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What is HTTP 402?

HTTP 402 is a standard, but rarely used, HTTP response status code indicating that payment is required to access a resource. In x402, this status code is activated to:
  • Inform clients (buyers or agents) that payment is required.
  • Communicate the details of the payment, such as amount, currency, and destination address.
  • Provide the information necessary to complete the payment programmatically.

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Why x402 Uses HTTP 402

The primary purpose of HTTP 402 is to enable frictionless, API-native payments for accessing web resources, especially for:
  • Machine-to-machine (M2M) payments (e.g., AI agents).
  • Pay-per-use models such as API calls or paywalled content.
  • Micropayments without account creation or traditional payment rails.
Using the 402 status code keeps x402 protocol natively web-compatible and easy to integrate into any HTTP-based service.

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Payment Headers in V2

x402 V2 uses two standardized headers for payment communication:
  • PAYMENT-SIGNATURE: Contains the Base64-encoded payment payload from the client. This header is sent by the client when retrying a request after receiving a 402 response, proving they have authorized payment.
  • PAYMENT-RESPONSE: Contains the Base64-encoded settlement response from the server. This header is returned by the server in the successful response, confirming the payment was verified and settled.
Both headers must contain valid Base64-encoded JSON strings. This encoding ensures compatibility across different HTTP implementations and prevents issues with special characters in JSON payloads.

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Summary

HTTP 402 is the foundation of the x402 protocol, enabling services to declare payment requirements directly within HTTP responses. It:
  • Signals payment is required
  • Communicates necessary payment details
  • Integrates seamlessly with standard HTTP workflows