ACP vs AP2 vs x402 vs MPP: agent payment protocols compared

The names arrived close together and are often listed as if they compete. Mostly they don't. A shopping agent buying shoes, an agent proving to a bank that the user really approved a purchase, and an agent paying a cent for an API call are three different transactions, with different parties and different risks. Picking the wrong one means building checkout flows for an API or signing payloads for a merchant that wants a card token.

1. ACP: agent-to-merchant checkout. The Agentic Commerce Protocol, developed by Stripe and OpenAI and open source under Apache 2.0, covers checkout and sharing payment credentials as secure tokens. The business stays merchant of record. OpenAI implements it in ChatGPT and Stripe's Shared Payment Token is the first compatible processor.
2. AP2: proof of what the user authorised. The Agent Payments Protocol, published by Google, uses verifiable digital credentials called mandates, a checkout mandate and a payment mandate, each open (constraints) or closed (a specific authorisation). It is offered as an extension for A2A and the Universal Commerce Protocol.
3. x402: pay for an HTTP request. A server answers 402 Payment Required with terms; the client retries with a signed payment; a facilitator verifies and settles. Stablecoin-based, with schemes for several networks.
4. MPP: Stripe's machine payments. Stripe describes the Machine Payments Protocol as working with card payments through Shared Payment Tokens and stablecoin payments, for charging per tool call or HTTP request.
5. Policy sits beside all of them. None of the four decides how much a particular agent may spend or on whom. That is the job of whatever holds the agent's credential.

• Supporting one doesn't rule out the others. A merchant could accept ACP checkouts and an API could accept x402.
• None of these is a wallet. They describe messages between parties; something still has to hold funds and sign.
• Launch status changes quickly. Check each protocol's own site for what is live where.

• Agentic Commerce Protocol
• AP2: Agent Payments Protocol
• x402 specification and reference implementation (GitHub)
• Stripe docs: Machine payments (MPP and x402)
• Agent2Agent (A2A) protocol

• Shared Payment Token (SPT)A Shared Payment Token (SPT) is a Stripe credential that grants an agent or seller scoped use of a customer's payment method, such as a card, with usage and expiration limits.
• x402 facilitatorAn x402 facilitator is a service that verifies x402 payment payloads and settles them on-chain on behalf of a resource server, through /verify and /settle endpoints.
• MCP serverAn MCP server is a service that offers tools, resources or prompts to AI applications using the Model Context Protocol, an open protocol built on JSON-RPC 2.0.

• What is agentic commerce?Agentic commerce is AI agents finding, buying and paying on someone's behalf. The main approaches, the protocols behind them, and where control sits.
• What is the x402 protocol?x402 uses the HTTP 402 status code so a server can ask for payment and an agent can pay in the same request. How it works and what it needs.
• How can an AI agent pay for an API?Prepaid keys, pay-per-call protocols like x402 and MPP, or paying a provider directly from a controlled wallet. Trade-offs, and what to limit in each.
• Virtual cards for AI agents: how do they work?How issuing a virtual card to an agent works, which spending controls card issuers like Stripe Issuing offer, and where card limits fall short.

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