x402 SDKs & Tooling

x402 SDKs & Tooling

Everything you need to pay for, or charge for, HTTP APIs and MCP tools in stablecoin over x402. All packages are non-custodial and open source.

Buyer side

• @cryptoapis-io/x402-buyer-sdk (github) — the buyer capability as an npm SDK for apps and agent frameworks: register a wallet, authorize, sign locally, retry. Its /mcp export (createX402ToolCaller) pays x402-gated MCP tools, not just HTTP endpoints.
• @cryptoapis-io/mcp-x402-pay (github) — an MCP server exposing an x402_pay tool: fetch a URL and, on a 402, pay it automatically and return the paid response. Also ships x402_discover for finding paid APIs. Works in any MCP-capable agent.
• x402 agent plugin — a Claude Code plugin bundling the buyer tool and a merchant-integration skill. Install with /plugin marketplace add CryptoAPIs-io/cryptoapis-x402-agent-plugin.

Merchant side

• @cryptoapis-io/x402-merchant-sdk (github) — zero-dependency middleware for Express / Hono / Next.js that adds a per-request paywall and runs the verify/settle round-trip.
• Charging for an MCP tool — the same SDK's /mcp export wraps a tool handler so agents pay per call; your handler runs only after settlement, so an unpaid call never reaches your code. A complete runnable server is in examples/mcp-paid-tool.
• Already run an MCP server you cannot modify? @cryptoapis-io/mcp-x402-accept is a self-hosted proxy that fronts an existing MCP server and paywalls chosen tools without changing its code. You run it yourself, so tool arguments and results never leave your machine.

Signing

• @cryptoapis-io/offline-signer (github) — offline, local transaction and EIP-712 signing across EVM, UTXO, Tron, XRP, Kaspa and Solana. Pairs with the buyer flow so your key signs locally, never over the wire.

Protocol

x402 is an open standard — see x402.org and the v2 specification. Crypto APIs implements both sides, across both the HTTP and MCP transports, so you can adopt it without running your own facilitator.