This guide is part of AI agent protocols, within The MCP Guide. Want your website ready for AI agents? See WebMCP development.
Six names come up again and again in AI agent conversations: MCP, A2A, WebMCP, UCP, ACP and AP2. They are not rivals for the same job. MCP connects agents to tools, A2A connects agents to each other, WebMCP puts tools in web pages, UCP and ACP define how agents shop, and AP2 proves a user approved a payment.
They also differ a lot in maturity and ownership. Two are stable and hosted by a Linux Foundation fund. Others are company-led, in beta or still browser experiments. This guide puts them side by side, checked against each project's own pages on 6 November 2026.
Key Takeaways
- MCP (spec 2026-07-28) and A2A (v1.0 since March 2026) are stable and both hosted by the Agentic AI Foundation under the Linux Foundation.
- WebMCP is a W3C community group draft with origin trials in Chrome 149 and Edge 150; Safari's engine opposes it.
- UCP is led by Google and Shopify and powers checkout in Google AI Mode and Gemini; its latest release is v2026-08-25.
- ACP, from OpenAI and Stripe, is "currently in beta," and ChatGPT now sends buyers to merchant-owned checkout.
- AP2, Google's payment mandate protocol, is at v0.2 and was donated to the FIDO Alliance in April 2026.
The comparison table
| Protocol | What it connects | Who runs it | Status, 6 Nov 2026 | Who needs it |
|---|---|---|---|---|
| MCP | AI apps and agents to tools and data | Agentic AI Foundation (Linux Foundation) | Stable; spec 2026-07-28 | Any business connecting AI to its systems or selling a tool to AI users |
| A2A | Agent to agent | AAIF hosted project; Technical Steering Committee | Stable; v1.0 (Mar 2026), v1.0.1 latest | Teams running several agents or working with partner agents |
| WebMCP | Web page to browser agent | W3C Web Machine Learning Community Group | Draft report; Chrome/Edge origin trials | Websites with key tasks agents should do in the page |
| UCP | Agents and platforms to merchants (catalog to orders) | Google and Shopify-led councils | Dated releases; v2026-08-25 | Merchants selling via Google AI Mode, Gemini, Shopify agents |
| ACP | Agents to merchant checkout | OpenAI and Stripe (Meta also a lead maintainer) | Beta; 2026-04-17 latest stable | Merchants and platforms selling via ChatGPT |
| AP2 | Proof of user consent for agent payments | Google, donated to FIDO Alliance | v0.2 (Apr 2026) | Payment providers, wallets, agent builders handling money |
The protocols one by one
MCP: agents to tools
The MCP specification, version 2026-07-28, defines how AI apps connect to servers offering resources, prompts and tools over JSON-RPC 2.0. Anthropic donated MCP to the Agentic AI Foundation, a directed fund under the Linux Foundation, in December 2025. Start with what MCP is.
A2A: agents to agents
The A2A project calls itself "an open standard for seamless communication and collaboration between AI agents." Agents publish Agent Cards and exchange tasks, messages and artifacts. The AAIF announcement of 17 August 2026 says Google launched A2A in April 2025, that it joins AAIF "as a hosted project," and that "A2A v1.0, the first stable specification, shipped in March 2026." Read what A2A is.
WebMCP: web pages to browser agents
WebMCP lets pages register JavaScript tools with document.modelContext.registerTool(). The WebMCP spec is a Draft Community Group Report dated 2 November 2026 and is not a W3C Standard. The implementation status page lists origin trials in Chrome 149 and Edge 150, experimental support in Brave's Leo and support in ChatGPT Desktop. WebKit opposes it; Mozilla is neutral. Read what WebMCP is.
UCP: the commerce journey
Google's launch post of 11 January 2026 says UCP "standardizes the full commerce journey - from discovery and consideration to purchase and order management," and offers integration "via APIs, Agent2Agent (A2A), and the Model Context Protocol (MCP)." It was developed with Shopify, Etsy, Wayfair, Target and Walmart, and its governance gives Google and Shopify permanent council seats. Read the UCP explainer.
ACP: checkout for ChatGPT
The ACP repository says the spec is "maintained by OpenAI and Stripe and is currently in beta." The ChatGPT merchants page says OpenAI is "moving away from a standalone Instant Checkout experience" and that purchases now finish on merchant sites or apps, while ACP remains "the open infrastructure for AI-native commerce in ChatGPT." Read the ACP explainer.
AP2: proof that the user said yes
The AP2 site describes signed mandates and says AP2 is "available as an extension for the open-source Agent2Agent (A2A) protocol." It links the v0.2 release and Google's donation to the FIDO Alliance, and says standardization will continue in FIDO working groups. Read the AP2 explainer.
How the layers fit together
Think of a shopping trip handled by agents:
- A shopper asks a browser agent to find a product. The store's page offers a search tool through WebMCP.
- Elsewhere, an assistant searches catalogs through MCP tools that implement UCP, as Shopify's
/api/ucp/mcpendpoint does. - The assistant hands a delivery booking to a courier's agent over A2A.
- At checkout, UCP or ACP defines the checkout session.
- AP2 mandates prove the shopper approved this cart and payment.
Real deployments will use only some of these. But the picture shows that they are layers, not competitors. UCP even lists MCP and A2A as transports, and AP2 is built as an A2A extension.
Governance at a glance
| Neutral foundation? | Main decision makers | |
|---|---|---|
| MCP | Yes: AAIF (Linux Foundation) | MCP maintainers under AAIF |
| A2A | Yes: AAIF (Linux Foundation) | Technical Steering Committee: AWS, Cisco, Google, IBM Research, Microsoft, Salesforce, SAP, ServiceNow |
| WebMCP | W3C community group (not standards track) | Editors from Microsoft and Google; browsers decide whether to ship |
| UCP | No | Governing Council with permanent Google and Shopify seats; Google holds proxy votes for open seats until Dec 2028 |
| ACP | No (foundation named as future path) | OpenAI and Stripe as Founding Maintainers |
| AP2 | Yes, moving into FIDO Alliance | FIDO working groups |
Sources: the A2A site, the UCP governance document (detailed in our UCP explainer), the ACP README and the AP2 site, all linked above.
Which ones does your business need?
| You are... | Start with | Later, maybe |
|---|---|---|
| A small business connecting AI to your tools | MCP | A2A |
| A SaaS or AI-tool company | MCP server for Claude, ChatGPT and others | A2A Agent Card, WebMCP in your web app |
| An online store | Product feeds plus your platform's UCP and ChatGPT options | WebMCP on product and cart pages |
| A large retailer with a custom stack | UCP and ACP, chosen by channel | AP2 as payment partners adopt it |
| A company running many internal agents | MCP plus A2A | AP2 if agents spend money |
| A payment provider or wallet | AP2, UCP payment handlers, ACP delegated payment | x402 for machine payments |
Common mistakes to avoid
- Treating drafts as standards. WebMCP and AP2 are early. Build prototypes, not core systems, on them.
- Mixing up the acronyms. "ACP" can also mean Zed's Agent Client Protocol or IBM's Agent Communication Protocol, now merged into A2A.
- Building what your platform already gives you. Shopify merchants get UCP through Shopify's tools.
- Skipping consent. Every one of these protocols leaves the final yes on money to a human or to signed user limits.
Next step
eSEOspace can help you pick the right protocol and build it. Our MCP development service builds MCP servers, and we also prototype WebMCP page tools and plan multi-agent setups. Every project is quoted individually. Bookmark the agent protocols hub for updates.
More in the MCP Guide
Latest research: State of MCP: October 2026
Frequently asked questions
Which AI agent protocol is most important?
For most businesses, MCP. It is the stable, widely supported way to connect AI apps to your tools and data. The others matter for specific jobs: multi-agent work, shopping, browser agents or payments.
Are MCP and A2A owned by the same group?
Both are hosted by the Agentic AI Foundation, a directed fund under the Linux Foundation. A2A keeps its own Technical Steering Committee.
Is WebMCP part of MCP?
No. WebMCP is a separate browser API from a W3C community group. It borrows MCP's vocabulary of tools and schemas but does not use the MCP protocol itself.
Do UCP and ACP do the same thing?
Mostly the same job, for different channels. UCP is led by Google and Shopify and used in Google AI Mode and Gemini. ACP comes from OpenAI and Stripe and is used in ChatGPT.
Where does x402 fit?
x402 is an HTTP payment standard for paying per request, often with stablecoins. AP2 can work alongside it, and MCP servers can use it to charge for tool calls. Our AP2 explainer covers how the two relate.
How often does this change?
Often. UCP shipped four releases in 2026, WebMCP's spec changed on 2 November 2026, and ChatGPT's checkout model has already changed once. Recheck status before you build.
