Comparing Payment Protocols
Overview
Three payment protocols let AI agents pay for API access. Each has different trade-offs:
| x402 | MPP | L402 | |
|---|---|---|---|
| Network | Base (Ethereum L2) | Tempo | Bitcoin Lightning |
| Currency | USDC | pathUSD | BTC (satoshis) |
| Speed | ~2 seconds | Sub-second | Milliseconds |
| Fees | Low (L2 gas) | Very low | Very low |
| SDK | @coinbase/x402-express | mppx | Custom (LND/CLN) |
| Facilitator | Coinbase | Tempo network | Lightning node |
| Stablecoin | Yes (USDC) | Yes (pathUSD) | No (BTC volatility) |
When to use x402
Best for: Production services that want the broadest ecosystem support.
- Backed by Coinbase with official SDKs in TypeScript, Go, and Python
- USDC stablecoin means predictable pricing
- Base L2 keeps gas fees low
- Largest growing ecosystem of x402-enabled services
- Includes Bazaar discovery for service catalogs
When to use MPP
Best for: High-frequency micropayments where speed matters.
- Sub-second finality on the Tempo chain
- Designed specifically for machine-to-machine payments
- pathUSD stablecoin (1:1 with USD)
- Simple SDK (mppx) with Express middleware
- Lowest overhead per transaction
When to use L402
Best for: Bitcoin-native services and Lightning ecosystem integration.
- Instant settlement via Lightning Network
- No stablecoin — payments in satoshis (BTC)
- Macaroon-based auth tokens enable flexible access control
- Established ecosystem (since 2020, originally "LSAT")
- Requires running or connecting to a Lightning node
Can I support multiple protocols?
Yes. AgentGrade checks for all three independently. Supporting multiple protocols gives agents more payment options. The agent chooses which to use based on what wallets/funds it has available.
Publish all supported protocols in your /.well-known/x402.json service catalog so agents can discover them.
Which should I start with?
If you're unsure, start with x402. It has the most complete SDK, Coinbase backing, and stablecoin pricing. Add MPP or L402 later if your use case benefits from their specific strengths.
The commerce layer above the rails
The protocols above answer "how does an agent pay?" A second family answers "how does an agent shop?" — carts, offers, order handoff, checkout. They sit a layer up and delegate the actual money movement to rails like the ones on this page.
- UCP (Universal Commerce Protocol) — co-developed by Google, Shopify, Etsy, Wayfair, Target, and Walmart (announced January 2026; spec at ucp.dev). JSON over REST and JSON-RPC, with A2A and MCP transports, and discovery via a
/.well-known/ucpmanifest. It powers Google's Universal Cart across Search, Gemini, and YouTube; Shopify's UCP-based agent layer has been self-serve for developers since June 2026. - ACP (Agentic Commerce Protocol) — maintained by OpenAI and Stripe (agenticcommerce.dev, Apache 2.0, beta). It defines product feeds for discovery plus a checkout API; payment is delegated via Stripe-developed Shared Payment Tokens — PSP-agnostic by design, Stripe first. Note the trajectory: OpenAI pulled back in-chat Instant Checkout in March 2026 ("the initial version… did not offer the level of flexibility that we aspire to provide"), refocusing ACP on product discovery with merchant-owned checkout; the checkout spec remains published and gated to approved partners.
- AP2 (Agent Payments Protocol) — the authorization layer: tamper-proof, cryptographically signed mandates proving a human authorized an agent's purchase. Announced by Google in September 2025 with 60+ partners and donated to the FIDO Alliance in April 2026, where two working groups (chaired by CVS Health, Google, and OpenAI; Mastercard and Visa) are standardizing it. AP2 is an extension of A2A and UCP and is payment-method-agnostic — cards first, crypto via the A2A x402 extension. As of mid-2026 it is a spec with broad endorsement and no publicly named production deployment.
The layering matters when you plan: a 402 rail (this page) makes you payable today with a few endpoints; UCP/ACP integration is a platform decision that mostly applies to catalog commerce; AP2 is the trust layer arriving underneath both. What AgentGrade verifies on the wire is the rail layer — a live, decodable payment challenge — because that is the part any site can ship independently.