x402
The native HTTP 402 Payment Required standard for machine-to-machine payments. From every instrumented agent, Harpd captures each before/after-payment hook.
- Native HTTP integration
- Facilitator-agnostic
- Any EVM chain
Supported protocols
Control agent spend across every payment protocol
Use one set of budgets, approvals and alerts across x402, MPP, AP2 and Kite — without changing who holds funds or keys.
About this page
How to use this page
- Read the four protocol cards — x402, MPP, AP2 and Kite — to see which payment model fits your agents.
- Use the comparison table to compare capabilities (payment model, reservations, disputes, on-chain verification) side by side.
- Check the code sample to see how the
@harpd/observeSDK drops into any stack. - Read the Quickstart for existing integration instructions.
What you'll find here
- One-line summaries of each protocol, a capability comparison table, an “above the facilitator” explainer, and a FAQ.
MPP
Machine Payments Protocol — an HTTP-native payment flow for agents paying for API and tool calls.
- HTTP-native requests
- Pay-per-call metering
- Machine-readable receipts
AP2
Agent Payments Protocol — signed mandates and verifiable authorization for agent-initiated payments.
- Signed mandates
- Verifiable user intent
- Configurable guardrails
Kite
Kite AI Control Plane — agent-passport settlement that normalizes on-chain Kite payments into the same event schema.
- Agent Passport settlement
- Avalanche-native
- Per-settlement metering
Different rails, one spending policy
Your agents may pay in different ways. Finance still gets one view of spend, one budget model and one reconciliation process.
| Capability | x402 | MPP | AP2 | Kite |
|---|---|---|---|---|
| Payment model | Per-request | Session / multi-party | Recurring mandate | Per-settlement |
| Best for | API metered calls | Multi-agent workflows | Subscriptions & billing | Kite AI agent economy |
| Reservations | — | Yes | Yes | — |
| Disputes | — | — | Built-in | — |
| On-chain verify | Always | Always | Always | Always |
Keep your payment stack. Add the guardrails.
Harpd checks policy and budget before your existing facilitator settles a payment, then independently verifies the result. Funds and keys never touch us.
- Drop-in SDK for any stack
- Read-only on-chain verification
- Non-custodial by design
// strict settlement: policy + budget + facilitator + ledger
import { settlePayment } from '@harpd/observe'
await settlePayment({ apiKey: process.env.HARPD_KEY, endpoint: 'https://api.harpd.com' }, {
agentId: 'agent-1', url: 'https://api.example.com/premium-data',
amount: 1000000, asset: 'eip155:8453/erc20:0xusdc', chain: 'eip155:8453',
idempotencyKey: 'order-123',
})
Frequently asked questions
What is x402?
x402 is the HTTP 402 "Payment Required" status — a native web standard for machine-to-machine payments. Harpd observes x402 flows across any facilitator and any EVM chain.
What is MPP (Machine Payments Protocol)?
MPP is an HTTP-native protocol for machine-to-machine payments, letting agents pay for API and tool calls with a standard payment flow. Harpd records each request and settlement as a payment event.
What is AP2 (Agent Payments Protocol)?
AP2 is an authorization protocol for agent-initiated payments. Its signed mandates provide verifiable proof of user intent and configurable guardrails; Harpd records and reconciles the resulting payment events.
What is Kite?
Kite is an agent-passport settlement layer (Avalanche-native) that normalizes on-chain Kite payments into Harpd's unified event schema.
Do I have to use a specific facilitator?
No. Harpd is facilitator-agnostic — Coinbase, Stripe, Circle, Alchemy, AltLayer, or your own. It unifies observability above all of them.
Which blockchains does Harpd reconcile on?
Ethereum, Base, Arbitrum, Polygon, Avalanche, and other EVM chains.