Overview

x402 is an open standard for internet native payments. A server returns HTTP 402 Payment Required response with a machine-readable description of what it costs; a client attaches a signed payment message and retries; a facilitator settles the payment on chain and the server returns a receipt along with the purchased content to the client.

On Mezo, the default settlement asset is MUSD. A seller names a price, such as $0.01, the x402 SDK resolves that to an MUSD amount at runtime (18 decimals, permit2 + EIP-2612), and the buyer’s wallet signs a message to authorize the transfer within a timebox.

The payment flow at a glance

Sequence diagram of an x402 payment flow on Mezo: Client/Buyer, Merchant/Server, Facilitator (facilitator.vativ.io), and Mezo Blockchain (eip155:31611).

Client makes an HTTP request to a server.
Server responds with a 402 Payment Required status and a PaymentRequired b64 object return as a PAYMENT-REQUIRED HTTP header.
Client selects one of the PaymentRequirements returned by the server response and creates a PaymentPayload based on the scheme & network of the PaymentRequirements they have selected.
Client retries the HTTP request, now including the PAYMENT-SIGNATURE HTTP header containing the PaymentPayload to the server.
Server verifies the PaymentPayload is valid either via local verification or by POSTing the PaymentPayload and PaymentRequirements to the /verify endpoint of a facilitator.
Facilitator performs verification of the object based on the scheme and network of the PaymentPayload and returns a Verification Response.
If the Verification Response is valid, the server performs the work to fulfill the request. If the Verification Response is invalid, the resource server returns a 402 Payment Required status and a Payment Required Response JSON object in the response body.
Server either settles the payment by interacting with a blockchain directly, or by POSTing the Payment Payload and Payment PaymentRequirements to the /settle endpoint of a facilitator.
Facilitator submits the payment to the blockchain based on the scheme and network of the Payment Payload.
Facilitator waits for the payment to be confirmed on the blockchain.
Facilitator returns a Payment Execution Response to the server.
Server returns a 200 OK response to the Client with the resource they requested as the body of the HTTP response, and a PAYMENT-RESPONSE header containing the Settlement Response as Base64 encoded JSON if the payment was executed successfully.

How MUSD fits into x402

Network identifier: CAIP-2 strings identify the Mezo network — Mezo Testnet is eip155:31611, Mezo Mainnet is eip155:31612 (x402 will not accept short names like "mezo-testnet").
Asset resolution: x402 prices are dollar strings (price: "$0.01"). @x402/evm’s DEFAULT_STABLECOINS registry resolves that to the correct MUSD token for the chosen network at runtime — you do not hardcode an asset address in route configs.
Signing: MUSD is a permit2 + EIP-2612 token, so payment authorization is a single EIP-712 signature, with no on-chain approval transaction and no gas on the buyer’s side.
Facilitator: both networks are settled by the same facilitator, https://facilitator.vativ.io, which serves Testnet and Mainnet from one endpoint.

Mezo x402 reference

A current SDK resolves the asset and settlement proxy for you at runtime; the values below are for reference and on-chain verification — you don’t hardcode them. Only the network id and the MUSD asset differ between Testnet and Mainnet:

Network	CAIP-2	MUSD asset (18 decimals)
Mezo Testnet	`eip155:31611`	`0x118917a40FAF1CD7a13dB0Ef56C86De7973Ac503`
Mezo Mainnet	`eip155:31612`	`0xdD468A1DDc392dcdbEf6db6e34E89AA338F9F186`

Everything else is identical on both networks:

Fact	Value
Facilitator	`https://facilitator.vativ.io` (serves both networks)
Settlement proxy	`0x402085c248EeA27D92E8b30b2C58ed07f9E20001` — canonical CREATE2 deploy, auto-resolved by the SDK
EIP-712 domain	name `Mezo USD`, version `1` — distinct from the `MUSD` symbol
Transfer method	`permit2` + EIP-2612

SDK support

Mezo Mainnet (eip155:31612) shipped in the following canonical x402 SDK

SDK	First Mezo Mainnet release
TypeScript `@x402/evm`	`v2.15.0`
Go `github.com/x402-foundation/x402/go/v2`	`v2.15.0`
Python `x402` (PyPI)	`v2.13.0`

Mezo Testnet (eip155:31611) shipped in the following canonical x402 SDK

SDK	First Mezo Testnet release
TypeScript `@x402/evm`	`v2.10.0`
Go `github.com/x402-foundation/x402/go/v2`	`v2.9.0`
Python `x402` (PyPI)	`v2.7.0`

A Java SDK is not yet available; use the TypeScript, Go, or Python SDK above.

Why Mezo for x402

Most chains that support x402 settle in USDC. Mezo’s pitch is different:

Bitcoin-native settlement asset. MUSD is minted against BTC collateral in the Mezo economy. Revenue denominated in MUSD stays in the Bitcoin ecosystem without a round-trip through ETH or centralized stablecoin issuers.
18-decimal precision for true micropayments. A $0.001 charge is a first-class operation, not a rounding artifact. Metered APIs, per-inference AI pricing, and per-request content access can price realistically without batching.
Zero gas for buyers (facilitator pays in BTC). The Mezo facilitator at facilitator.vativ.io sponsors every on-chain settle transaction in native BTC. Buyers (including AI agents) only need MUSD to spend and a private key to sign an EIP-712 permit2 authorization; no BTC gas top-up, no bridged gas token, no pre-funding step before the first payment.
Predictable chain economics. Mezo is purpose-built around Bitcoin-backed financial primitives, so MUSD supply and borrowing costs follow rules anchored to the BTC collateral base rather than stablecoin issuer policy.

What you can build

x402 + MUSD on Mezo is a fit anywhere a seller wants HTTP-native pricing without managing subscriptions, API keys, or payment-processor accounts:

Paywalled content. Articles, streams, reports, research dumps. One 402 per protected URL, priced per asset.
Metered APIs. Charge per request for weather data, crypto feeds, analytics endpoints, or custom business logic. No rate-limit dance, no API-key lifecycle to manage.
Pay-per-inference AI services. Wrap an LLM, image generator, STT, embedding, or fine-tuned model in an x402 endpoint. Callers pay per invocation at true-cost pricing enabled by 18-decimal MUSD.
Machine-to-machine (M2M) payments. AI agents equipped with an @x402/fetch client and a private key transact autonomously: discover a paid tool, read the 402 requirements, settle, consume. No accounts, no credential exchange, no human-in-the-loop per payment. With facilitator-sponsored gas, the agent only needs MUSD and a signing key; no BTC top-up to start transacting.
Paid MCP servers. Expose agent tools via the Model Context Protocol and price them per call through x402. Pairs naturally with autonomous-agent consumers.
Usage-based SaaS substitution. Replace subscription billing with per-use pricing for tools, dashboards, or internal services where usage is bursty or uncertain.

Getting Started

For a step-by-step walkthrough (install the SDK packages, run a paywalled seller built on @x402/express and the bundled @x402/paywall UI, then pay it with a browser wallet), see the Quickstart for x402 guide.