x402

x402 is the open payment standard that puts money back into HTTP where it belongs: status 402 Payment Required. no account system required for the protocol itself. buyers can be humans or agents.

these docs stay vendor-neutral. facilitators are helpers, not the religion.

mental model

1. buyer requests a resource
2. seller returns 402 with payment instructions
3. buyer submits a payment payload
4. seller verifies/settles (optionally via facilitator /verify + /settle)
5. seller returns the resource

when to use

pay-per-request APIs. agent-consumable paid tools. content that should gate on settlement, not on yet another signup.

when not to confuse with k4d pay

k4d tip links today: USDC tip + k4d_pay_* cookie. not 402 headers. different product. see tip-link-unlock.

facilitators (quick)

hosted settlement helpers include CDP, QuickNode, Second State, Stellar public, and fiat-oriented options like AsterPay (stablecoin in → EUR via SEPA Instant). pick by chains, cost, and whether you need on-chain-only or bank settlement. deeper matrix lives in the x402-everything skill (facilitators.md).

gotchas

• keep the standard docs as normative: docs.x402.org
• agents can pay via paybox x402 MCP tools without reinventing settlement
• test on the network you actually settle on
• facilitators are interchangeable config; do not bake one vendor into your mental model

official wins

introduction · seller quickstart · buyer quickstart · HTTP 402

recipe: x402-paywalled-api · skill: skills