MPP (Machine Payments Protocol)

MPP is a machine-to-machine payment protocol for ongoing relationships between two services — think AI agent paying an API, or one service retaining another. Instead of a new authentication + payment per call (like x402), MPP opens a session with a pre-deposited budget; the consumer signs vouchers for each unit of work; the session reconciles at close. Best when you have recurring, high-frequency micropayments between the same two parties.

When to pick MPP vs. alternatives

Situation	Protocol
New consumer discovered your API and pays per call	x402
Recurring relationship, many calls per session	MPP
Pre-authorized budget envelope, no per-call voucher	AP2
Hosted checkout for human buyer	UCP / ACP

Anatomy

┌───────────┐         ┌─────────────────┐         ┌───────────┐
│ Consumer  │ 1. Open │   Session       │ 2. Pay  │  Service  │
│  agent    ├────────▶│   (budget held) │◀────────┤           │
│           │         └─────────────────┘         │           │
│           │              ▲   │                  │           │
│           │  3. Voucher  │   │  4. Stream cost  │           │
│           ├──────────────┘   └─────────────────▶│           │
│           │                                     │           │
│           │  5. Close + return unspent budget   │           │
│           │◀────────────────────────────────────│           │
└───────────┘                                     └───────────┘

One-shot payment (simplest case)

For a single micropayment without opening a session:

curl -X POST https://sandbox.getsly.ai/v1/mpp/pay \
  -H "Authorization: Bearer agent_..." \
  -d '{
    "service_url": "https://inference.example.com/v1/complete",
    "amount": "0.05",
    "currency": "USDC",
    "agent_id": "agt_...",
    "wallet_id": "wal_...",
    "description": "Completion request, 120 tokens"
  }'

Useful for first call to a new service. For subsequent calls, open a session.

Open a session

curl -X POST https://sandbox.getsly.ai/v1/mpp/sessions \
  -H "Authorization: Bearer agent_..." \
  -d '{
    "service_url": "https://inference.example.com",
    "deposit_amount": "50.00",
    "currency": "USDC",
    "agent_id": "agt_...",
    "wallet_id": "wal_..."
  }'

Response:

{
  "id": "mpp_sess_...",
  "status": "open",
  "budget_remaining": "50.00",
  "budget_used": "0.00",
  "currency": "USDC",
  "voucher_key": "<ed25519 public key for voucher verification>",
  "expires_at": "2026-04-23T18:00:00Z"
}

The deposit_amount is held from the agent’s wallet. Any unspent balance returns on close.

Sign a voucher

Each unit of work within the session produces a voucher:

curl -X POST https://sandbox.getsly.ai/v1/mpp/sessions/mpp_sess_.../voucher \
  -H "Authorization: Bearer sess_..." \
  -d '{
    "amount": "0.05",
    "description": "Completion request, 120 tokens",
    "request_id": "req_abc123"
  }'

Returns a signed voucher that the service verifies to confirm it’s authorized to deliver and be paid. Sessions enforce budget_remaining — once it hits zero, voucher requests return INSUFFICIENT_BALANCE.

Stream session costs

Useful when you want live metrics during a long session:

curl -N https://sandbox.getsly.ai/v1/mpp/sessions/mpp_sess_.../stream \
  -H "Authorization: Bearer sess_..."

Server-sent events stream back with each voucher:

event: voucher
data: {"voucher_id":"v_...","amount":"0.05","budget_remaining":"49.95"}

event: voucher
data: {"voucher_id":"v_...","amount":"0.02","budget_remaining":"49.93"}

Close a session

curl -X POST https://sandbox.getsly.ai/v1/mpp/sessions/mpp_sess_.../close \
  -H "Authorization: Bearer sess_..."

Settlement is atomic:

All signed vouchers finalize
Total paid amount debits from the agent’s wallet (vs. the deposit hold)
Unspent deposit returns to the agent’s wallet
Session transitions to closed

Service-side: verify a receipt

When a consumer shows you a voucher, verify it:

curl -X POST https://sandbox.getsly.ai/v1/mpp/receipts/verify \
  -H "Authorization: Bearer pk_live_..." \
  -d '{ "voucher_id": "v_..." }'

Returns { valid: true, session_id, amount, issued_at, expires_at } or a failure reason (replay, expired, revoked).

Discover MPP services

curl https://sandbox.getsly.ai/v1/mpp/services?category=inference \
  -H "Authorization: Bearer agent_..."

Returns MPP-enabled services with published pricing. Or probe a specific service:

curl https://sandbox.getsly.ai/v1/mpp/services/inference.example.com/pricing \
  -H "Authorization: Bearer agent_..."

Analytics + reconciliation

Per-tenant summary:

curl "https://sandbox.getsly.ai/v1/mpp/analytics?period=30d" \
  -H "Authorization: Bearer pk_live_..."

Returns transfer volume, fees, session count, budget utilization. Receipt-level reconciliation:

curl "https://sandbox.getsly.ai/v1/mpp/reconciliation?session_id=mpp_sess_..." \
  -H "Authorization: Bearer pk_live_..."

Provision a wallet for MPP

If an agent doesn’t have an MPP-capable wallet yet:

curl -X POST https://sandbox.getsly.ai/v1/mpp/wallets/provision \
  -H "Authorization: Bearer pk_live_..." \
  -d '{ "agent_id": "agt_..." }'

Provisions a Tempo wallet (MPP-native, supports voucher signing).

Endpoints

Endpoint	Purpose
`POST /v1/mpp/pay`	One-shot micropayment
`POST /v1/mpp/sessions`	Open session
`GET /v1/mpp/sessions`	List sessions
`GET /v1/mpp/sessions/:id`	Session detail + vouchers
`POST /v1/mpp/sessions/:id/voucher`	Sign voucher
`POST /v1/mpp/sessions/:id/close`	Close + settle
`GET /v1/mpp/sessions/:id/stream`	SSE cost stream
`POST /v1/mpp/receipts/verify`	Verify voucher
`GET /v1/mpp/services`	Service directory
`GET /v1/mpp/services/:domain/pricing`	Probe pricing
`POST /v1/mpp/wallets/provision`	Provision MPP wallet
`GET /v1/mpp/analytics`	Tenant summary
`GET /v1/mpp/reconciliation`	Receipt-level recon
`GET /v1/mpp/transfers`	List MPP transfers

Lifecycle states

Session: open → active → closed | exhausted (budget ran out) | error

When not to use MPP

One-off calls — overhead of opening a session isn’t worth it; use x402
Human buyers — MPP is machine-to-machine; for humans use UCP / ACP
Fixed-price subscriptions — use AP2 mandates instead

Get Started

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Core Concepts

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SDKs & Tools

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Resources

MPP (Machine Payments Protocol)

When to pick MPP vs. alternatives

Anatomy

One-shot payment (simplest case)

Open a session

Sign a voucher

Stream session costs

Close a session

Service-side: verify a receipt

Discover MPP services

Analytics + reconciliation

Provision a wallet for MPP

Endpoints

Lifecycle states

When not to use MPP

​When to pick MPP vs. alternatives

​Anatomy

​One-shot payment (simplest case)

​Open a session

​Sign a voucher

​Stream session costs

​Close a session

​Service-side: verify a receipt

​Discover MPP services

​Analytics + reconciliation

​Provision a wallet for MPP

​Endpoints

​Lifecycle states

​When not to use MPP

When to pick MPP vs. alternatives

Anatomy

One-shot payment (simplest case)

Open a session

Sign a voucher

Stream session costs

Close a session

Service-side: verify a receipt

Discover MPP services

Analytics + reconciliation

Provision a wallet for MPP

Endpoints

Lifecycle states

When not to use MPP