Between October 2025 and mid-2026, over a dozen payment protocols launched to let AI agents spend money autonomously — shipped by Coinbase, Stripe, Google, Visa, Mastercard, OpenAI, and Lightning Labs. If you're building an AI agent, an API, or an MCP server and wondering which payment rail to adopt, this is for you.
None of these protocols were designed to interoperate. But they're converging into a layered stack — settlement, authorization, commerce — rather than competing for a single winner. This comparison is based on primary sources: protocol specs, IETF drafts, foundation announcements, and live adoption data. No fluff. No hype.
Agent payment protocols aren't all doing the same thing. They sort into three functional layers, and a single agent transaction may touch all three:
| Layer | What It Does | Protocols |
|---|---|---|
| Settlement | How money actually moves between parties | x402, MPP, L402, FADP |
| Authorization / Identity | Who's paying, with what authority, within what limits | AP2, Visa TAP, Mastercard Agent Pay, Amex ACE |
| Commerce | Discovery, shopping carts, checkout, fulfillment | UCP, ACP, x402-mesh |
Think of it like the existing payments stack: settlement = card rails (VisaNet), authorization = 3D Secure + CVV, commerce = Shopify checkout. The agent payment ecosystem is building the same layers, just for machines instead of humans.
This is where most of the attention is right now. x402 and MPP are the two main settlement protocols, and they represent fundamentally different philosophies.
x402 revives the long-dormant HTTP 402 "Payment Required" status code, turning it into a stablecoin payment rail. It was created by Coinbase with Cloudflare and is now governed by the Linux Foundation x402 Foundation with 22 launch members including Google, Visa, Mastercard, Stripe, AWS, Circle, Anthropic, and Vercel.
How it works: Agent sends GET → Server returns HTTP 402 with payment terms (USDC amount, recipient address, chain, facilitator URL) → Agent signs an EIP-3009 authorization with its wallet → Agent retries with the signed proof → Facilitator verifies and settles on-chain → Server returns HTTP 200 with the data. Total time: ~2 seconds.
Key characteristics:
MPP was launched by Stripe and Tempo Labs in March 2026. It also uses HTTP 402, but adds session-based streaming payments, fiat rails, and built-in compliance. It's submitted as an IETF Internet-Draft.
Key characteristics:
| Dimension | x402 | MPP |
|---|---|---|
| Philosophy | Minimalist — one protocol, one job | Maximalist — full commercial stack |
| Payment Model | One-shot per request | One-shot + session streaming |
| Settlement | USDC stablecoins only | USDC + fiat (Stripe rails) |
| Min Payment | ~$0.001 | ~$0.0001 (session mode) |
| Gas | Facilitator pays (gasless for agent) | Batched settlement |
| Standardization | Apache 2.0, Linux Foundation | IETF Internet-Draft |
| Live Since | October 2025 | March 2026 |
| Backwards Compatible | — | Yes — MPP clients can consume x402 services |
MPP is backwards-compatible with x402. An MPP client can consume existing x402 endpoints. This means x402 services automatically get MPP client access — and vice versa is not true.
FADP is an open x402 alternative submitted as an IETF Internet-Draft. It also extends HTTP 402 but uses nonce-based replay protection instead of signatures, and supports any ERC-20 token (not just USDC).
Key differences from x402:
L402 is the oldest agent payment protocol (since 2020), created by Lightning Labs. It also uses HTTP 402 but adds "macaroons" — reusable credentials that let an agent pay once for time-boxed access (e.g., one hour of API calls for one Lightning payment).
| Protocol | Backers | What It Does |
|---|---|---|
| AP2 (Agent Payments Protocol) | Google (now FIDO Alliance) | Verifiable Digital Credentials (Mandates) — users delegate spending authority with budget limits to their agents |
| Visa TAP | Visa | RFC 9421 HTTP Message Signatures for agent identity verification — proves which agent is making a payment |
| Mastercard Agent Pay | Mastercard + Cloudflare | Layered SD-JWT credentials for privacy-preserving agent authentication — live in 9+ APAC markets |
| Amex ACE | American Express | Card-rail identity + purchase protection for agent transactions |
These authorization protocols aren't competing with x402/MPP — they compose on top of them. A payment could use Visa TAP for identity, AP2 for spending authority, and x402 for settlement. In April 2026, Visa launched Intelligent Commerce Connect (ICC) — a single endpoint that translates between TAP, MPP, ACP, and UCP. The convergence is happening at the infrastructure level.
| Protocol | Backers | What It Does |
|---|---|---|
| UCP (Universal Commerce Protocol) | Google + Shopify | Full-stack commerce for agents: discovery → cart → payment → fulfillment |
| ACP (Agentic Commerce Protocol) | OpenAI + Stripe + Meta | REST checkout sessions with delegate payment tokens |
| x402-mesh | StartupHub AI | Peer pricelist + referral layer on x402 — agents discover competing offers, referrers earn commission in USDC |
| If you're building… | Use | Why |
|---|---|---|
| An open, permissionless API for agents | x402 | Simplest path. No accounts, no intermediaries. Just wallet + middleware. |
| High-frequency, enterprise agent payments | MPP | Sessions, sub-cent pricing, fiat support, Stripe compliance. |
| Already on Stripe | Both via Stripe | Stripe supports both x402 and MPP — you get both without choosing. |
| On Base / Coinbase ecosystem | x402 via CDP | Canonical implementation from the team that built it. 3 lines of code. |
| Consumer commerce / checkout | Crossmint + UCP or ACP | Card issuance, agent checkout, widest chain coverage. |
| EU-based, deploying spending agents | Multi-protocol + compliance | EU AI Act Article 26 takes effect August 2, 2026 — human oversight, audit logs required. |
Pick a single protocol only if your use case is narrow. Otherwise, adopt a multi-protocol approach — accept whichever flow an arriving agent presents. This is the same logic as accepting both Visa and Mastercard.
Protocols handle payment. But agents need to know what to pay for. That's the discovery problem — and it's unsolved.
minia2a is the marketplace layer: a catalog of x402-enabled services that AI agents can browse, trial, and pay for. It doesn't compete with x402 or MPP — it sits on top of them, the same way an app store sits on top of payment processors.
When an agent needs to solve a captcha, check a token's security, verify an email, or fetch live market data — it doesn't need to know which protocol the service uses. It just needs to find a service that works, try it, and pay for it. That's the job minia2a does.
The agent payment protocol landscape is converging, not fragmenting. Key signals:
The infrastructure is being laid for a world where AI agents pay each other for services as routinely as humans swipe credit cards. The rails exist. The discovery layer — knowing what to pay for — is the next frontier.
Building an agent that needs to pay for services — or an API that agents should pay for?
minia2a is the marketplace where x402 services are listed, trialed, and paid for by AI agents.
Explore the Marketplace →