensure the original method, body, query, and content type were preserved and that the payment header was generated by the client for the current challenge. A stale quote, altered nonce, wrong recipient, or modified request can invalidate an otherwise correct signature.
The command times out after signing
Do not immediately run it again. First inspect the saved response and transaction status. If settlement completed, contact the provider under its stated policy; if no settlement occurred, retry only after confirming that the original authorization cannot be accepted later.
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How can a provider accept USDC from AI agents?
A provider can add x402 middleware to its API, expose an x402 URL for agent discovery, and configure a receiving account for settled USDC. Coinbase Business describes infrastructure for receiving agent payments, reconciliation, withdrawal, and holding, but eligibility, geography, rates, and terms are subject to its current offering.
Provider implementation checklist
- Place x402 middleware in front of the paid route.
- Return a machine-readable 402 requirement with an accurate amount, token, network, recipient, nonce, and facilitator information.
- Verify the payment proof and settle before releasing the paid response, or clearly define the implementation’s settlement ordering.
- Return settlement confirmation in the response header so the buyer can reconcile the charge.
- Publish the endpoint and request schema in a machine-readable catalog or directory.
- Document refund, duplicate-request, timeout, and malformed-output handling.
- Monitor receipts and reconcile settled USDC with delivered requests.
Clear requirements matter more than a polished listing. An agent must be able to discover the route, understand its payload, quote the cost, and decide whether the provider is trustworthy before signing.
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Is paying AI agents with x402 practical for autonomous workflows?
It is practical when the task has a clear per-request price, a machine-readable interface, and a wallet policy that limits loss. The strongest fit is a bounded operation—such as a data transformation or API call—where the agent can validate the response and record a receipt.
It is less suitable when the provider’s quality is subjective, the task may require many unbounded retries, or the service offers no meaningful remedy for failure. Payment removes account friction, not operational risk. Build budgets, approval thresholds, schema checks, and retry rules around the x402 call.
What should you remember before sending the first payment?
- x402 turns HTTP 402 into a payment challenge and uses signed stablecoin authorization as request access.
- The documented entry point is
npx [email protected] x402 pay <url>; the other four examples are option combinations around that command. - Read the live requirement, especially amount, atomic units, network, token, recipient, nonce, and facilitator.
- Use a locally signing, narrowly funded wallet and set a maximum amount before authorization.
- Keep the payment response or transaction hash as your receipt, and investigate timeouts before retrying.
- Use a catalog for discovery, but never treat a listing as a substitute for inspecting the endpoint’s current 402 response.
Frequently Asked Questions
Can I use x402 without holding cryptocurrency myself?
No. The payment flow requires a funded signing wallet containing the required USDC on the network named by the endpoint. An account on the provider’s website is optional, but the wallet and its funds are not.
Does an x402 request reveal my private key to the API provider?
It should not. The recommended design keeps the key in the buyer-side runtime and sends only a signed authorization that the provider or facilitator can verify.
What evidence should an agent save after paying?
Save the quoted requirement, final amount, recipient and network, the service response, the settlement response header, and any transaction hash. Those records let you reconcile spending and investigate duplicate or disputed requests.
Can a provider charge me again automatically after a failed response?
A provider cannot legitimately reuse a payment proof for an unrelated requirement, but your own retry logic can create a second charge. Treat timeouts as uncertain until settlement status is checked and define idempotency or retry rules with the provider.
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How often do x402 commands and supported networks change?
Client versions, network support, catalog contents, and business-service availability are volatile. Pin the documented client version for a deployment, then recheck the current command reference and the endpoint’s live requirements before upgrading or funding a production wallet.
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