MPP: machine payments over HTTP.

What it is

MPP's core is an IETF draft, draft-httpauth-payment-01 (2026-09-09, active)¹. A server asks for payment with a 402 and a WWW-Authenticate: Payment header;² the agent sends a payment credential, and a successful answer carries a Payment-Receipt² header.

Stripe takes MPP (cards and stablecoins), x402 (stablecoins)³.³ The agent pays, and Stripe settles to the seller's balance.³

How many products offer it

2 of 151 products we drove offer MPP, seen or paid. 26 of 151 offer at least one way for an agent to pay.

How to add it

1. 1

   Answer paid routes with a payment challenge.

   A 402 with WWW-Authenticate: Payment.² Stripe's machine payments do the challenge and the settling for you.³

2. 2

   Publish your prices.

   MPP discovery reads your OpenAPI document at /openapi.json,⁴ with x-payment-info on each paid operation.⁴ Each paid operation also lists its 402.⁵

3. 3

   Return a receipt.

   Send Payment-Receipt with each paid answer, so the agent can keep it.² The pay-per-call guide has the rest.

Check it yourself

1. Call a paid route without paying.

   You want a 402 with a WWW-Authenticate: Payment header.

   curl -si https://example.com/api/paid-route | grep -i -E "^HTTP|www-authenticate"

2. Check your discovery document.

   It should list the paid routes with their price.

   curl -s https://example.com/openapi.json | grep -c x-payment-info