AI payments protocols

The number of protocols and standards around AI payments is growing quickly. They are often mentioned together, even though in practice they solve different problems. This page groups them by function. Claims about real-world adoption move quickly, so they are stated here without exaggeration.

Six families

1. Mandate and authorization protocols

Carry cryptographic proof of user intent and agent authority.

AP2 — Agent Payments Protocol. Announced by Google in September 2025. Uses W3C Verifiable Credentials to express Intent Mandates, Cart Mandates, and Payment Mandates. Network-neutral by design. Its partner list spans card networks, PSPs, and stablecoin providers, but real working integrations are still at an early stage.

2. Agent-to-merchant checkout protocols

Define how agents discover products, build carts, and complete checkout with merchants.

ACP — Agentic Commerce Protocol. Jointly maintained by OpenAI and Stripe. The specification is maintained on GitHub with tracked versions; releases are planned through 2026. Used in ChatGPT Instant Checkout and Stripe Agentic Commerce Suite.
UCP — Universal Commerce Protocol. Announced by Google in January 2026 alongside Search AI Mode and Gemini. Modular by architecture; interoperates with AP2 for payment authorization and with A2A and MCP for agent coordination.

3. Machine-to-machine payment protocols

Embed payment directly into HTTP interactions between software systems.

x402. Open protocol from Coinbase, governed through a foundation co-created with Cloudflare. Reuses the HTTP 402 "Payment Required" status code. Current implementations are stablecoin-oriented on Base, Solana, Ethereum, Arbitrum, and Polygon; by design it is not tied to a single chain. Public materials cite notable on-chain transaction counts and annualized volumes, but those figures are reported numbers, not independently verified ones, and they move quickly.

4. Agent identity protocols

Establish who the agent is and on whose behalf it acts.

Visa Trusted Agent Protocol (TAP). Introduced in October 2025; identifies agents on merchant surfaces and links them to Visa cardholder identity.
Mastercard Agent Pay. A comparable scheme on the Mastercard side.
American Express ACE Developer Kit. Announced in April 2026. Adds issuer-side agent registration, tokenized credentials, and an Agent Purchase Protection program.
Skyfire KYAPay. An independent Know Your Agent protocol used by Skyfire's network; built around scoped credentials and decisioning at the moment of the transaction.
Cloudflare Web Bot Auth. Cryptographic authentication for bot and agent traffic at the network edge. It underlies parts of the current Visa TAP and Mastercard Agent Pay implementations.

5. Agent-to-agent and agent-to-tool protocols

Coordinate agents with other agents or with external tools.

A2A (Agent-to-Agent). A family of protocols for inter-agent interaction; includes an x402 extension for crypto payments between agents.
MCP (Model Context Protocol). A protocol from Anthropic for structured agent-to-tool access. Not itself a payment protocol, but widely used as the framework through which payment-capable tools are exposed to agents.

6. On-chain agent identity

ERC-8004. An Ethereum standard for on-chain agent registration and identity. Still early, but relevant for crypto-native agent flows.

How they fit together

No single protocol covers the full path from intent to settlement. A plausible composition for an AI-initiated consumer purchase looks like this:

Agent identity: Visa TAP, Mastercard Agent Pay, or Skyfire KYAPay.
Mandate: AP2 carrying cryptographically signed user consent.
Checkout: ACP or UCP structuring the merchant interaction.
Execution: a card rail, a stablecoin rail (including x402 for machine-to-machine components), or a bank rail.

For enterprise B2B and M2M flows the composition is different: x402 for pay-per-call, direct banking integrations or stablecoin rails for payouts, and a mandate or operator-policy confirmation as the authorization carrier.

Maturity, stated cautiously

All of these protocols exist publicly and have announced partners. The number with meaningful working volume is still smaller than press coverage may suggest. AP2 and x402 have the clearest specification material; ACP has the clearest working reference implementation, inside ChatGPT and Stripe; TAP and Agent Pay have the clearest network backing, although the networks themselves still describe them as pilots.

Where a decision layer fits

A payment decision layer consumes protocols rather than competing with them. It reads mandate evidence from AP2, scope claims from ACP or UCP, agent-identity signals from TAP, Agent Pay, ACE, or KYAPay, and M2M payment intent from x402 — and evaluates them against operator policy to produce a final decision before execution.

Change log

2026-04-18 — Initial publication.