As agents become part of products and services, a growing set of shared protocols is taking shape. A useful distinction is between those that help agents connect and coordinate, and those that support commerce and payment. Their roles overlap, and the relationships between them are still developing.
Connecting and coordinating
MCP (Model Context Protocol): defines how an agent can use external tools and data. Introduced by Anthropic in 2024, MCP is now part of the Linux Foundation’s Agentic AI Foundation and has been adopted across products including ChatGPT, Gemini, Microsoft Copilot and VS Code.
A2A (Agent-to-Agent Protocol): launched by Google in April 2025 and contributed to the Linux Foundation in June 2025, A2A defines how agents discover capabilities, communicate and coordinate work.
Transacting and paying
ACP (Agentic Commerce Protocol): connects agents and merchants across product discovery and checkout. OpenAI launched it with Stripe in ChatGPT’s 'Instant Checkout' in September 2025, initially for ChatGPT users in the United States buying from US Etsy sellers. In March 2026, OpenAI expanded ACP into product discovery and shifted its emphasis towards merchants using their own checkout experiences.
UCP (Universal Commerce Protocol): introduced by Google and retail partners in January 2026, UCP provides a shared structure for commerce interactions, from product discovery to checkout and post-purchase support. It is designed to work with MCP and A2A, and with AP2 for payment authorisation.
AP2 (Agent Payments Protocol): Google announced AP2 in September 2025 with payments and technology partners, then donated it to the FIDO Alliance in April 2026. It is best understood as a trust and authorisation layer for agent-led payments, not a new payment rail. It also supports purchases authorised in advance, where the person is absent when the transaction takes place. Signed mandates record what the person has authorised and the limits within which the agent may act.
MPP (Machine Payments Protocol): introduced by Stripe and Tempo in March 2026, MPP lets agents and services coordinate payments programmatically. This includes an agent paying for API access or other services as part of its work.
A simple way to think of them is this: MCP and A2A help agents connect and coordinate; ACP and UCP structure commerce interactions; AP2 provides evidence of payment authority; and MPP helps agents pay for services. These are useful distinctions, but some roles overlap. The protocols are evolving, and how they work together will depend on the services and systems that adopt them.
Sources
OpenAI, Powering Product Discovery in ChatGPT
https://openai.com/index/powering-product-discovery-in-chatgpt/
Google, Under the Hood: Universal Commerce Protocol (UCP)
https://developers.googleblog.com/under-the-hood-universal-commerce-protocol-ucp/
AP2 specification
https://ap2-protocol.org/ap2/specification/
Stripe, Introducing the Machine Payments Protocol
https://stripe.com/blog/machine-payments-protocol
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