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x402 is a payment protocol built on the HTTP 402 Payment Required status code. It enables programmatic payments between any HTTP client and server without accounts, sessions, or custom authentication.

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The payment flow

1. Client sends a normal HTTP request
2. Server responds with 402 + payment requirements
3. Client signs a payment locally
4. Client retries the request with X-PAYMENT header
5. Server verifies payment via a facilitator
6. Server returns the requested resource

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Step by step

1. Initial request The client makes a standard HTTP request to a protected resource. 
GET /api/data HTTP/1.1
Host: api.example.com
2. Payment required The server responds with 402 Payment Required and includes payment requirements in the response body: 
{
  "x402Version": 1,
  "accepts": [
    {
      "scheme": "exact",
      "network": "solana-devnet",
      "asset": "USDC",
      "maxAmountRequired": "10000",
      "payTo": "7xKX...",
      "resource": "https://api.example.com/api/data",
      "description": "API access",
      "mimeType": "application/json",
      "maxTimeoutSeconds": 60
    }
  ]
}
3. Payment construction The client inspects the requirements, selects a compatible payment method, and constructs a payment payload. The payload is signed locally using the client’s wallet. 4. Retry with payment The client retries the original request with the payment proof attached: 
GET /api/data HTTP/1.1
Host: api.example.com
X-PAYMENT: eyJ4NDAyVmVyc2lvbiI6MSwi...
The X-PAYMENT header contains a base64-encoded JSON payload with the payment proof. 5. Verification and settlement The server forwards the payment to a facilitator, which verifies the signature and settles the transaction on-chain. The facilitator returns a confirmation. 6. Resource delivery Once the payment is confirmed, the server returns the requested resource with a 200 OK response.

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Payment requirements

A payment requirement describes what the server accepts. Key fields:
FieldDescription
schemePayment method (e.g., "exact")
networkBlockchain network (e.g., "solana-devnet", "base-sepolia")
assetToken to pay with (e.g., "USDC")
maxAmountRequiredMaximum payment amount (in smallest unit)
payToRecipient address
resourceThe URL being paid for
maxTimeoutSecondsHow long the payment proof is valid
A server can accept multiple payment methods simultaneously. The client chooses one it can fulfill.

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Payment payload

The X-PAYMENT header contains a base64-encoded JSON object:
FieldDescription
x402VersionProtocol version (currently 1)
schemeWhich scheme was used
networkWhich network was used
payloadScheme-specific proof (signatures, transaction data)

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Further reading