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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.

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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:

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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).

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How it works

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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.

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Facilitator responsibilities

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Verification

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

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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)

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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