"1 line of code to accept digital dollars. No fee, 2 second settlement, $0.001 minimum payment."

app.use(
  "/your-endpoint",
  // How much you want to charge, and where you want the funds to land
  paymentMiddleware("$0.10", "0x209693Bc6afc0C5328bA36FaF03C514EF312287C")
);
// Thats it! See example/resource.ts for a complete example. Instruction below for running on base-sepolia.

• resource: Something on the internet. This could be a webpage, file server, RPC service, API, any resource on the internet that accepts HTTP / HTTPS requests.
• client: An entity wanting to pay for a resource.
• facilitator server: A server that facilitates verification and execution of on-chain payments.
• resource server: An HTTP server that provides an API or other resource for a client.

• Permissionless and secure for clients and servers
• Gassless for client and resource servers
• Minimal integration for the resource server and client (1 line for the server, 1 function for the client)
• Ability to trade off speed of response for guarantee of payment
• Extensible to different payment flows and chains

The x402 protocol is a chain agnostic standard for payments on top of HTTP, leverage the existing 402 Payment Required HTTP status code to indicate that a payment is required for access to the resource.

It specifies:

1. A schema for how servers can respond to clients to facilitate payment for a resource (PaymentDetails)
2. A standard header X-PAYMENT that is set by clients paying for resources
3. A standard schema and encoding method for data in the X-PAYMENT header
4. A recommended flow for how payments should be verified and settled by a resource server
5. A REST specification for how a resource server can perform verification and settlement against a remote 3rd party server (facilitator)
6. A specification for a X-PAYMENT-RESPONSE header that can be used by resource servers to communicate blockchain transactions details to the client in their HTTP response

The following outlines the flow of a payment using the x402 protocol. Note that steps (1) and (2) are optional if the client already knows the payment details accepted for a resource.

1. Client makes an HTTP request to a resource server

2. Resource server responds with a 402 Payment Required status and a Payment Required Response JSON object in the response body.

3. Client selects one of the paymentDetails returned by the accepts field of the server response and creates a Payment Payload based on the scheme of the paymentDetails they have selected.

4. Client sends the HTTP request with the X-PAYMENT header containing the Payment Payload to the resource server

5. Resource server verifies the Payment Payload is valid either via local verification or by POSTing the Payment Payload and Payment Details to the /verify endpoint of a facilitator server.

6. Facilitator server performs verification of the object based on the scheme and networkId of the Payment Payload and returns a Verification Response

7. If the Verification Response is valid, the resource server performs the work to fulfill the request. If the Verification Response is invalid, the resource server returns a 402 Payment Required status and a Payment Required Response JSON object in the response body.

8. Resource server either settles the payment by interacting with a blockchain directly, or by POSTing the Payment Payload and Payment Details to the /settle endpoint of a facilitator server.

9. Facilitator server submits the payment to the blockchain based on the scheme and networkId of the Payment Payload.

10. Facilitator server waits for the payment to be confirmed on the blockchain.

11. Facilitator server returns a Payment Execution Response to the resource server.

12. Resource server returns a 200 OK response to the Client with the resource they requested as the body of the HTTP response, and a X-PAYMENT-RESPONSE header containing the Settlement Response as Base64 encoded JSON if the payment was executed successfully.

// Payment Required Response
{
  // Version of the x402 payment protocol
  x402Version: int,

  // List of payment details that the resource server accepts. A resource server may accept on multiple chains.
  accepts: [paymentDetails]

  // Message from the resource server to the client to communicate errors in processing payment
  error: string
}

// paymentDetails
{
  // Scheme of the payment protocol to use
  scheme: string;

  // Network of the blockchain to send payment on
  networkId: string;

  // Maximum amount required to pay for the resource as usdc dollars x 10**6
  maxAmountRequired: uint256 as string;

  // URL of resource to pay for
  resource: string;

  // Description of the resource
  description: string;

  // MIME type of the resource response
  mimeType: string;

  // Output schema of the resource response
  outputSchema?: object | null;

  // Address to pay value to
  payToAddress: string;

  // Maximum time in seconds for the resource server to respond
  maxTimeoutSeconds: number;

  // Address of the USDC contract
  usdcAddress: string;

  // Extra information about the payment details specific to the scheme
  extra: object | null;
}

// `Payment Payload` (included as the `X-PAYMENT` header as base64 encoded json)
{
  // Version of the x402 payment protocol
  x402Version: number;

  // scheme is the scheme value of the accepted `paymentDetails` the client is using to pay
  scheme: string;

  // networkId is the network id of the accepted `paymentDetails` the client is using to pay
  networkId: string;

  // payload is scheme dependent
  payload: <scheme dependent>;

  // resource the client is paying for
  resource: string;
}

A facilitator server is a 3rd party service that can be used by a resource server to verify and settle payments, without the resource server needing to have access to a blockchain node or wallet.

// Verify a payment with a supported scheme and network
POST /verify
Request body JSON:
{
  paymentHeader: string;
  paymentDetails: paymentDetails;
}

Response:
{
  isValid: boolean;
  invalidReason: string | null;
}

// Settle a payment with a supported scheme and network
POST /settle
Request body JSON:
{
  paymentHeader: string;
  paymentDetails: paymentDetails;
}

Response:
{
  // Whether the payment was successful
  success: boolean;

  // Error message from the facilitator server
  error: string | null;

  // Transaction hash of the settled payment
  txHash: string | null;

  // Network id of the blockchain the payment was settled on
  networkId: string | null;
}

// Get supported payment schemes and networks
GET /supported
Response:
{
  kinds: [
    {
      "scheme": string,
      "networkId": string,
    }
  ]
}

A scheme is a logical way of moving money.

Blockchains allow for a large number of flexible ways to move money. To help facilitate an expanding number of payment use cases, the x402 protocol is extensible to different ways of settling payments via its scheme field.

Each payment scheme may have different operational functionality depending on what actions are necessary to fulfill the payment. For example exact, the first scheme shipping as part of the protocol, would have different behavior than upto. exact transfers a specific amount (ex: pay $1 to read an article) while a theoretical upto would transfer up to an amount, based on the resources consumed during a request (ex: generating tokens from an LLM).

See specs/schemes for more details on schemes, and see specs/schemes/exact/scheme_exact_evm.md to see the first proposed scheme for exact payment on EVM chains.

Because a scheme is a logical way of moving money, the way a scheme is implemented can be different for different blockchains. (ex: the way you need to implement exact on Ethereum is very different than the way you need to implement exact on Solana)

Clients and facilitator must explicitly support different (scheme, networkId) pairs in order to be able to create proper payloads and verify / settle payments.

cd example

1. create .env cp ../packages/typescript/x402/.env.example .env and follow instruction in the file to create wallets

2. npm install to install dependencies

3. in 3 separate terminals, run npm run facilitator, npm run resource, then finally npm run client. You should see things happen across all 3 terminals, and get a joke at the end in the client terminal.

cd packages/typescript

1. npm install to install dependencies
2. Create .env with funded keys as above
3. npm run test to run tests

• have tests run on an anvil fork

The x402 protocol is licensed under the Apache-2.0 license.