Agentic commerce protocols: ACP, UCP, AP2, MCP and llms.txt
Agentic commerce protocols give an AI agent a channel beside a store's pages. ACP and UCP cover checkout and the shopping journey. AP2 covers payment authorization. MCP connects AI applications to tools and data. llms.txt is a proposed file that summarizes a site for language models. The agents Cromanion observed still browsed the pages.
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What are the main agentic commerce protocols?
Five names come up most. Three are commerce-specific: ACP, UCP and AP2. One is general: MCP, which some of the others build on. And one is a file rather than a protocol: llms.txt. Each is published openly, and each is described here from its own publisher's page.
| Name | What it is | Published by |
|---|---|---|
| ACP — Agentic Commerce Protocol | An open standard for commerce flows between buyers, AI agents and businesses, including checkout | Stripe and OpenAI, under an open-source license |
| UCP — Universal Commerce Protocol | An open standard for the commerce journey: discovery, checkout, order management and payment | Google, with retailers and platforms including Shopify, Etsy, Wayfair, Target and Walmart |
| AP2 — Agent Payments Protocol | An open protocol for an agent to initiate and authorize payments for a user, using signed mandates | Google, with more than 60 payments and technology companies |
| MCP — Model Context Protocol | An open standard for connecting AI applications to external data, tools and workflows | Introduced by Anthropic; supported across many AI clients |
| llms.txt | A proposed Markdown file at a site's root, summarizing it for language models | Proposed by Jeremy Howard on llmstxt.org; not a formal standard |
How do these protocols fit together?
They sit at different layers. MCP is a general way for an AI application to call tools. UCP and ACP describe the shopping and checkout steps. AP2 describes how a payment is authorized. UCP's publisher says it can be integrated through MCP and is compatible with AP2.
A UCP business publishes what it supports in a JSON file at a fixed address, /.well-known/ucp. Shopify describes its agent tools as implementing UCP across the buyer journey.
AP2 introduces mandates: signed records of what the person asked for and of the exact cart they approved. It builds on MCP and on Agent2Agent (A2A), another agent protocol.
What did the agents Cromanion observed actually use?
The store's pages. On 2026-09-21, Meta's Muse was tested once on this point: it did not use a Shopify store's commerce endpoint or its llms.txt, and navigated the storefront. In three runs on 2026-09-25, xAI's Grok shopping agent loaded the storefront pages and filled the checkout form.
That is a narrow observation, and it is stated as one. Muse was tested once. Whether Grok's agent would use a protocol channel was not tested. The bench agents drove a browser from screenshots and were never offered a protocol either.
It is enough to say that these two agents made the visit. It is not enough to say how often agents in general use a protocol instead.
Which assistants browse a store, and which buy through a feed?
Personal assistants shop in two ways. Some browse: Instinct, Muse, Grok, Perplexity Comet and ChatGPT's browser agent load a store's pages and act on them. Others buy through a feed or a protocol: Perplexity Shopping and Copilot Checkout let the person buy from the assistant's own interface.
For a browsing assistant, the pages are the source. Instinct's company says its assistant loads web pages and apps and interacts with them directly, and a review describes it ordering groceries from its own computer. Muse and Grok's agent were observed doing so on Cromanion's test store. Perplexity presents Comet as a browser with its own assistant. OpenAI closed its Atlas browser on August 9, 2026; its browser-based agent work now runs in the ChatGPT desktop app and the ChatGPT Chrome extension.
For a feed assistant, the product comes from the merchant's catalog data, and the person never opens the store's pages. Perplexity Shopping has offered Instant Buy since November 2025: a checkout in the chat, through PayPal. Copilot Checkout, launched in January 2026, lets a shopper pay without leaving Copilot.
Some sit between the two. Google's “buy for me” starts in Google: the person confirms there, and Google then adds the item to the cart on the merchant's site and completes the checkout on that site with Google Pay.
Feed checkouts change quickly. ChatGPT Instant Checkout launched in September 2025 on the Agentic Commerce Protocol. OpenAI moved away from Instant Checkout in March 2026 and now lets merchants use their own checkout.
Does a protocol replace the store visit?
For an agent that uses it, a protocol can carry the steps a page would. For an agent that does not, the pages are the only source. The two can coexist on one store, and the observed agents took the page route.
Common questions
Is llms.txt a standard?
No. It is a proposal published on llmstxt.org in September 2024, open for community input. Some sites publish one. In Cromanion's single test on this point, Muse did not use it.
Do these protocols let an agent pay without its person?
They define how an agent can check out or initiate a payment for a user. AP2 records the person's instructions and the approved cart as signed mandates. Whether a given agent may pay alone depends on that agent and on what its person allowed.
Sources
- Agentic Commerce Protocol — OpenAI and Stripe. What ACP is, who develops it, and its open-source license. Checked .
- Under the Hood: Universal Commerce Protocol (UCP) — Google for Developers. What UCP covers, who co-developed it, /.well-known/ucp, and its relation to AP2 and MCP. Checked .
- Build commerce agents with UCP — Shopify. Shopify's agent tools implementing UCP across the buyer journey. Checked .
- Announcing Agent Payments Protocol (AP2) — Google Cloud. What AP2 is, its partners, its mandates, and that it extends A2A and MCP. Checked .
- What is the Model Context Protocol (MCP)? — Model Context Protocol. What MCP is, and which AI clients support it. Checked .
- Introducing the Model Context Protocol — Anthropic. Who introduced MCP. Checked .
- The /llms.txt file — llmstxt.org. The llms.txt proposal, its author, date and status. Checked .
- Everyday personal AI assistant startup Instinct raises $1B at $10B valuation — SiliconANGLE. That Instinct's assistant loads web pages and apps and interacts with them directly. Checked .
- Instinct AI review 2026: what the texting agent does and the catch — eesel AI. That Instinct works on its own computer with its user's accounts and orders groceries, and that users shop through it. Checked .
- Comet Browser: a Personal AI Assistant — Perplexity. Comet as a browser with its own assistant. Checked .
- Evolving Atlas into ChatGPT for browser-based agentic work — OpenAI Help Center. Atlas stopping on August 9, 2026, and browser-based agent work moving to the ChatGPT desktop app and Chrome extension. Checked .
- PayPal and Perplexity Launch Instant Buy Ahead of Black Friday — PayPal. Perplexity's shopping experience offering checkout in the chat through PayPal, November 2025. Checked .
- Stripe powers Instant Checkout in ChatGPT and releases Agentic Commerce Protocol co-developed with OpenAI — Stripe. The launch of ChatGPT Instant Checkout on the Agentic Commerce Protocol, 29 September 2025. Checked .
- OpenAI shifts checkout plans in its agentic commerce strategy — Digital Commerce 360. OpenAI's statement that Instant Checkout is moving to Apps, March 2026. Checked .
- OpenAI revamps shopping experience in ChatGPT after struggling with Instant Checkout offering — CNBC. OpenAI moving away from Instant Checkout and letting merchants use their own checkout, March 2026. Checked .
- Shop with AI Mode, use AI to buy and try clothes on yourself virtually — Google. How “buy for me” works: the cart on the merchant's site, checkout with Google Pay. Checked .
- Conversations that Convert: Copilot Checkout and Brand Agents — Microsoft Advertising. The launch of Copilot Checkout, January 2026. Checked .
- PayPal Powers Microsoft's Launch of Copilot Checkout — PayPal. That Copilot Checkout lets a shopper pay without leaving Copilot. Checked .
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