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x402 turns an ordinary HTTP request into a pay-per-use transaction. An AI agent requests an API, receives 402 Payment Required with the price, token, network, and recipient address, signs a payment authorization with its wallet, then retries the request with proof. The API or a facilitator verifies and settles that payment before returning the data. This lets software buy one metered result without first creating an account, negotiating a subscription, or receiving a long-lived API key.
The useful mental model is: HTTP request → 402 challenge → signed retry → verified settlement → paid response. x402 is most practical for deterministic, machine-readable work such as inference, embeddings, data lookups, content delivery, and tool calls.
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What is x402?
x402 is an open HTTP payment standard introduced by Coinbase. It gives the previously unused HTTP 402 Payment Required status a machine-to-machine meaning: the server can tell a client exactly what payment is needed, and the client can authorize that payment and retry automatically. Coinbase describes the goal as letting “developers and AI agents pay for APIs, services, and software directly with stablecoins over HTTP.”
The payment is coupled to a particular HTTP request rather than to a permanent customer account. A client may be an AI agent, a conventional application, or another service. The server remains responsible for its price and access policy; x402 supplies a standardized challenge-and-proof exchange.
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How does an x402 request work?
Each stage has a distinct job. A failed signature, unsupported network, or missing funds stops the flow before the protected response is delivered.
- Send the initial request. The agent makes a normal HTTP request to the protected endpoint, just as it would for a free API.
- Read the payment challenge. The server responds with HTTP
402 Payment Required. ItsPAYMENT-REQUIREDheader contains encoded requirements, including the amount, accepted token, blockchain network, and merchant address. - Construct and sign locally. The agent’s wallet creates the payment payload and signs it. The private key should remain under the wallet’s control; the API does not need to receive that key.
- Retry with proof. The agent repeats the original request and adds a
PAYMENT-SIGNATUREheader containing the signed authorization. - Verify and settle. The server can perform verification and settlement itself or call a facilitator. Cloudflare documents facilitator
POST /verifyandPOST /settleoperations. The facilitator broadcasts the client’s pre-signed transaction but does not hold the funds. - Return the result and receipt. After successful settlement, the API returns the requested data and a
PAYMENT-RESPONSEheader with settlement confirmation.
Implementations must bind the authorization to the intended request and handle duplicate retries safely. A client should not blindly pay again after a timeout; it should inspect the receipt or settlement state first.
Do x402 payments need an API key or service account?
Normally, no pre-created service account, session token, subscription, or pre-shared API key is required. The signed payment credential is presented only when the endpoint asks for it, so access can be granted per request.
That does not mean authentication disappears. An operator may still require identity, quotas, abuse controls, or an allowlist for expensive operations. x402 solves payment authorization, not every security or governance problem. A wallet also needs sufficient funds in the accepted token on the accepted network.
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Which parts make up an x402 system?
Client
The client is an AI agent or application that can interpret the challenge, choose an approved payment option, and sign with a compatible crypto wallet. Production clients should enforce spending limits, permitted domains, token allowlists, and per-request approval rules rather than allowing unrestricted autonomous purchases.
Server
The server publishes its price and payment options, validates the signed credential, settles the transfer, and serves the response. It should reject expired, malformed, replayed, or incorrectly targeted authorizations and preserve enough receipt data to explain a disputed request.
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Facilitator
A facilitator is optional infrastructure for verification and settlement. It reduces the operator’s need to maintain direct blockchain connectivity, but creates a dependency on that service’s availability, supported networks, and operational policies. Direct settlement removes that dependency while increasing the server’s infrastructure responsibility.
Payment scheme
Cloudflare documents an exact scheme for fixed-amount transfers across EVM and several non-EVM networks. It also documents an upto EVM scheme, where the final charge is determined at settlement. The latter is useful when usage is metered, but the client must understand the maximum authorization and the conditions that determine the final amount.
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Which networks and stablecoins can x402 use?
Support depends on the implementation and can change as SDKs and facilitators evolve. Cloudflare’s documentation lists Base, Ethereum, Polygon, Optimism, Arbitrum, Avalanche, Solana, Aptos, Stellar, Sui, and Algorand. It documents base-sepolia for testing and uses base in production examples.
Do not infer that every endpoint accepts every listed network or token. The authoritative choice is the options encoded in that endpoint’s PAYMENT-REQUIRED header. The wallet must hold the specified stablecoin, on the specified network, and be able to pay any applicable network fee.
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Coinbase identifies API access, pay-per-use AI models, on-demand content, and machine-to-machine services as use cases. The strongest fit is a request that produces a clearly defined result after one call or one metered unit of work.
- Model inference or an embedding for a submitted input
- A data lookup, structured record, or current feed query
- Web content or a file delivered to the requesting agent
- A tool call that performs one bounded operation
- Usage-metered processing where an
uptoauthorization caps exposure
This pricing model can be easier to reason about than a monthly plan when demand is irregular: an agent pays only when it obtains a result. Operators still need to price failed work, refunds, latency, and partial completion explicitly.
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How should you compare x402 implementations?
“x402-compatible” is not a complete comparison. Check the following differences before selecting an SDK, proxy, or facilitator.
| Decision area | Questions to ask |
|---|---|
| Headers and version | Does the client and server agree on the payment-header format and protocol version? |
| Scheme and networks | Are exact or upto payments available, and are the required EVM or non-EVM networks supported? |
| Settlement | Can the server settle directly, or is a facilitator mandatory? |
| Wallet controls | Where are keys held, and can spending limits, domains, tokens, and maximum amounts be enforced? |
| Reliability | How are replay, nonce, timeout, duplicate retry, failed settlement, and receipt lookup handled? |
| Integration | Does the implementation provide middleware, SDKs, or a proxy for the server framework you use? |
Cloudflare supplies SDK, middleware, and proxy examples, while Coinbase supplies protocol and facilitator references. Treat network lists, header names, SDK behavior, and facilitator availability as version-sensitive details and verify them against current documentation before deployment.
What are x402’s practical limitations?
- A funded wallet is still required. Removing account onboarding does not remove custody, key management, token funding, or network-fee concerns.
- Payment is not a safety policy. An autonomous agent can spend money on a technically valid but unwanted request unless its operator sets authorization boundaries.
- Facilitators add dependency. They simplify blockchain operations but can become an availability or compatibility bottleneck.
- Retries can cost twice. Timeouts and ambiguous settlement states require idempotency and receipt checks before another signed payment is issued.
- Support is not universal. A listed chain or stablecoin may not be enabled by a particular endpoint, facilitator, wallet, or SDK.
When is x402 a good fit?
Choose x402 when an API operator wants metered access and the caller needs exactly one machine-readable result without negotiating a subscription first. It is less suitable when a workflow needs long-running state, complex refunds, human approval at every stage, or a conventional enterprise billing relationship. In those cases, x402 can still be one payment rail, but it should not be mistaken for a complete account, invoicing, or identity system.
Frequently Asked Questions
Can an x402 client pay with a normal bank card?
The protocol flow described here uses a wallet-signed payment authorization and supported blockchain tokens. A card-funded wallet or intermediary could exist outside the protocol, but card payment itself is not the x402 credential sent in the retry.
Who pays the blockchain transaction fee in x402?
The payment requirements and implementation determine how fees are handled. The client must at least use a wallet and network combination capable of completing the transaction; the endpoint’s documentation should state whether the quoted amount includes or excludes additional network costs.
Can an API refund an x402 payment?
A successful settlement is an on-chain transfer, so a refund is a separate operation rather than an automatic reversal. The API operator must define refund conditions and a return mechanism independently of the request’s payment proof.
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Is x402 limited to AI agents?
No. Any software that can parse the challenge and sign an accepted payment can act as the client. AI agents are a prominent use case because they can select and purchase tools dynamically, but the protocol is an HTTP pattern for programs generally.
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