Abstract

HTTP 402 Payment Required has been a reserved status code since 1999, waiting for a practical implementation. We present Vanta SDK, a complete toolkit for implementing x402 payments—a protocol that enables native payment flows directly within HTTP requests and responses using blockchain technology.

The x402 protocol leverages the trustless, programmable nature of blockchain payments to finally realize the vision of HTTP 402. When a client requests a protected resource, the server responds with a payment challenge containing price, recipient, and network details. The client completes payment on-chain, retries the request with proof, and receives the resource. This simple flow enables powerful new use cases.

Key Contributions

• Protocol Specification: A complete specification for implementing HTTP 402 using EVM-compatible blockchains, including challenge generation, payment verification, and error handling.
• Reference Implementation: Production-ready client and server SDKs for JavaScript/TypeScript with support for major web frameworks.
• Security Model: Comprehensive threat analysis and mitigations for replay attacks, front-running, and other attack vectors.
• Access Token Layer: Optional session management for improved UX and reduced on-chain verification overhead.

Applications

x402 payments enable several compelling use cases:

• AI Agent Commerce: Autonomous agents can programmatically pay for API access, compute resources, and data without human intervention.
• API Monetization: Developers can monetize APIs with pay-per-request pricing without managing API keys or subscriptions.
• Micropayment Content: Content creators can gate individual pieces behind micropayments instead of subscription walls.
• Metered Services: Usage-based billing with per-token, per-byte, or per-compute-second pricing models.

By standardizing on HTTP 402 and blockchain payments, Vanta SDK provides a universal, permissionless payment layer for the programmable web.