Agentic Payment Protocols Compared: AP2, ACP, MPP, UCP, and Trusted Agent Protocol

Bottom line: Five protocols define how AI agents pay merchants as of May 2026. Google's AP2 is the cryptographic mandate layer. Stripe/OpenAI's ACP is the checkout API. Stripe/Tempo's MPP is server-to-server machine payments. Google's UCP (Mastercard-backed) is the agent-to-merchant interoperability layer. Visa's Trusted Agent Protocol lets merchants verify trusted agents. Visa's Intelligent Commerce Connect supports all four major rails through a single integration. Most merchants will accept multiple protocols, not pick one.

The agentic payments stack went from concept to five competing protocols in twelve months. This is the practitioner's map: who built what, what each one actually does, and how to think about which to adopt.

Side-by-side comparison

What problem does each one solve?

The five protocols are not competitors in the same lane. They solve overlapping but distinct problems.

AP2 — Verifiable mandates as the trust layer

Agent Payments Protocol (AP2) is Google's open, payment-agnostic protocol for binding user intent to a transaction. It defines three verifiable digital credentials, all SD-JWTs:

• Cart Mandate — generated by the merchant, signed by the user via a hardware-backed key, used when the human is present at purchase. Contains payer/payee identities, tokenized payment method, exact transaction details.
• Intent Mandate — generated by the shopping agent, signed by the user, used when the human is not present (e.g., "buy these tickets when they go on sale"). Contains the agent's natural language understanding of the user's prompt, authorized payment methods, and a TTL.
• Payment Mandate — separate VDC shared with the network and issuer to signal AI agent presence and human-present vs. human-not-present modality.

Per the AP2 specification, the Merchant Payment Processor MUST verify the Payment Credential is appropriately scoped to the Checkout. Specification work is moving to FIDO. Mandates are bound to a specific checkout via cryptographic hash, preventing rainbow-table attacks. This is the dispute-resolution layer for the next decade — the cryptographic proof that "the user authorized this exact thing."

ACP — The actual checkout API

Agentic Commerce Protocol (ACP) is the open standard Stripe and OpenAI co-developed for AI agents to complete purchases on behalf of buyers. Released September 29, 2025, the latest stable spec is dated 2026-04-17 and covers checkout, payment delegation, cart, feed, orders, authentication, and Model Context Protocol integration.

ACP's primitive is the Shared Payment Token (SPT) — a single-use token issued by Stripe (or another compliant PSP) that's scoped to a specific merchant and basket total. The agent passes the SPT to the merchant via API; the merchant creates a PaymentIntent. Card numbers, CVVs, and other sensitive data never leave Stripe's infrastructure.

ACP is governed by OpenAI and Stripe as Founding Maintainers via a Specification Enhancement Proposal (SEP) process documented in the GitHub repo. PayPal joined as a payment provider on October 28, 2025. Walmart shipped full in-ChatGPT purchasing. Stripe's Agentic Commerce Suite (December 11, 2025) makes ACP one-line for existing Stripe merchants and brought Coach, Kate Spade, URBN, Squarespace, Wix, and BigCommerce into the standard.

MPP — Machine-to-machine payments

Machine Payments Protocol (MPP) is the Stripe/Tempo standard for server-to-server payments where there's no human-facing checkout. Visa's Intelligent Commerce Connect lists MPP as one of four supported agent protocols, alongside Trusted Agent Protocol, ACP, and UCP. MPP focuses on backend payment flows — invoice payments, programmatic API billing, agent-to-agent settlements — where neither side is a consumer browsing.

UCP — Cross-platform interoperability

Google's Universal Commerce Protocol (UCP) is the agent-to-merchant interoperability layer. At NRF in January 2026, Mastercard joined Google on UCP. The protocol is designed to let any AI agent talk to any merchant without bespoke per-merchant integration. Per Mastercard's blog, Mastercard supports UCP alongside Google's AP2 and Agent2Agent Protocol, and OpenAI's ACP. Per AP2's FAQ, AP2 is positioned as the specialized payments layer that plugs into UCP for agent-driven transactions.

TAP — Verifying the agent itself

Visa's Trusted Agent Protocol solves a different problem: how does a merchant tell a legitimate AI agent (acting with consumer consent) from a malicious bot? TAP gives participating agents a way to prove their identity and consent at the merchant edge. Merchants can keep bot defenses high while still accepting trusted agent traffic — fewer false declines, fewer disputes. TAP is built to slot into existing acceptance infrastructure that merchants already use.

How do the protocols stack?

This is the most important thing to understand: they are layered, not exclusive.

A real-world flow can look like this:

1. A consumer asks ChatGPT to buy something.
2. ChatGPT (the agent) hits the merchant via ACP (REST/MCP checkout).
3. The merchant verifies the agent identity via TAP at the network edge.
4. The user signs an AP2 Cart Mandate (or Intent Mandate, if not present) with a hardware-backed passkey.
5. Stripe issues an ACP Shared Payment Token scoped to merchant + amount.
6. The transaction crosses the network with an AP2 Payment Mandate signaling agent presence to the issuer.
7. The merchant settles through Visa Intelligent Commerce Connect, which routes through the appropriate card network.

In this stack, ACP is the wire protocol, AP2 is the trust layer, TAP is the agent attestation, and Visa ICC is the acceptance layer. UCP would handle a multi-merchant comparison flow before ACP narrows to a single checkout.

Visa Intelligent Commerce Connect: the meta-layer

Visa launched Intelligent Commerce Connect (ICC) on April 8, 2026 — currently in pilot with Aldar, AWS, Diddo, Highnote, Mesh, Payabli, and Sumvin, with general availability expected by end of June 2026. ICC is a protocol-agnostic, network-agnostic, token-vault-agnostic on-ramp. Through a single integration, merchants accept agent-initiated payments across all four major protocols: Trusted Agent Protocol, MPP, ACP, and UCP.

Per Don Apgar at Javelin Strategy, ICC "effectively enables every VAP-connected processor and acquirer to offer a turnkey agentic commerce platform that's interoperable with all of the current standards." Visa is positioning itself as the neutral payment layer underneath protocol competition — wherever the standards land, transactions still flow through Visa.

Mastercard's parallel play: Verifiable Intent

Mastercard didn't ship a competing wire protocol. Instead, in March 2026, Mastercard open-sourced Verifiable Intent on GitHub — a tamper-resistant cryptographic audit trail linking consumer identity, AI agent instructions, and transaction outcomes. Partners include Google, Fiserv, IBM, Checkout.com, and Basis Theory. The framework uses Selective Disclosure to share only minimum-necessary information.

Where Visa's TAP focuses on agent authentication at the merchant edge, Verifiable Intent focuses on dispute resolution after the fact. Fiserv integrated the Acceptance Framework into its merchant infrastructure, putting agent payments at millions of merchants through existing terminals and gateways.

What should merchants do today?

A few practical recommendations based on adoption signals as of May 2026:

1. Don't pick one protocol. Major rails (Visa ICC, Mastercard Acceptance Framework, Stripe Agentic Commerce Suite) all support multiple. Pick a PSP that supports the protocols your buyers' agents speak.
2. Add ACP first if you sell DTC. It has the largest installed base via Stripe + OpenAI + PayPal + Walmart and one-line activation for existing Stripe users.
3. Add UCP if you want cross-platform reach. Google + Mastercard support means your catalog reaches Gemini, ChatGPT (via ACP bridges), and Mastercard-acquired merchants.
4. Implement TAP / Trusted Agent Protocol at the edge. Even if you don't accept agent payments yet, identifying trusted agent traffic is now a fraud-prevention discipline.
5. Treat AP2 mandates as the audit trail. Even if your wire protocol is ACP, AP2's cryptographic mandates are becoming the dispute evidence standard.

Where Ovra fits

Ovra issues per-agent virtual Visa cards under programmable spending policies — and the architecture is protocol-agnostic at the application layer. Today, in private beta on a sandbox card issuer, Ovra's Intent → Grant → Issue state machine implements the same pre-authorization pattern that AP2 standardized at the network level. Tomorrow, as the regulated EMI partnership ships and TAP, AP2, and ACP support land, the developer interface stays identical. One MCP URL, one REST API, every emerging rail underneath.

The protocols matter — but for builders, the question is which infrastructure layer abstracts them so you don't have to rewrite your agent every six months. That's the bet Ovra is making.

Further reading

• AP2 Core Concepts — Google's official primer on Cart, Intent, and Payment Mandates.
• ACP Specification (2026-04-17) — full OpenAPI + JSON Schema for the latest stable cut.
• Visa Intelligent Commerce — Visa's developer portal, including the MCP server bridge.
• Mastercard Agent Pay — Acceptance Framework + Verifiable Intent overview.
• Stripe Agentic Commerce — implementation guide for ACP and Shared Payment Tokens.
• McKinsey: Agentic commerce opportunity — market sizing and ecosystem analysis.