x402 Facilitator Overview

The Nevermined x402 Facilitator verifies and settles x402 payments with smart accounts and programmable contracts

The Nevermined x402 Facilitator is an enforcement and settlement engine for x402. It lets any API, agent, MCP tool, or protected resource accept HTTP-native payments without running on-chain infrastructure.

It supports both:

• Standard x402 settlement (“pay-per-request” style)
• Nevermined’s programmable x402 extension (nvm:erc4337 scheme with smart accounts + session keys + contract settlement)

For the complete technical specification, see the x402 Smart Accounts Extension Spec.

What is x402?

x402 standardizes a payment-enforced HTTP flow:

1. Client calls an endpoint normally
2. Server responds with HTTP 402 Payment Required and payment-required header
3. Client builds a payment authorization (locally signed)
4. Client retries the request including the x402 token in the payment-signature header
5. Server delegates verification and settlement to the Facilitator
6. Server returns response with payment-response header (settlement receipt)

For more background and examples:

• Making x402 programmable
• Building Agentic Payments with Nevermined, x402, A2A, and AP2

Why use a Facilitator?

A facilitator is the third party that:

• verifies payment proofs
• simulates/enforces what is allowed (amount, plan, merchant/agent binding)
• executes settlement on-chain
• returns a canonical receipt (e.g., transaction hash)

This is particularly important for programmable x402, where settlement may be more than a single ERC-20 transfer (credits, subscriptions, policy-based settlement).

Facilitator API

Environments

Environment	URL	Purpose
Sandbox	https://facilitator.sandbox.nevermined.app	Testing and development with testnets
Production	https://facilitator.live.nevermined.app	Live mainnet transactions

Use the sandbox environment for development and testing. Switch to production only when you’re ready to process real payments.

Core Endpoints

The Nevermined x402 Facilitator exposes two core endpoints for payment verification and settlement:

Verify Endpoint

POST /api/v1/x402/verify

Validates payment authorization, checks permissions, and simulates on-chain settlement before workload execution.

Settle Endpoint

POST /api/v1/x402/settle

Executes on-chain settlement after workload completion and returns transaction receipt.

How it works

Nevermined’s programmable x402 extension

Standard x402 is often implemented as an “exact transfer” authorization (e.g., EIP-3009). Nevermined extends x402 to support:

• Smart Accounts (ERC-4337) and delegated session keys
• Smart-contract settlement (credits, subscriptions, PAYG, dynamic charging)
• Policy enforcement (merchant allowlists, spend caps, validity windows)

This keeps the HTTP handshake the same, but upgrades settlement from “transfer” to programmable execution.

Facilitator responsibilities

Verification

• x402 envelope structure/version
• signature authenticity
• session key validity + scoped permissions
• plan state + subscriber balance
• simulation of allowed on-chain actions (UserOps)

Settlement

After the server completes its work, the facilitator can execute the settlement action permitted by the payment payload, such as:

• order (purchase/top-up)
• redeem / burn (consume credits)
• “exact” transfers (when using standard x402)

Getting started

Express.js Integration

One-line payment protection with Express middleware

How It Works

End-to-end flow (client + server) with x402 headers and facilitator calls

Payment Models

Credits, subscriptions, and dynamic pricing using programmable settlement

x402 Protocol

Integrate x402 into your API/agent

Google A2A

Use x402 with A2A + AP2-style payment intent messaging