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AI Agent payment protocols overview · a 7-protocol survey

Confidence: Likely Updated 2026-05-26 Review by 2026-09-22 Sources 6 Machine-translated Original (JA)
#agent-economy#ai-agent#payments#protocol#ap2#x402
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Wiki route

This entry sits under agent-economy INDEX. Read it against AI Agent payment 7-protocol layering table for peer / contrast context and payments index for the broader system / regulatory boundary.

Key facts

  • The industry shorthand “4 protocols” refers to AP2 + x402 + MCP + A2A — the 4 most frequently cited
  • At the AP2 launch (2025.09) there were already 60+ partners (Coinbase / MetaMask / Salesforce / American Express)
  • x402 is not chain-specific but rides the HTTP standard — any stablecoin, any chain can connect
  • MCP is the LLM ↔ tools protocol released by Anthropic in 2024.11 , followed by OpenAI in 2025
  • In mid-2026 年 the FIDO Alliance took over AP2 , renaming it AAIF (Agentic AI Identity Federation)

Mechanism / How it works

The 7 protocols divide labor by layer. The intent layer (where the user expresses “I want to buy Y for X yen”) is handled by AP2 / SPT / ACP, which process the mandate’s 3-layer trust chain (Intent → Cart → Payment), fixed in place with VC + DPK signatures. The transport layer (the actual money transfer) is handled by x402 , which revives the HTTP 402 Payment Required header to realize per-call USDC micropayments; the Lite version has already landed in Cloudflare Agents / Vercel AI SDK. The context layer (how the LLM invokes tools) is handled by MCP, which standardizes the LLM ↔ tools / data-sources interface, while A2A handles agent-to-agent communication. The 3 layers are complementary, not mutually exclusive. A complete agent payment flow usually invokes AP2(intent) + x402(transport) + MCP (context) simultaneously. The lower layer depends on ERC-4337 Overview · Application-Layer Implementation of Account Abstraction and ERC-7702 Overview · The Pectra Upgrade Where EOAs Temporarily Gain SCW Functionality, which provide programmable wallet capability.

Origin & evolution

2024.11 MCP debuted, establishing the de facto standard for the “LLM interface.” 2025.05 Coinbase launched x402 , reviving the 30-year-old HTTP 402 status code and binding it to USDC (for USDC’s role see USD Stablecoin Interchange Market). 2025.09 Google, together with 60+ partners, launched AP2 , positioning it as “the standard for agent payment intent.” Within 2025 年, secondary competing protocols such as SPT (Skyfire) / ACP (Catena Labs · Sequoia-backed) / TAP (xpay) appeared one after another. In mid-2026 年 FIDO took over AP2 and renamed it AAIF; Apple / Microsoft / Mastercard joined and the federation expanded to 100+ — the protocol layer entered the channel of commoditization, analogous to WebAuthn / OAuth / OpenID Connect (for detail see Commoditization of Agent payment protocols and the upward migration of value).

Sources