Glossary
Agentic commerce glossary
The protocols and terms behind AI-agent shopping, in plain language. Start with the agentic commerce guide for how they fit together.
- Agent Payments Protocol (AP2)
- The Agent Payments Protocol (AP2) is Google's specification for cryptographic proof of payment authorization — who approved a purchase, spending limits, and which merchants are allowed — kept separate from what was ordered. It plugs into UCP.
- Agent-to-Agent Protocol (A2A)
- Agent-to-Agent (A2A) is the open protocol for one AI agent to communicate with another — a buyer's agent negotiating with a merchant's agent. Agents publish an Agent Card at /.well-known/agent.json declaring their capabilities and how to reach them.
- Agentic checkout
- Agentic checkout is an AI agent completing a purchase on a buyer's behalf — creating a cart, applying payment, and placing the order through a commerce protocol (ACP or UCP) instead of a human filling out a web form.
- Agentic Commerce Protocol (ACP)
- The Agentic Commerce Protocol (ACP) is OpenAI and Stripe's specification for AI-agent shopping in ChatGPT. It has three parts: a product feed merchants push, an agentic-checkout API, and a delegated-payment flow built on Stripe's Shared Payment Token.
- GTIN (Global Trade Item Number)
- A GTIN is the global product identifier — UPC, EAN, or ISBN — that AI agents use to match your product across surfaces. It's mandatory for Perplexity and a high-confidence signal everywhere; adding GTINs is one of the highest-ROI product-data actions.
- llms.txt
- An /llms.txt file is a plain-text manifest at a site's root (the llmstxt.org convention) that points AI agents at key resources — the product feed, policy pages, taxonomy — so they don't have to guess. It's a voluntary signal, not a requirement.
- Model Context Protocol (MCP)
- The Model Context Protocol (MCP) is the open standard — originally from Anthropic, now under the Linux Foundation — for connecting AI agents to tools and data: search a catalog, read inventory, create a cart. It's the lowest-level plumbing the commerce protocols ride on.
- Universal Commerce Protocol (UCP)
- The Universal Commerce Protocol (UCP) is the Google- and Shopify-led specification for the full agent commerce lifecycle — discovery, checkout, payment, and post-purchase. Merchants declare what they support at /.well-known/ucp, and it powers Google AI Mode and Gemini shopping (Microsoft Copilot aligns to it).