x402 Software Development Kit

Download stable releases, command-line toolsets, and configuration templates to secure agent payments today.

NodeJS Protocol SDK (npm)

Easily intercept server calls, parse X-402 headers, construct invoices, and verify off-chain stablecoin payment proofs within your Express, Nest, or Koa backend.

npm install @x402/sdk

MCP Gateway Config (JSON)

Get our standardized JSON schemas to configure Model Context Protocol servers. Enforce automatic spending limits and wallet pre-authorization policies.

Get Schema JSON

Frequently Asked Questions

Are private keys processed by the x402 SDK?

Absolutely not. Private keys remain inside your local runtime/wallet context. The SDK only formats public transaction proposals and verifies cryptographic invoice proofs.

Which block networks does the relay support?

It supports compliant, ultra-low cost EVM chains (such as Polygon and Base) where micro-payments of less than a cent are commercially viable.