Five Protocols, Five Different Architectures
By April 2026 the agent payment protocol landscape has consolidated around five major players: Coinbase's x402, Stripe's MPP, Google's AP2, Visa TAP, and Mastercard Agent Pay. They share the high-level goal of letting AI agents transact reliably on behalf of users. They differ in almost every architectural choice: settlement layer, authorisation model, target rail, and standardisation philosophy.
At a Glance
| Protocol | Sponsor | Layer | Settlement rail | Status (April 2026) |
|---|---|---|---|---|
| x402 | Coinbase / Linux Foundation | HTTP-level signalling + crypto settlement | USDC on Base, generalising | Production, expanding |
| MPP | Stripe | Managed credentials | Card and bank rails through Stripe | Production |
| AP2 | Cross-rail intent and authorisation | Rail-neutral; works above any rail | Active development, early production | |
| Visa TAP | Visa | Card-network agent credentials | Visa card rails | Pilot, broader rollout 2026 |
| Mastercard Agent Pay | Mastercard | Card-network agent credentials | Mastercard card rails | Pilot, broader rollout 2026 |
x402: The Crypto-Native Layer
x402 is the HTTP-level protocol Coinbase shipped in 2025 (and donated to the Linux Foundation as an open standard) that lets servers respond to requests with HTTP 402 Payment Required, including a settlement instruction the requesting client can act on automatically. x402 is the only protocol of the five that is rail-defined: it specifies USDC-on-Base as the canonical settlement, with extensions to other stablecoins and chains. The strength is operational simplicity for crypto-native flows. The constraint is that the rail is crypto-only by default, which limits applicability for fiat-only flows.
MPP: The Card-Rail Managed Credentials Layer
Stripe's Managed Payments Profile is not a new rail but a managed configuration on top of card and bank rails. It moves authorisation upstream so agents can transact within user-defined constraints (caps, categories, time bounds) without interactive auth. The strength is operational maturity on the largest existing rail. The constraint is Stripe-specificity: MPP is not interoperable with other card processors out of the box.
AP2: The Cross-Rail Protocol
Google's AP2 is the most ambitious in scope: a rail-neutral protocol that expresses agent payment intent and authorisation in a way that any underlying rail (cards, bank, stablecoin) can settle. The strength is composability. AP2 can sit above x402, MPP, TAP, and Agent Pay simultaneously, providing a unified agent-side interface. The constraint is adoption velocity: cross-rail neutrality requires buy-in from each underlying rail, which is slower than rail-specific deployment.
Visa TAP and Mastercard Agent Pay: The Card-Network Counterparts
Visa's Trusted Agent Protocol and Mastercard's Agent Pay are the card-network counterparts to MPP. They operate at the network level (Visa, Mastercard) rather than the processor level (Stripe), which means they apply to any issuer participating in the network rather than being processor-specific. Both are in pilot as of April 2026 with broader rollout through the year. Functionally they are similar but card-network-bound; merchants accepting both card networks need to handle both protocols if they want full agent-context metadata on transactions.
How They Compose
The five protocols are not strictly competitive. They sit at different layers and can compose. A typical mature stack in 2027 might look like: ai.txt declares pricing intent at the publisher level, the publisher returns x402 (or marketplace-mediated 402) for crypto-native settlement, AP2 mediates the agent's intent and authorisation, MPP or TAP/Agent Pay provides card-rail settlement when the agent uses fiat, and ERC-8004 attestations create a verifiable record of what was paid. Each layer is necessary; few of them substitute for each other.
What Brands and Publishers Should Care About
For publishers, the practical concern is making sure your monetization stack accepts payments through whichever rail an AI agent uses. As of April 2026, this means at minimum supporting x402 for crypto-native flows and accepting card-rail settlement (which any standard merchant processor handles by default) for the card-rail protocols. The cross-rail AP2 layer is mostly invisible to merchants because it abstracts to the underlying rail.
For brands buying agent-mediated visibility, the concern is making sure your commerce surfaces handle agent traffic cleanly across all five protocols, since you cannot predict which one an arriving agent will use. The agentic commerce readiness scorecard (six dimensions: render without auth, schema, price/stock consistency, anti-bot infrastructure, order review brand identity, payment authorization) covers what works for all five.