x402 Leading the Machine Economy! Three Major Protocols Collaborate to Build a New Era of AI Agent Payments

As AI Agents evolve from tools to autonomous economic entities, traditional payment infrastructure can no longer meet their needs for autonomous transactions, cross-ecosystem interactions, and verifiable identities. To address this, three emerging protocols—x402, AP2, and ERC-8004—have jointly constructed the payment foundation for the machine economy. The x402 protocol utilizes the HTTP 402 status code to achieve “API call as payment”, leading the machine economy.

x402 protocol: Activate HTTP 402 to achieve native API payments

x402 is launched by the largest compliant cryptocurrency exchange in the United States, and its core innovation lies in activating the underutilized HTTP 402 status code (“Payment Required”), embedding payment logic natively into the web request-response process, achieving “API call means payment”. This seemingly simple concept actually addresses the most fundamental issue in the AI Agent economy: how to complete high-frequency micropayments without relying on human intervention.

The x402 protocol is based on a client/server architecture. The client is the buyer who purchases services/products, and on the server side, the seller provides the services/products. On this architectural basis, the largest compliant cryptocurrency exchange in the United States offers Facilitators' services to simplify the process of verifying and settling payments between buyers and sellers. Taking the server Canza (which provides trading information AI) ranked first in x402scan as an example, its operating process is as follows:

First, the user initiates a request on the client side to access Canza's paid services. Subsequently, the Canza server uses an HTTP 402 Response to define the payment requirements: the client needs to provide an X-PAYMENT Header and make the payment via USDC on the Base chain. After the client parses the 402 Response JSON content, the wallet will prompt to sign a TransferWithAuthorization message (implemented via ERC-3009). This message allows the signer to transfer funds from the signer's address to a delegated third-party EOA address or contract address without incurring Gas fees.

Subsequently, the user signs the message, and the client submits the Payload using the base64 encoded X-PAYMENT Header. After the Canza server receives the incoming Payload, it will be verified by the facilitator, and the server will settle the payment on the blockchain. Once the Canza server confirms the payment, Canza provides the requested services to the user.

Three Major Advantages of the x402 Protocol:

Multi-chain Support: Supports EVM chains such as Base, Avalanche, and Solana, with flexible server-side configuration.

Low-Friction Payment: Achieve gas-free transfers through ERC-3009, reducing transaction costs.

Native HTTP Integration: No additional payment layer is required, and any application that supports HTTP can easily connect.

The x402 protocol is particularly noteworthy as it supports multiple blockchains (EVM chains such as Base, Avalanche, and Solana) and various crypto assets (must support ERC-3009, default is USDC) for payments, requiring configuration only on the server side. This flexibility allows x402 to adapt to different application scenarios and user preferences.

AP2 protocol: Digital Authorization Constructing a Trustworthy Transaction Framework

AP2 (Agent Payments Protocol) is an open payment framework based on the Agent to Agent (A2A) communication protocol and the Model Context Protocol (MCP) extension. Its core objective is to address three major core issues in Agent commerce: authorization verification (proving that the Agent has obtained user consent), authenticity (ensuring that transactions reflect the user's genuine needs), and transaction accountability (clarifying responsibility in case of disputes).

The workflow of the AP2 protocol revolves around the core concept of digital mandates, which are tamper-proof, cryptographically signed digital contracts that serve as verifiable evidence of user instructions. They are specifically divided into three types of mandates:

The Intent Mandate is applicable for automated trading when the user is not present. The user provides operational instructions to the AI Agent in advance, including clear conditional constraints, such as “Purchase concert tickets, with a budget not exceeding 500.” This mandate provides both legal and technical safeguards for the AI Agent's autonomous decision-making, ensuring that its actions are always within the scope of the user's authorization.

The Cart Mandate is applicable for transactions confirmed by users on site. It is generated when the agent is ready with specific products and prices for user confirmation. The user's approval will sign the Cart Mandate, establishing a secure and immutable record regarding the exact products and prices, ensuring that what you see is what you pay. This addresses the risk of “order tampering” in traditional e-commerce.

The Payment Mandate is an independent credential shared between the payment network and the issuer, designed to convey information regarding the participation of the AI Agent and the existence of the user, helping to resolve transaction disputes, conduct risk assessments, and regulatory compliance. This design ensures that the transaction behavior of the AI Agent has the same traceability and compliance as traditional financial transactions.

The three types of authorization designs of the AP2 protocol essentially digitize and automate the authorization delegation mechanism found in human society. It provides a secure and standardized framework for the x402 payment protocol, ensuring that each payment has a clear basis for authorization and accountability.

ERC-8004: Decentralized Identity Laying the Foundation for the Machine Economy

ERC-8004 is a decentralized AI Agent identity solution on Ethereum, aimed at solving the issues of authenticity of AI Agent identities, reliability of behavior records, and verifiability. Unlike AP2, ERC-8004 focuses on building trust in interactions between AI Agents, rather than the transactional trust among users, AI Agents, and merchants.

The design of ERC-8004 revolves around three lightweight registries, each responsible for different aspects of the trust model. The Identity Registry is implemented based on the ERC-721 standard and extends the URIStorage functionality, allowing the AI Agent identity to be compatible with the existing NFT ecosystem. Each AI Agent registers by calling the register function, obtaining a unique agentId (i.e., the tokenId of ERC-721).

The Reputation Registry provides a standard interface for publishing and obtaining service feedback for AI Agents, supporting a scoring feedback system from 0 to 100, label classification, and payment proof association. This registry employs a hybrid on-chain and off-chain architecture, ensuring the on-chain composability of core data while leaving complex aggregation calculations to off-chain processing for improved efficiency. The contract structure of the Reputation Registry is closely related to the Identity Registry, ensuring that only registered AI Agents can access reputation records.

The Validation Registry provides a universal Hook for requesting and recording independent validation results, supporting various validation mechanisms including economic staking (validators re-running tasks) and cryptographic proofs (TEE proofs, zkML validation, etc.). This design allows validation mechanisms with different security requirements to coexist within the same ecosystem.

Building the Foundation of Machine Economy Payment with Trinity

x402, AP2, and ERC-8004 together constitute a complete AI Agent payment system. ERC-8004 addresses the identity issue of AI Agents, ensuring that each AI Agent participating in economic activities has a verifiable identity and credit record. x402 solves the problem of “how to use cryptocurrency for high-frequency micropayments,” seamlessly embedding payments into the API call process. AP2 provides a secure and standardized framework for the x402 payment protocol, establishing independent economic behavior boundaries for AI Agents.

The synergy of these three is far greater than using them individually. Imagine a scenario where an AI Agent needs to invoke the data analysis service of another AI Agent. First, it verifies the identity and reputation of the other party through ERC-8004 to ensure the service provider is reliable. Then, it creates an intent authorization document using AP2, setting budget limits and service requirements. Finally, it completes the instant payment through the x402 protocol, with the entire process requiring no human intervention.