# What is x402?

x402 is an open protocol for HTTP-native payments. A resource can require payment as part of the request flow, allowing software to pay for a capability without a traditional subscription or long-lived customer account.

Preflight Stack uses x402 as the payment rail for three focused utilities: dependency preflight decisions, random selection, and temporary file handoff.

## Machine discovery

- Start from https://preflightstack.com/llms.txt for the curated agent index.
- Use service-origin OpenAPI documents as the authoritative API contracts.
- Use https://preflightstack.com/.well-known/x402 for exact paid resource URLs and methods.
- Service pages expose standard `service-doc`, `service-desc`, and `status` link relations.

Discovery metadata says what a host claims to provide. It does not transfer trust or guarantee security.