x402

Why it's ranked here

It is a standard with real implementations behind it, not a whitepaper. Three language SDKs share one server design, the TypeScript side covers Express, Hono, Next, Fastify, Fetch and Axios, and chain packages span EVM, Solana, Aptos, Stellar, Hedera, Casper and more. The code handles the unglamorous failure cases: a settlement whose receipt times out returns a retryable pending state with the transaction hash, and the server retries once while reconciling against that hash instead of paying twice. Apache 2.0 licensed.

What's good

Server integration is a middleware call with a route map, and the same route config shape appears in all three languages. Three payment schemes cover distinct needs: a fixed price, a capped maximum where the seller settles actual usage, and batched settlement through escrow and offchain vouchers for many tiny charges. Lifecycle hooks before and after verify and settle let a seller abort, recover or grant free access. A dedicated wrapper puts paid tools behind the Model Context Protocol, the standard AI assistants use to call tools.

Tradeoffs

The public facilitator is meant for testnets; the docs say not to assume it is a production path for mainnet, so sellers must pick a provider, run their own, or settle in-process. The default pending-settlement store lives in memory and only works when the retry hits the same process, so replicated facilitators need a shared store such as Redis. On Solana, sellers settling without a facilitator must build their own duplicate-payment detection. Fiat support is an aim in the principles, not something the fetched code implements.

How to use it well

Start on a testnet with the public facilitator and the fixed-price scheme on one route. Before mainnet, choose a facilitator deliberately. If yours runs on serverless, set the confirmation timeout a few seconds under the platform deadline, since a killed process returns an error with no hash to reconcile. Reach for the capped scheme when cost depends on output, such as metered model calls. Use the upfront flow when a slow handler risks the signed transaction expiring. It does not provide wallets, key custody or accounting.

Observed

License

Apache License 2.0

SDK languages

TypeScript (npm @x402 packages), Python (pip install x402), Go module

Server frameworks (TypeScript)

Express, Hono, Next, Fastify; clients for Fetch and Axios

Payment schemes

exact, upto, batch-settlement (batch-settlement is EVM only)

Chain packages

EVM, SVM (Solana), AVM, Aptos, Casper, Stellar, TVM, Hedera, Keeta

Interfaces

HTTP middleware, HTTP facilitator client, MCP tool payment wrapper

Transport

Three base64-encoded JSON headers on HTTP status 402 responses and retries

Public facilitator scope

Documented as intended for development and testnet workflows