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.

What is the one-sentence difference between x402, AP2, ACP and UCP?

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.

How do x402, AP2, ACP and UCP compare at a glance?

Only x402 is a plain settlement rail; AP2 authorizes, ACP bundles checkout with a delegated payment, and UCP standardizes the journey around them. The table’s four rows mirror four records in the Almanac’s own protocols dataset (data/almanac/protocols.json: 28 records, 7 in the payments layer, each with source URLs and a last_verified date, served at /api/protocols); transaction volumes appear only as reported by the named tracker with its as-of date, never as audited fact.

Disambiguation: ACP on this page means the Agentic Commerce Protocol (OpenAI + Stripe, payments). The Atlas also catalogues a second ACP — the Agent Communication Protocol (IBM/AGNTCY), an interop protocol that has nothing to do with checkout.

What does each protocol actually do?

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, with no account, card form or API key. Coinbase published the x402 whitepaper on May 6, 2025 (Coinbase, 2025); since July 14, 2026 the spec is stewarded by the x402 Foundation under the Linux Foundation, whose operational launch counted 40 member organizations (Linux Foundation, 2026), and settlement is network-agnostic: any EVM chain (Base, Polygon, Arbitrum and others), plus Solana, Stellar, XRPL, NEAR, TON, Aptos, Algorand, Hedera, Keeta and Concordium — which of those is actually live depends on the facilitator (x402 project, 2026). 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 published methodology warns that raw settlement counts include ecosystem testing, self-dealing and infrastructure activity, so genuine commerce is a subset of the total (realdora, 2026). No second tracker corroborates that figure, and Coinbase’s ecosystem separately reported 165M+ by April 2026 on a different, non-comparable basis. Treat every volume figure here as reported, not audited. Read x402's full settlement record in the Atlas or see x402 defined in the Lexicon.

AP2 — the authorization layer

AP2 authorizes. It answers the question no settlement rail answers: can this agent prove a human allowed this spend? Its mandates — Checkout Mandate and Payment Mandate in the v0.2 spec of April 28, 2026 (the launch spec named them Intent/Cart/Payment) — travel as verifiable digital credentials (VDCs; SD-JWTs in the v0.2 spec), then AP2 routes the authorized payment to cards, real-time bank transfers or stablecoins (AP2 project, 2026). It is deliberately rail-agnostic: Google's companion a2a-x402 extension plugs x402 in as the crypto rail. Google announced AP2 on September 16, 2025 with more than 60 organizations — merchants, payment networks and tech partners — including Mastercard and PayPal (Google, 2025). The specification site states that standardization will continue within FIDO's Agentic Authentication Technical and Payments Technical Working Groups (AP2 project, retrieved September 2026). AP2's mandate model is specified in the Atlas.

ACP — in-conversation checkout

ACP checks out. It defines the REST contract between a buyer, the buyer's AI agent, a merchant and a payment provider, so a purchase completes inside the conversation: the agent passes a narrowly scoped delegated payment token, and the merchant — who stays merchant of record — charges it through any compliant provider (Stripe, 2025). Launched September 29, 2025 by OpenAI and Stripe as the protocol behind ChatGPT Instant Checkout, with US Etsy sellers live on day one (Stripe, 2025). That standalone in-chat checkout was wound down on March 24, 2026, when OpenAI shifted to product discovery inside ChatGPT plus merchant- and app-owned checkout (Modern Retail, 2026); ACP itself continues in that role: its fifth dated spec release (2026-04-17) came after the wind-down and is still published by OpenAI and Stripe (OpenAI and Stripe, 2026). ACP uses date-based spec versions (YYYY-MM-DD) and has shipped five: 2025-09-29 (initial), 2025-12-12 (fulfilment), 2026-01-16 (capability negotiation), 2026-01-30 (extensions, discounts, payment handlers) and 2026-04-17 (cart, feed, orders, authentication and MCP) — the latest stable, which supersedes 2026-01-30 and is still labelled beta (OpenAI and Stripe, 2026). The ACP spec record sits in the Atlas.

UCP — the commerce interoperability layer

UCP interoperates. Where the other three move or authorize money, UCP standardizes the surrounding journey — product discovery, cart building, checkout — so an agent can shop across retailers without bespoke integrations. It is transport-agnostic (a business can expose UCP capabilities over REST, MCP or A2A) and it does define its own payment model: the Checkout capability separates the instruments a buyer pays with from the payment handlers that process them, with the business staying merchant of record (Google, 2026). AP2 sits on top as an optional extension, not as the base authorization layer — a business opts in by declaring support for the AP2 extension in its discovery document, which adds cryptographically signed mandates (VDC / SD-JWT) (UCP project, 2026). Sundar Pichai introduced it at NRF on January 11, 2026 as open and agnostic, built with Shopify, Etsy, Wayfair, Target and Walmart and endorsed by 20+ more (Google, 2026); ucp.dev now lists co-developers across shopping, lodging and food, including Amazon, Microsoft, Meta, Salesforce and Stripe (UCP project, retrieved September 2026). Governance runs on the protocol's own GitHub organization — a five-seat Governing Council plus per-domain Tech Councils (Shopping, Food, Lodging and Payments) and chartered domain working groups — and it is vendor-led rather than neutral: Google and Shopify each hold a permanent seat on the Governing Council, Google holds the proxy vote for all open seats until December 2028 and acts as custodian of the ucp.dev domain (UCP project, 2026). It is not hosted by the Linux Foundation. Latest version: v2026-08-25, released August 25, 2026 (UCP project, 2026). UCP's interoperability spec is catalogued in the Atlas.

How do x402, AP2, ACP and UCP fit together?

The four protocols compose into a stack rather than a knockout fight: UCP describes the shopping journey, AP2 proves the agent may spend, and x402 or a card rail settles the money — while ACP is the vertically integrated alternative for chat-native retail, bundling checkout and delegated payment into one contract. Two compositions are explicit in the primary specs: AP2 plugs in x402 as its crypto rail (the a2a-x402 extension), and UCP offers AP2 as an optional payment-authorization extension. The real fork in the road is ACP vs the UCP/AP2 stack — one conversation-native and OpenAI/Stripe-shaped, one interoperability-first and consortium-shaped. Adjacent, not covered here: MPP, Stripe and Tempo's machine-billing protocol (announced March 18, 2026 — Stripe, 2026), and Visa TAP, which verifies the agent's identity at checkout (announced October 14, 2025 — Visa, 2025) — see the full payments layer of the Atlas.

Which protocol should your site support?

Match the protocol to what you sell. If agents pay for your resources — API calls, content, per-request access — implement x402: it is live, HTTP-native and needs no checkout. If you run retail checkout and want ChatGPT-surface demand, implement ACP: it is how ChatGPT surfaces your catalogue for discovery and how a purchase completes in your own app or store. If you are building for the interoperable long game, watch UCP and expect AP2 mandates to arrive with it. The honest sequencing for most sites as of September 2026: x402 first (it is the only one a small site can ship alone), ACP where chat-commerce demand already exists, UCP/AP2 as the consortium stack matures. This site practises the first step itself: POST /api/pay issues a demo token and GET /api/premium/playbook answers HTTP 402 with x402-style terms until the X-Payment header arrives — a SIMULATED flow that moves no money. The commerce readiness dimension covers what agents need from your site either way — and the Agent-Readiness Audit 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 (APIs, content, per-request access), 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 across commerce systems — 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 on this page were verified against their primary specs on July 30, 2026 and re-checked against the sources listed below on September 7, 2026. Transaction-volume figures are not from a primary spec and are not audited: each is attributed in the text to the tracker or ecosystem that published it, with its as-of date.

Related: how value moves on the agentic web · the payments layer of the Atlas · x402 · AP2 · ACP · UCP · agentic commerce, defined · audit your site's commerce readiness

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