# x402 vs AP2 vs ACP vs UCP: Agent Payment Protocols Compared

> x402 settles a payment inline over HTTP, AP2 authorizes an agent to spend, ACP completes a checkout inside a chat, and UCP standardizes the whole shopping journey — four protocols answering four different questions, verified against their primary specs on 2026-07-30.

## The one-sentence answer

**x402 settles, AP2 authorizes, ACP checks out, UCP interoperates.** x402 (Coinbase, 2025) moves stablecoin money inline over plain HTTP; AP2 (Google, 2025) gives an agent a cryptographically signed permission to spend across any rail; ACP (OpenAI + Stripe, 2025) completes a retail checkout inside the conversation; and UCP (Google, Shopify, Walmart + consortium, 2026) standardizes the full shopping journey — discovery, cart, checkout — across commerce systems. They are not four competitors for one job: AP2 plugs in x402 as its crypto rail (the a2a-x402 extension), and UCP can layer AP2's signed mandates as an optional per-business extension on top of its own Checkout payment model.

## The four protocols at a glance

| Protocol | Created by · launched | The question it answers | How it works | Status (as of Jul 2026) |
|---|---|---|---|---|
| [x402](/protocols/x402) | Coinbase · May 6, 2025 | How does an agent **pay for this request**? | Revives HTTP 402: server answers with payment terms, agent pays in stablecoin on any supported network (any EVM chain, plus Solana, Stellar, XRPL and more), retries — no account, no API key | Live; agenteconomy.to (community dashboard) counted 159.6M cumulative settled transactions across the 7 chains it tracks, Jul 30, 2026 — includes tests/self-dealing, uncorroborated; Coinbase ecosystem separately reported 165M+ by Apr 2026 on a different basis; x402 Foundation (40 member orgs) under the Linux Foundation since Jul 14, 2026 |
| [AP2](/protocols/ap2) | Google + 60+ orgs (incl. Mastercard, PayPal) · Sep 16, 2025 | Was the agent **allowed to spend this**? | Signed mandates as verifiable digital credentials (v0.2: Checkout + Payment Mandate) prove user intent, then route to cards, bank transfers or stablecoins | Emerging; spec v0.2.0 (Apr 28, 2026); open extension for A2A, MCP and UCP; Apache-2.0 |
| [ACP](/protocols/acp-commerce) | OpenAI + Stripe · Sep 29, 2025 | How does a purchase **complete inside a chat**? | Agent hands the merchant a scoped delegated payment token over REST; merchant stays merchant of record, charges via any compliant provider | Live and maintained; was the rail behind ChatGPT Instant Checkout (Etsy live at launch) until the standalone in-chat checkout was wound down Mar 24, 2026 — now discovery + merchant/app-owned checkout; spec version 2026-04-17 (5th, latest stable, beta); Apache-2.0 |
| [UCP](/protocols/ucp) | Google, Shopify, Walmart + consortium · NRF, Jan 2026 | How do commerce systems **interoperate for agents at all**? | Open standard for discovery, carts and checkout, transport-agnostic over REST, MCP or A2A; own payment-instrument/handler model in Checkout, AP2 mandates optional | Active; v2026-04-08; Google/Shopify-founded council governance (Governing Council + per-domain Tech Councils), vendor-led, not under the Linux Foundation; Apache-2.0 |

**Disambiguation:** ACP on this page means the [Agentic Commerce Protocol](/protocols/acp-commerce) (OpenAI + Stripe, payments). The Atlas also catalogues a second ACP — the [Agent Communication Protocol](/protocols/acp) (IBM/AGNTCY), an interop protocol unrelated to checkout.

## What each protocol actually does

### x402 — inline HTTP settlement

x402 **settles**. A server answers an unpaid request with HTTP 402 and machine-readable payment terms; the agent pays in stablecoin and retries — the whole loop lives in plain HTTP. Coinbase launched it May 6, 2025; since July 14, 2026 it is stewarded by the x402 Foundation under the Linux Foundation (40 member organizations at launch), settling network-agnostically: any EVM chain (Base, Polygon, Arbitrum and others), plus Solana, Stellar, XRPL, NEAR, TON, Aptos, Algorand, Hedera, Keeta and Concordium — which are live depends on the facilitator. The community on-chain dashboard agenteconomy.to counted 159,604,144 cumulative settled x402 transactions and $41.1M in stablecoin volume across the 7 chains and 18 facilitators it tracks (July 30, 2026); its own methodology warns raw counts include tests, infrastructure traffic and self-directed usage, so genuine commerce is a subset. No second tracker corroborates it; Coinbase's ecosystem separately reported 165M+ by April 2026 on a different basis. Reported, not audited. See [x402 in the Atlas](/protocols/x402) and [defined in the Lexicon](/glossary/x402).

### AP2 — the authorization layer

AP2 **authorizes**: can this agent prove a human allowed this spend? Its mandates — Checkout Mandate and Payment Mandate in spec v0.2 (Apr 28, 2026; the launch spec named them Intent/Cart/Payment) — travel as verifiable digital credentials (VDCs; SD-JWTs in the v0.2 spec); AP2 then routes to cards, real-time bank transfers or stablecoins. Rail-agnostic by design: the a2a-x402 extension plugs x402 in as the crypto rail. Announced Sep 16, 2025 with 60+ organizations (merchants, payment networks, tech partners) incl. Mastercard and PayPal. [AP2 in the Atlas](/protocols/ap2).

### ACP — in-conversation checkout

ACP **checks out**: a REST contract among buyer, agent, merchant and payment provider. The agent passes a narrowly scoped delegated payment token; the merchant stays merchant of record and charges it through any compliant provider. Launched Sep 29, 2025 by OpenAI + Stripe as the protocol behind ChatGPT Instant Checkout (US Etsy sellers live day one). OpenAI wound the standalone in-chat checkout down on Mar 24, 2026, shifting to product discovery in ChatGPT plus merchant- and app-owned checkout; ACP continues in that role. Date-based spec versions, five so far: 2025-09-29, 2025-12-12, 2026-01-16, 2026-01-30 and 2026-04-17 (latest stable, supersedes 2026-01-30, still beta). [ACP in the Atlas](/protocols/acp-commerce).

### UCP — the commerce interoperability layer

UCP **interoperates**: it standardizes discovery, carts and checkout so an agent can shop across retailers without bespoke integrations. Transport-agnostic (REST, MCP or A2A). UCP defines its own payment model in Checkout — payment instruments separated from payment handlers, business stays merchant of record — and offers the AP2 Mandates extension as an optional, per-business opt-in for signed authorization. Announced by Sundar Pichai at NRF, Jan 2026, with Shopify, Walmart and a broader consortium; governed on its own GitHub org by a Governing Council plus per-domain Tech Councils (currently Shopping and Food) — vendor-led, not neutral: Google and Shopify hold permanent Governing Council seats, Google holds the proxy vote for all open seats until Dec 2028 and is custodian of ucp.dev; not under the Linux Foundation. Latest version v2026-04-08. [UCP in the Atlas](/protocols/ucp).

## How the four fit together

The four compose into a stack: UCP describes the shopping journey, AP2 proves the agent may spend, x402 or a card rail settles — while ACP is the vertically integrated alternative for chat-native retail. Two compositions are explicit in the primary specs: AP2 plugs in x402 as its crypto rail (a2a-x402), and UCP offers AP2 as an optional payment-authorization extension. The real fork is ACP vs the UCP/AP2 stack. Adjacent: [MPP](/protocols/mpp) (Stripe + Tempo machine billing) and [Visa TAP](/protocols/visa-tap) (agent identity at checkout) — see the [payments layer](/protocols/payments).

## Which protocol should your site support?

Match the protocol to what you sell. Agents paying for **resources** (APIs, content, per-request access) → x402: live, HTTP-native, no checkout needed. **Retail checkout** with ChatGPT-surface demand → ACP: it is how ChatGPT surfaces your catalogue for discovery and how a purchase completes in your own app or store. Building for the **interoperable long game** → watch UCP, expect AP2 mandates with it. Honest mid-2026 sequencing: x402 first (the only one a small site ships alone), ACP where chat-commerce demand exists, UCP/AP2 as the stack matures. The [commerce readiness dimension](/agent-readiness/commerce) covers what agents need either way — and the [Agent-Readiness Audit](/services) checks whether your site answers HTTP 402 with a valid payment offer today.

## x402, AP2, ACP and UCP — frequently asked questions

### Are AP2 and x402 competitors?

No. AP2 authorizes — it proves an agent had permission to spend — and x402 settles. Google's own AP2 materials ship x402 as a supported crypto rail via the a2a-x402 extension, so the two are explicitly complementary.

### What is the difference between ACP and AP2?

ACP is a complete checkout contract — delegated payment token, merchant of record stays the merchant — shipped by OpenAI and Stripe. It was the rail behind ChatGPT Instant Checkout from September 2025 until OpenAI wound the standalone in-chat checkout down on 24 March 2026; ACP now carries product discovery in ChatGPT and checkout inside connected merchant apps. AP2 is not a checkout: it is a mandate framework proving user authorization, designed to route any rail, shipped by Google with 60+ partners.

### Which protocol should a website support first?

For machine-payable resources, x402 — it works today over plain HTTP with no consortium dependency. For retail checkout, ACP where ChatGPT demand exists. UCP and AP2 matter as the interoperable stack matures.

### What does UCP add on top of AP2 and x402?

The journey. UCP standardizes discovery, carts and checkout — transport-agnostic over REST, MCP or A2A — and defines its own payment-instrument / payment-handler model in Checkout, with AP2's signed mandates available as an optional extension on top. AP2 and x402 move money; UCP makes the shopping interoperable.

Protocol facts verified against their primary specs on 2026-07-30. Transaction-volume figures are not from a primary spec and are not audited — each is attributed in the text to its publisher and as-of date.

Related: [how value moves on the agentic web](/commerce) · [the payments layer of the Atlas](/protocols/payments) · [x402](/protocols/x402) · [AP2](/protocols/ap2) · [ACP](/protocols/acp-commerce) · [UCP](/protocols/ucp) · [agentic commerce, defined](/glossary/agentic-commerce) · [audit your site's commerce readiness](/services)