This is a sourced chronology of the agentic-commerce events that shape how AI shopping agents select stores, products, and offers: protocol launches, market shifts, and the research we cite across this handbook. Each entry is typed by evidence, sits under the year it happened, and links its primary source. The list runs newest first; it is not exhaustive, and it grows as verified events land. For the running log of smaller changes to this site and to how agents choose, see the updates log.
2026
llms.txt specification v2 replaces the 2024 proposal
v2 removes the Optional-section context-expansion semantics, allows extension-replacement .md URLs alongside appended ones, and adds Markdown link relations (rel=alternate and rel=describedby) as the discovery mechanism, so llms.txt guidance written against v1 needs rechecking.
Adyen announces Adyen Agentic, a payment layer spanning UCP, AP2, and ACP
A major payment service provider now abstracts across the competing agentic rails in one API suite, first-party evidence that merchants can support every rail rather than betting on one.
Source: Adyen press release: Adyen Announces Adyen Agentic (opens in new tab)
Google announces Universal Cart and UCP checkout expansion to Canada, Australia, then the UK
UCP-powered checkout moves beyond the US, and Universal Cart spans Search, Gemini, and other Google surfaces, widening where a UCP-ready catalog can be bought from.
Source: Google (blog.google): shopping updates at Google Marketing Live 2026 (opens in new tab)
Microsoft makes UCP-ready Merchant Center feeds generally available in the US
UCP's consuming surfaces now include Microsoft Copilot, not only Google's, so one UCP feed reaches a second major agent ecosystem.
Source: Microsoft Advertising blog: Win across all three eras of the web (opens in new tab)
Adobe reports retail AI-driven traffic up 393% year over year in Q1 2026
Agent-referred demand is already arriving, and in March 2026 it converted 42% better, so being selectable by agents is a present concern, not a future one.
Adobe finds retail product pages average 66 out of 100 for machine readability
Roughly a third of a typical product page is not machine-readable to an agent, so closing that gap is a concrete advantage most catalogs have not yet taken.
American Express debuts Agentic Commerce Experiences with Agent Purchase Protection
A fourth card-network rail arrives, and it is the first to pair a developer kit with a consumer protection commitment: Amex says it will protect Card Members from errors made by registered agents (some kit services are still in development).
Google announces UCP Cart, Catalog, and Identity Linking capabilities
UCP grows past single-item buying: multi-item carts, real-time variant and inventory retrieval, loyalty-account linking, and a simplified Merchant Center onboarding path.
OpenAI moves away from standalone in-chat Instant Checkout in ChatGPT
Purchases that start in ChatGPT now complete on merchant-owned sites and apps rather than in-chat, and OpenAI's own merchant documentation describes shopping as discovery with merchant-owned checkout, which moves the selection battleground squarely to the product feed.
Google introduces the Universal Commerce Protocol (UCP) at NRF 2026
UCP proposes one open standard spanning discovery, checkout, and post-purchase, with the retailer remaining the merchant of record.
Source: Google (blog.google): Sundar Pichai's NRF 2026 remarks (opens in new tab)
Microsoft launches Copilot Checkout on the Agentic Commerce Protocol with Stripe
Shoppers can complete purchases inside Copilot, with Stripe running the payment leg over ACP and the merchant remaining seller of record, which makes Copilot the second major surface consuming ACP.
2025
Visa and Cloudflare announce the Trusted Agent Protocol
Trusted Agent Protocol lets a merchant verify that an AI agent is legitimate, an identity rail that gates whether an agent's purchase is accepted at all.
Source: Visa: Trusted Agent Protocol (investor.visa.com) (opens in new tab)
OpenAI and Stripe launch the Agentic Commerce Protocol (ACP) and Instant Checkout
ACP launched as the open checkout standard behind ChatGPT Instant Checkout, one of the first mainstream agent-completed purchase flows.
Source: Stripe Newsroom: Stripe and OpenAI bring Instant Checkout to ChatGPT (opens in new tab)
Google announces the Agent Payments Protocol (AP2)
AP2 defines the payment-authorization and trust layer, with signed mandates, that agent checkouts build on.
Source: Google Cloud: Announcing the Agent Payments Protocol (AP2) (opens in new tab)
ACES agent-shopping study first appears on arXiv
The study documents the selection levers this handbook tracks: grid-position bias, a sponsored-tag penalty, an endorsement lift, per-model price sensitivity, and seller-side description rewrites.
Source: What Is Your AI Agent Buying? (ACES, arXiv:2508.02630) (opens in new tab)
Mastercard and Microsoft introduce Mastercard Agent Pay
Agent Pay introduces Mastercard Agentic Tokens: registered, verified agents transact with tokenized credentials under controls the consumer sets, part of the payment-rail layer a store needs to complete an agent's purchase.
Source: Mastercard newsroom: Mastercard unveils Agent Pay (opens in new tab)
2024
Jeremy Howard proposes the llms.txt standard
The llms.txt format gives sites a root-level Markdown index for language models; its only prioritization levers are link order and an Optional section, which shapes what agents read first.
Source: llms.txt proposal (llmstxt.org, Jeremy Howard) (opens in new tab)