Every agentic payment protocol that matters — and where xpay sits.
Six open specs are reshaping how AI agents pay for APIs, services, and merchant goods. No single protocol will win — they solve different problems at different layers. This is the operator-grade map of x402, MPP, UCP, ACP, plus the supporting governance and identity standards (AP2, TAP, A2A). xpay runs the horizontal control plane that works across all of them.
The protocol landscape
| Protocol | Owner | Category | Status | What it does |
|---|---|---|---|---|
| x402HTTP 402 Payment Required | Coinbase + Cloudflare (open spec) | API Economy | Live | Native HTTP payments for APIs and AI agents. |
| MPPMachine Payments Protocol | Stripe + Tempo | API Economy | Pilot | A card-rail protocol for machine-to-machine payments. |
| UCPUniversal Commerce Protocol | Google + Shopify (open spec) | Commerce | Live | Open standard for agent-driven shopping discovery. |
| ACPAgent Commerce Protocol | Stripe + OpenAI | Commerce | Live | Conversational checkout for ChatGPT and beyond. |
| AP2Agent Payments Protocol | Governance | Spec | Governance standards for agent payments — multi-rail. | |
| TAPTrusted Agent Protocol | Visa | Identity | Live | Cryptographic identity for shopping agents. |
| A2AAgent-to-Agent Protocol | Google + Linux Foundation | Agent-to-Agent | Pilot | How agents discover and delegate to other agents. |
Pick the layer you are working at.
The same agent often touches multiple protocols — paying an API via x402, checking out at a merchant via ACP, delegating to another agent via A2A. Filter by where in the stack you are working.
x402
Native HTTP payments for APIs and AI agents.
MPP
A card-rail protocol for machine-to-machine payments.
UCP
Open standard for agent-driven shopping discovery.
ACP
Conversational checkout for ChatGPT and beyond.
AP2
Governance standards for agent payments — multi-rail.
TAP
Cryptographic identity for shopping agents.
A2A
How agents discover and delegate to other agents.
Discovery → Control → Settlement.
Read the agentic payments map as three horizontal layers. Different protocols compete within a layer; the layers themselves are complementary — and most production agents will span all three.
Discovery & Commerce
How the agent finds a merchant or another agent.
Control plane
Spending policies, audit trail, governance — independent of rail. xpay lives here.
Settlement rails
Where the money actually moves.
The comparisons people actually search for.
Pragmatic, no-fluff takes on how the protocols line up against each other.
x402 vs MPP
Stablecoin rail vs card rail for machine payments. When to pick one, when you actually need both.
Read the full comparison →x402 vs ACP
Per-call API settlement vs conversational checkout. They live at different layers of the stack.
Read the full comparison →ACP vs UCP
Stripe + OpenAI's in-chat checkout vs Google + Shopify's open discovery protocol — and why most merchants will publish to both.
Read the full comparison →x402 vs AP2
A settlement rail vs a governance spec. AP2 implementations frequently use x402 underneath.
Read the full comparison →One control plane. Every rail. Same policies.
Operators do not pick a single protocol. They pick the protocol per workflow and need the same governance surface across them. xpay is that surface — a working implementation of AP2-aligned policies, settlement-rail-agnostic.
Frequently asked, plainly answered.
- What are agentic payment protocols?
- Agentic payment protocols are open standards that let AI agents pay for APIs, services, or merchant goods on behalf of a human or another agent. The four primary protocols today are x402 (HTTP-native stablecoin payments), MPP (Stripe + Tempo card-rail machine payments), UCP (open commerce discovery from Google + Shopify), and ACP (Stripe + OpenAI conversational checkout). MCP is sometimes lumped in but is a tool-calling protocol, not a payment one.
- Is x402 the same as MPP?
- No. x402 is HTTP-native and settles in stablecoins (typically USDC on Base) — designed for per-call API payments. MPP rides the existing card network with Tempo clearing — designed for merchant-friendly machine-to-machine card payments. They are complementary: most production agents will speak both.
- How does ACP relate to UCP?
- ACP is conversational-first, anchored to Stripe and OpenAI, and shines inside ChatGPT. UCP is discovery-first, open, anchored to Google and Shopify, and lets any agent shop at any UCP merchant. They overlap at the merchant boundary but optimise for different surfaces — most merchants will publish to both.
- Why is MCP not on this page?
- MCP (Model Context Protocol, by Anthropic) standardises how an LLM calls tools and fetches resources — not how it pays for them. We cover MCP separately in our MCP monetisation product. On this page we focus on protocols whose primary purpose is payment, identity, or commerce settlement.
- What does xpay do across these protocols?
- xpay is the horizontal control plane: one set of spending policies, one audit trail, and one operator surface that works whether the underlying rail is x402, MPP, ACP, or a future protocol. Where a protocol explicitly defers governance (UCP) or limits it to one provider (ACP), we fill the gap.
- Which protocols are live in production today?
- x402 is live on Base mainnet via several facilitators including xpay. ACP is live in ChatGPT with Etsy, Glossier, Vuori, Spanx, SKIMS and a growing Shopify merchant set. UCP launched in January 2026 with Google AI Mode and Shopify Sidekick. MPP is in Stripe + Tempo pilot. AP2 is published as a spec; TAP is live with Visa.
Primary sources we cross-checked.
- UCP — ucp.dev ↗
- UCP — Google Developers ↗
- Building UCP — Shopify Engineering ↗
- x402 — Coinbase Developer Platform ↗
- ACP — Stripe Agentic Commerce ↗
- A2A — Google Developers Blog ↗
- TAP — Oscilar overview ↗
- AP2 — Google Cloud announcement ↗
- Nekuda — UCP analysis ↗