AI agent payment protocols compared: x402, MPP, AP2, ACP and L402
Updated 2026-10-21
Short answer: x402 and L402 charge for individual API requests over HTTP (stablecoins vs Bitcoin Lightning). AP2 and ACP are about agents buying from merchants: authorizing and checking out a purchase. MPP is a payment-method-agnostic standard for machine payments. They overlap and can work together; for example x402 is described as a crypto rail within AP2.
Side-by-side
| Protocol | Who is behind it | What it is for | How money moves |
|---|---|---|---|
| x402 | Created by Coinbase; governed by the x402 Foundation (Linux Foundation) | Pay for a single HTTP request | Stablecoins (mostly USDC) on Base, Solana and other chains |
| MPP (Machine Payments Protocol) | Co-authored by Tempo and Stripe | Machine-to-machine payments, payment-method agnostic | Tempo tokens, Stripe, Visa network tokens, Lightning (per mpp.dev) |
| AP2 (Agent Payments Protocol) | Google; backed by 60+ organizations | Agent-led purchases with signed “mandates” (intent, cart, payment) | Payment-agnostic; x402 can serve as its crypto rail |
| ACP (Agentic Commerce Protocol) | OpenAI and Stripe | Agents checking out from merchants (powers ChatGPT Instant Checkout) | Payment-infrastructure agnostic; Stripe integration available |
| L402 | Lightning Labs | Pay-per-request API access with macaroons + HTTP 402 | Bitcoin over the Lightning Network |
x402
An HTTP-native protocol: the server answers 402 Payment Required with a price, the client signs a stablecoin payment and retries. No account or API key. It is by far the most common protocol in our directory (239 of 242 listings). Deep dive: What is x402?
MPP
The Machine Payments Protocol is an open standard co-authored by Tempo and Stripe. Its site describes it as extensible and agnostic to payment method, with support for payments over Tempo, the Stripe API, Visa network tokens and Lightning. It also lists MPP-enabled services.
AP2
Google’s Agent Payments Protocol focuses on trust in agent-led purchases. It uses verifiable digital credentials and a three-mandate model (intent, cart, payment) so a merchant can prove the user authorized what the agent bought. It is payment-agnostic, covering cards and, through x402, crypto.
ACP
The Agentic Commerce Protocol, by OpenAI and Stripe, standardizes how an agent checks out from a merchant. It powers Instant Checkout in ChatGPT and is open source under Apache 2.0.
L402
Lightning Labs’ L402 pairs HTTP 402 with Lightning invoices and macaroons (bearer credentials). It is the Bitcoin-native counterpart to x402 for pay-per-request APIs; Aperture is its production reverse proxy.
Which one should you use?
- Selling API calls or data to agents: x402 (stablecoins) or L402 (Bitcoin). See how to accept USDC for your API.
- Letting agents buy physical or retail goods from your store: look at ACP and AP2 (and Google’s UCP for the merchant side).
- Mixed fiat and crypto machine payments: MPP.
Browse tools by protocol support in the protocols and payments categories.
Sources
- x402: official documentation
- MPP (Machine Payments Protocol): official documentation
- AP2 (Agent Payments Protocol): official documentation
- ACP (Agentic Commerce Protocol): official documentation
- L402: official documentation
- Awesome Agentic Payments (Bitrefill), protocol summaries
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