Nevermined x402

Nevermined provides first-class support for the x402 payment protocol, enabling AI agents, APIs, and services to charge per-request using secure, locally-signed payment authorizations.

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

Overview

This section explains:

The x402 HTTP 402 handshake and PAYMENT-SIGNATURE retry pattern
How Nevermined extends x402 with Smart Account-based settlement
How subscribers generate and sign x402 payment proofs
How permissions, session keys, and delegated execution work
How the facilitator verifies and settles requests
How to use the Python and TypeScript x402 client libraries
Advanced integration with Google A2A/AP2

For pricing and plan configuration (credits, time-based access, dynamic pricing), see:

Payment Models

Nevermined’s x402 implementation is compatible with the standard protocol while adding programmable settlement layers powered by Nevermined smart contracts.

Background: What x402 Solves

The x402 protocol defines a payment-enforced HTTP 402 mechanism:

A client calls an API.
The server responds with HTTP 402 Payment Required and instructions.
The client signs a payment authorization locally (no private key leaves the device).
The signed authorization is included in the next request.
The server forwards it to a facilitator, which:
- Verifies the signature
- Checks balance/permissions
- Settles payment on-chain (EIP-3009 or equivalent)

Nevermined extends this with ERC-4337 Smart Accounts, session keys, and programmable billing models, allowing much more complex behavior than standard EIP-3009-based transfers.

Why Nevermined Extends x402

x402 itself focuses on single ERC-20, pay-per-request flows. Nevermined introduces:

Area	x402	Nevermined
Payment authorization	EIP-3009	ERC-4337 UserOps + session keys
Wallet model	EOA signatures	Smart Accounts with granular permissions
Billing models	Pay-per-request	Subscriptions, credits, time windows, credit-burning
Delegated execution	Basic	Fully programmable “burn”, “order”, “redeem”, plan-specific actions
Settlement layer	ERC-20	Nevermined smart-contract settlement

This means a subscriber can grant a server the ability to redeem credits or burn usage units while maintaining strict control over what the server can do.

High-Level Architecture

Roles:

Subscriber: owns a Smart Account; delegates permissions through smart account policies
Server/Agent: exposes an API secured by x402
Facilitator: Nevermined component that verifies and settles payments
Blockchain: executes credit burns, orders, or other plan-specific actions

The Nevermined x402 Extensions

Nevermined introduces two x402 schemes for different payment rails:

Scheme	Network	Use Case	Settlement
`nvm:erc4337`	`eip155:84532`	Crypto payments	ERC-4337 UserOps + session keys
`nvm:card-delegation`	`stripe`	Fiat/credit card	Stripe PaymentIntent + credit burn

For the complete card-delegation specification, see the Card Delegation Spec.

`nvm:erc4337` — Smart Account Extension

The nvm:erc4337 scheme uses ERC-4337 smart accounts instead of EOA wallets, enabling programmable settlement through session keys and UserOperations. Instead of embedding an EIP-3009 transfer, the payload includes:

An EIP-712 signature
One or more session keys
Encoded UserOperations representing actions like:
- order (purchase credits if balance is low)
- burn (burn credits for usage)
- redeem (convert plan entitlements into usage)

PaymentRequired Response (402)

When a server requires payment, it returns a 402 response with a payment-required header. The scheme depends on the plan’s pricing configuration: Crypto plan (nvm:erc4337):

{
  "x402Version": 2,
  "error": "Payment required to access resource",
  "resource": {
    "url": "/api/v1/agents/80918427023170428029540261117198154464497879145267720259488529685089104529015/tasks",
    "description": "AI agent task execution",
    "mimeType": "application/json"
  },
  "accepts": [{
    "scheme": "nvm:erc4337",
    "network": "eip155:84532",
    "planId": "44742763076047497640080230236781474129970992727896593861997347135613135571071",
    "extra": {
      "version": "1",
      "agentId": "80918427023170428029540261117198154464497879145267720259488529685089104529015"
    }
  }],
  "extensions": {}
}

Fiat plan (nvm:card-delegation):

{
  "x402Version": 2,
  "error": "Payment required to access resource",
  "resource": {
    "url": "/api/v1/agents/80918427023170428029540261117198154464497879145267720259488529685089104529015/tasks",
    "description": "AI agent task execution",
    "mimeType": "application/json"
  },
  "accepts": [{
    "scheme": "nvm:card-delegation",
    "network": "stripe",
    "planId": "44742763076047497640080230236781474129970992727896593861997347135613135571071",
    "extra": {
      "version": "1",
      "agentId": "80918427023170428029540261117198154464497879145267720259488529685089104529015"
    }
  }],
  "extensions": {}
}

PaymentPayload (Client Response)

The client responds with a payment-signature header containing the payment payload:

{
  "x402Version": 2,
  "resource": {
    "url": "/api/v1/agents/80918427023170428029540261117198154464497879145267720259488529685089104529015/tasks"
  },
  "accepted": {
    "scheme": "nvm:erc4337",
    "network": "eip155:84532",
    "planId": "44742763076047497640080230236781474129970992727896593861997347135613135571071",
    "extra": {
      "version": "1",
      "agentId": "80918427023170428029540261117198154464497879145267720259488529685089104529015"
    }
  },
  "payload": {
    "signature": "0x01845ADb2C711129d4f3966735eD98a9F09fC4cE...",
    "authorization": {
      "from": "0xD4f58B60330bC59cB0A07eE6A1A66ad64244eC8c",
      "sessionKeysProvider": "zerodev",
      "sessionKeys": [
        { "id": "order", "data": "0x20a13d82dd9ee289fc5e5a90f4011c8dc03f8f5d..." },
        { "id": "redeem", "data": "0x68e8e34d659149087451cafc89a7320114072e49..." }
      ]
    }
  },
  "extensions": {}
}

What the subscriber delegates

Permission	Meaning
order	Allows facilitator to top-up credits (e.g., auto-purchase)
redeem	Allows facilitator to deduct credits per request
<custom>	Additional session-key-scoped behaviors you define

Complete Payment & Execution Flow

Below is the Nevermined x402 Smart Account flow (verification + settlement).

Facilitator Responsibilities

Verification

The facilitator validates:

x402 envelope structure
EIP-712 signature
Session key authenticity (data or hash)
UserOperation validity (simulation)
Permission requirements (e.g., burn MUST be delegated)
Subscriber balance and plan state
If verification fails, server returns 402 PAYMENT-FAILED.

Settlement

Settlement runs after the server performs the work:

Execute order (if needed) to top up credits
Execute burn to deduct usage
Submit UserOps on-chain
Return tx hashes to the server

Developer Guide: Subscriber Flow

Step 1 — Discover payment requirements

When the server returns 402 Payment Required, it includes the payment-required header (base64-encoded) with:

Supported schemes (nvm:erc4337)
Plan and agent IDs
Network information

Step 2 — Build a payment payload

Using Nevermined Payments libraries (Python or TS), you generate an x402 access token:

TypeScript
Python

import { Payments } from '@nevermined-io/payments'
import { X402_HEADERS } from '@nevermined-io/payments/express'

const payments = Payments.getInstance({
  nvmApiKey: 'nvm:subscriber-key',
  environment: 'sandbox',
})

// Fetch protected resource to get the payment requirements
const response = await fetch('https://api.example.com/protected')

if (response.status === 402) {
  // Decode the payment-required header
  const paymentRequired = JSON.parse(
    Buffer.from(response.headers.get(X402_HEADERS.PAYMENT_REQUIRED)!, 'base64').toString()
  )

  // Extract planId and agentId from accepts array
  const { planId, agentId } = paymentRequired.accepts[0]

  // Generate the x402 access token
  const { accessToken } = await payments.x402.getX402AccessToken(planId, agentId)
}

import base64
import json
import requests
from payments_py import Payments, PaymentOptions

payments = Payments.get_instance(
    PaymentOptions(nvm_api_key="nvm:subscriber-key", environment="sandbox")
)

# Fetch protected resource to get the payment requirements
response = requests.get("https://api.example.com/protected")

if response.status_code == 402:
    # Decode the payment-required header
    payment_required = json.loads(
        base64.b64decode(response.headers.get("payment-required")).decode()
    )

    # Extract planId and agentId from accepts array
    plan_id = payment_required["accepts"][0]["planId"]
    agent_id = payment_required["accepts"][0].get("extra", {}).get("agentId")

    # Generate the x402 access token
    token_res = payments.x402.get_x402_access_token(plan_id, agent_id)
    access_token = token_res["accessToken"]

The SDK auto-detects the payment scheme from plan metadata. For fiat plans (isCrypto: false), use resolveScheme() (TypeScript) or resolve_scheme() (Python) to determine the correct scheme. Middleware handles this automatically — see the Express.js and FastAPI integration guides.

Step 3 — Submit with HTTP header

Clients include the x402 access token in the payment-signature header:

payment-signature: <x402-access-token>

Developer Guide: Agent Flow

Quick Integration: Framework Middleware

Both TypeScript (Express.js) and Python (FastAPI) have built-in middleware that handles x402 automatically:

Express.js (TypeScript)
FastAPI (Python)

For Express.js applications, use the paymentMiddleware from @nevermined-io/payments/express:

import { Payments } from '@nevermined-io/payments'
import { paymentMiddleware } from '@nevermined-io/payments/express'

const payments = Payments.getInstance({
  nvmApiKey: process.env.NVM_API_KEY!,
  environment: 'sandbox'
})

// One line to protect routes
app.use(
  paymentMiddleware(payments, {
    'POST /ask': { planId: PLAN_ID, credits: 1 }
  })
)

// Route handler - no payment logic needed
app.post('/ask', async (req, res) => {
  const result = await processRequest(req.body)
  res.json({ result })
})

See the Express.js Integration Guide for full details.

For FastAPI applications, use the PaymentMiddleware from payments_py.x402.fastapi:

from fastapi import FastAPI
from payments_py import Payments, PaymentOptions
from payments_py.x402.fastapi import PaymentMiddleware

app = FastAPI()

payments = Payments.get_instance(
    PaymentOptions(nvm_api_key=os.environ["NVM_API_KEY"], environment="sandbox")
)

# One line to protect routes
app.add_middleware(
    PaymentMiddleware,
    payments=payments,
    routes={
        "POST /ask": {"plan_id": PLAN_ID, "credits": 1}
    }
)

# Route handler - no payment logic needed
@app.post("/ask")
async def ask(request: Request):
    body = await request.json()
    result = await process_request(body)
    return {"result": result}

See the FastAPI Integration Guide for full details.

The middleware handles verification, settlement, and all x402 headers automatically.

The middleware automatically detects the payment scheme from plan metadata. Plans with fiat pricing (isCrypto: false) use nvm:card-delegation (Stripe). No code changes are needed on the agent side. You can explicitly override with the scheme parameter in the route configuration.

Manual Integration

For other frameworks or custom implementations, follow these steps:

Step 1 — Receive and parse

Read the x402 token from the payment-signature header (x402 v2).
If no token is present, return a 402 response with payment requirements.

Step 2 — Verify with the facilitator

TypeScript
Python

import { Payments, decodeAccessToken } from '@nevermined-io/payments'

const payments = Payments.getInstance({
  nvmApiKey: 'nvm:agent-key',
  environment: 'sandbox',
})

// Get token from payment-signature header
const x402Token = req.headers['payment-signature']

if (!x402Token) {
  // Return 402 with payment requirements
  return res.status(402).json({ error: 'Payment Required' })
}

// Verify permissions - facilitator extracts planId and subscriberAddress from token
const verification = await payments.facilitator.verifyPermissions({
  x402AccessToken: x402Token,
  maxAmount: BigInt(creditsRequired),
})

if (!verification.isValid) {
  return res.status(402).json({ error: 'Payment verification failed' })
}

from payments_py import Payments, PaymentOptions

payments = Payments.get_instance(
    PaymentOptions(nvm_api_key="nvm:agent-key", environment="sandbox")
)

# Get token from payment-signature header
x402_token = request.headers.get("payment-signature")

if not x402_token:
    # Return 402 with payment requirements
    return {"error": "Payment Required"}, 402

# Verify permissions - facilitator extracts planId and subscriberAddress from token
verification = payments.facilitator.verify_permissions(
    x402_access_token=x402_token,
    max_amount=credits_required,
)

if not verification.is_valid:
    return {"error": "Payment verification failed"}, 402

Step 3 — Execute your workload

Perform the paid operation only after verification succeeds.

Step 4 — Settle

TypeScript
Python

// Settle after work is complete - burns the actual credits used
const settlement = await payments.facilitator.settlePermissions({
  x402AccessToken: x402Token,
  maxAmount: BigInt(actualCreditsUsed),
})

// Return response with settlement receipt in payment-response header
const settlementReceipt = Buffer.from(JSON.stringify({
  success: true,
  creditsRedeemed: actualCreditsUsed,
  transactionHash: settlement.txHash,
})).toString('base64')

res.set('payment-response', settlementReceipt)
res.json({ result })

import base64
import json

# Settle after work is complete - burns the actual credits used
settlement = payments.facilitator.settle_permissions(
    x402_access_token=x402_token,
    max_amount=actual_credits_used,
)

# Return response with settlement receipt in payment-response header
settlement_receipt = base64.b64encode(json.dumps({
    "success": True,
    "creditsRedeemed": actual_credits_used,
    "transactionHash": settlement.tx_hash,
}).encode()).decode()

response.headers["payment-response"] = settlement_receipt
return {"result": result}

Summary

This section provides a comprehensive guide for developers integrating Nevermined with x402:

x402 gives a universal payment-required protocol
Nevermined enriches it with Smart Accounts, UserOps, and advanced billing models using the nvm:erc4337 and nvm:card-delegation schemes
Card delegation enables fiat/credit card payments via nvm:card-delegation, with auto-detection from plan metadata
Subscribers delegate controlled permissions using session keys
Servers use payment-signature headers and verify/settle via the facilitator
Facilitators verify and settle on-chain
Python & TypeScript libraries provide turnkey developer tooling
Express.js and FastAPI middleware handle the entire flow automatically

Start here

Use cases

Platform / Guides

Nevermined x402

Nevermined x402

Overview

Background: What x402 Solves

Why Nevermined Extends x402

High-Level Architecture

The Nevermined x402 Extensions

`nvm:erc4337` — Smart Account Extension

PaymentRequired Response (402)

PaymentPayload (Client Response)

What the subscriber delegates

Complete Payment & Execution Flow

Facilitator Responsibilities

Verification

Settlement

Developer Guide: Subscriber Flow

Step 1 — Discover payment requirements

Step 2 — Build a payment payload

Step 3 — Submit with HTTP header

Developer Guide: Agent Flow

Quick Integration: Framework Middleware

Manual Integration

Step 1 — Receive and parse

Step 2 — Verify with the facilitator

Step 3 — Execute your workload

Step 4 — Settle

Summary

Start here

Use cases

Platform / Guides

​Nevermined x402

​Overview

​Background: What x402 Solves

​Why Nevermined Extends x402

​High-Level Architecture

​The Nevermined x402 Extensions

​nvm:erc4337 — Smart Account Extension

​PaymentRequired Response (402)

​PaymentPayload (Client Response)

​What the subscriber delegates

​Complete Payment & Execution Flow

​Facilitator Responsibilities

​Verification

​Settlement

​Developer Guide: Subscriber Flow

​Step 1 — Discover payment requirements

​Step 2 — Build a payment payload

​Step 3 — Submit with HTTP header

​Developer Guide: Agent Flow

​Quick Integration: Framework Middleware

​Manual Integration

​Step 1 — Receive and parse

​Step 2 — Verify with the facilitator

​Step 3 — Execute your workload

​Step 4 — Settle

​Summary

Nevermined x402

Overview

Background: What x402 Solves

Why Nevermined Extends x402

High-Level Architecture

The Nevermined x402 Extensions

`nvm:erc4337` — Smart Account Extension

PaymentRequired Response (402)

PaymentPayload (Client Response)

What the subscriber delegates

Complete Payment & Execution Flow

Facilitator Responsibilities

Verification

Settlement

Developer Guide: Subscriber Flow

Step 1 — Discover payment requirements

Step 2 — Build a payment payload

Step 3 — Submit with HTTP header

Developer Guide: Agent Flow

Quick Integration: Framework Middleware

Manual Integration

Step 1 — Receive and parse

Step 2 — Verify with the facilitator

Step 3 — Execute your workload

Step 4 — Settle

Summary