MPP (Machine Payments Protocol)

MPP is an open payment protocol co-authored by Stripe and Tempo, now on the IETF standardization track. It extends HTTP 402 with standard authentication headers and supports multiple payment methods — stablecoins, fiat cards, Lightning, and more.

MPP is backwards-compatible with x402. Tollbooth’s mpp settlement strategy lets you accept payments from both x402 and MPP clients through the same gateway.

Why MPP

• Multiple payment methods — accept USDC stablecoins and Stripe card payments through one config
• Standard headers — uses WWW-Authenticate / Authorization / Payment-Receipt instead of custom headers
• 50+ integrations — growing ecosystem of MPP-compatible agents and services
• IETF track — on the path to becoming an internet standard

Configuration

settlement:
  strategy: mpp
  methods:
    - type: tempo        # stablecoins via x402 facilitator
    - type: stripe       # fiat card payments via Stripe
      secretKey: "${STRIPE_SECRET_KEY}"

Each method in the methods array adds a payment option. Clients pick whichever method they support.

Tempo method

Wraps the existing x402 facilitator for stablecoin payments. This is the same verify/settle flow as the default facilitator strategy, but advertised via MPP headers.

settlement:
  strategy: mpp
  methods:
    - type: tempo

No additional config needed — it uses the same facilitator URL resolution (global facilitator field, per-route overrides, etc.).

Stripe method

Accepts fiat card payments via Stripe’s Shared Payment Tokens (SPT). Agents with a Stripe payment method can pay without crypto.

settlement:
  strategy: mpp
  methods:
    - type: stripe
      secretKey: "${STRIPE_SECRET_KEY}"

The gateway creates a Stripe PaymentIntent for each challenge, and the client confirms it with their SPT credential.

How it works

Payment flow

Agent                     Tollbooth                   Facilitator / Stripe
  |                          |                              |
  |  GET /weather            |                              |
  |─────────────────────────>|                              |
  |                          |                              |
  |  402 Payment Required    |                              |
  |  + payment-required      |  (x402 header)               |
  |  + WWW-Authenticate:     |  (MPP header, one per method) |
  |    Payment method="tempo"|                              |
  |    Payment method="stripe"|                             |
  |<─────────────────────────|                              |
  |                          |                              |
  |  (agent picks a method   |                              |
  |   and creates credential)|                              |
  |                          |                              |
  |  GET /weather            |                              |
  |  + Authorization:        |                              |
  |    Payment id=".."       |                              |
  |    payload=".."          |                              |
  |─────────────────────────>|                              |
  |                          |  verify + settle             |
  |                          |─────────────────────────────>|
  |                          |  settlement info             |
  |                          |<─────────────────────────────|
  |                          |                              |
  |  200 OK                  |                              |
  |  + payment-response      |  (x402 receipt)              |
  |  + Payment-Receipt       |  (MPP receipt)               |
  |<─────────────────────────|                              |

Header formats

Challenge (402 response):

WWW-Authenticate: Payment id="abc", method="tempo", intent="charge",
    amount="0.01", currency="usd", decimals=6, description="GET /weather",
    request="<base64url-encoded-json>"

Credential (request):

Authorization: Payment id="abc", payload="<base64url-encoded-json>"

Receipt (success response):

Payment-Receipt: id="abc", receipt="<base64url-encoded-json>"

Backward compatibility

When using the mpp strategy, tollbooth always returns both x402 and MPP headers:

• 402 responses include both payment-required and WWW-Authenticate: Payment
• Success responses include both payment-response and Payment-Receipt
• Incoming requests are accepted with either payment-signature (x402) or Authorization: Payment (MPP)

This means existing x402 clients continue to work unchanged.

Comparison with other strategies

Strategy	Payment methods	Headers	Best for
facilitator	USDC stablecoins	x402 only	Simple x402 setups
nanopayments	USDC (batched)	x402 only	Sub-cent micropayments
mpp	Stablecoins + fiat cards	x402 + MPP	Universal agent payments
Custom	Anything	x402 only	Specialized logic

Learn more

• MPP specification
• Stripe MPP documentation
• Settlement strategies — full comparison of all tollbooth settlement options