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.
Quick answer
Which agent payment protocols does Harpd support?
Harpd observes and reconciles payments across four protocols: x402 (the native HTTP 402 Payment Required standard), MPP (Machine Payments Protocol), AP2 (Agent Payments Protocol, with signed mandates), and Kite (an Avalanche-native agent-passport settlement layer). One set of budgets, approvals and alerts applies above all of them, regardless of facilitator or chain.
- Protocols
- x402 · MPP · AP2 · Kite
- Facilitators
- Any (Coinbase, Stripe, Circle…)
- Chains
- EVM (ETH, Base, Arbitrum…)
Data source: Harpd protocol referenceLast updated:
Why it matters
Agents increasingly pay for API and tool calls across different protocols. Finance still needs one view of spend, one budget model and one reconciliation process. Supporting x402, MPP, AP2 and Kite through a single policy layer means teams can adopt new payment rails without rebuilding their controls each time.
Limitations
- This page is a protocol reference, not a step-by-step integration guide; see the Quickstart for setup.
- Harpd is non-custodial — it observes and reconciles payments but never holds funds or keys.
- On-chain verification covers EVM-based networks; support for a given facilitator or chain depends on the connector you deploy.
Data source: Harpd protocol reference
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.