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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.

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When to pick MPP vs. alternatives

SituationProtocol
New consumer discovered your API and pays per callx402
Recurring relationship, many calls per sessionMPP
Pre-authorized budget envelope, no per-call voucherAP2
Hosted checkout for human buyerUCP / ACP

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Anatomy

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

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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.

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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.

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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.

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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"}

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Close a session

curl -X POST https://sandbox.getsly.ai/v1/mpp/sessions/mpp_sess_.../close \
  -H "Authorization: Bearer sess_..."
Settlement is atomic:
  1. All signed vouchers finalize
  2. Total paid amount debits from the agent’s wallet (vs. the deposit hold)
  3. Unspent deposit returns to the agent’s wallet
  4. Session transitions to closed

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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).

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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_..."

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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_..."

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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).

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Endpoints

EndpointPurpose
POST /v1/mpp/payOne-shot micropayment
POST /v1/mpp/sessionsOpen session
GET /v1/mpp/sessionsList sessions
GET /v1/mpp/sessions/:idSession detail + vouchers
POST /v1/mpp/sessions/:id/voucherSign voucher
POST /v1/mpp/sessions/:id/closeClose + settle
GET /v1/mpp/sessions/:id/streamSSE cost stream
POST /v1/mpp/receipts/verifyVerify voucher
GET /v1/mpp/servicesService directory
GET /v1/mpp/services/:domain/pricingProbe pricing
POST /v1/mpp/wallets/provisionProvision MPP wallet
GET /v1/mpp/analyticsTenant summary
GET /v1/mpp/reconciliationReceipt-level recon
GET /v1/mpp/transfersList MPP transfers

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Lifecycle states

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

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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