Express.js Integration

Start here: need to register a service and create a plan first? Follow the 5-minute setup.

Add payment protection to your Express.js API using the x402 protocol. The paymentMiddleware handles verification and settlement automatically.

x402 Payment Flow

┌─────────┐                              ┌─────────┐
│  Client │                              │  Agent  │
└────┬────┘                              └────┬────┘
     │                                        │
     │  1. POST /ask (no token)               │
     │───────────────────────────────────────>│
     │                                        │
     │  2. 402 Payment Required               │
     │     Header: payment-required (base64)  │
     │<───────────────────────────────────────│
     │                                        │
     │  3. Generate x402 token via SDK        │
     │                                        │
     │  4. POST /ask                          │
     │     Header: payment-signature (token)  │
     │───────────────────────────────────────>│
     │                                        │
     │     - Verify permissions               │
     │     - Execute request                  │
     │     - Settle (burn credits)            │
     │                                        │
     │  5. 200 OK + AI response               │
     │     Header: payment-response (base64)  │
     │<───────────────────────────────────────│
     │                                        │

Installation

npm install @nevermined-io/payments express

Quick Start: One-Line Payment Protection

The paymentMiddleware from @nevermined-io/payments/express handles the entire x402 flow:

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

const app = express()
app.use(express.json())

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

// Protect routes with one line
app.use(
  paymentMiddleware(payments, {
    'POST /ask': {
      planId: process.env.NVM_PLAN_ID!,
      credits: 1
    }
  })
)

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

app.listen(3000, () => console.log('Server running on http://localhost:3000'))

That’s it! The middleware automatically:

Returns 402 with payment-required header when no token is provided
Verifies the x402 token via the Nevermined facilitator
Burns credits after request completion
Returns payment-response header with settlement receipt

x402 Headers

The middleware follows the x402 HTTP transport spec:

Header	Direction	Description
`payment-signature`	Client → Server	Base64-encoded x402 access token
`payment-required`	Server → Client (402)	Base64-encoded payment requirements
`payment-response`	Server → Client (200)	Base64-encoded settlement receipt

Route Configuration

Fixed Credits

paymentMiddleware(payments, {
  'POST /ask': { planId: PLAN_ID, credits: 1 },
  'POST /generate': { planId: PLAN_ID, credits: 5 }
})

Dynamic Credits

Calculate credits based on request/response:

paymentMiddleware(payments, {
  'POST /generate': {
    planId: PLAN_ID,
    credits: (req, res) => {
      // Charge based on token count in response
      const tokens = res.locals.tokenCount || 100
      return Math.ceil(tokens / 100)
    }
  }
})

Path Parameters

paymentMiddleware(payments, {
  'GET /users/:id': { planId: PLAN_ID, credits: 1 },
  'POST /agents/:agentId/task': { planId: PLAN_ID, credits: 2 }
})

With Agent ID

paymentMiddleware(payments, {
  'POST /task': {
    planId: PLAN_ID,
    agentId: AGENT_ID,  // Required for plans with multiple agents
    credits: 5
  }
})

Payment Scheme (Crypto vs. Fiat)

The middleware auto-detects the payment scheme from plan metadata. Plans with fiat pricing (isCrypto: false) automatically use nvm:card-delegation (Stripe), while crypto plans use nvm:erc4337. You can explicitly override the scheme in the route configuration:

paymentMiddleware(payments, {
  'POST /ask': {
    planId: PLAN_ID,
    credits: 1,
    scheme: 'nvm:card-delegation'  // Force fiat/Stripe scheme
  }
})

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.

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

Middleware Options

paymentMiddleware(payments, routes, {
  // Custom token header(s) - default: 'payment-signature' (x402 v2)
  tokenHeader: 'payment-signature',

  // Hook before verification
  onBeforeVerify: (req, paymentRequired) => {
    console.log(`Verifying payment for ${req.path}`)
  },

  // Hook after verification (for observability setup)
  onAfterVerify: (req, verification) => {
    // Access agentRequest for observability
    const agentRequest = verification.agentRequest
    if (agentRequest) {
      console.log(`Agent: ${agentRequest.agentName}`)
    }
  },

  // Hook after settlement
  onAfterSettle: (req, creditsUsed, settlement) => {
    console.log(`Settled ${creditsUsed} credits, tx: ${settlement.txHash}`)
  },

  // Custom error handler
  onPaymentError: (error, req, res) => {
    res.status(402).json({ error: error.message })
  }
})

Complete Example

See the complete working example in the http-simple-agent tutorial on GitHub.

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

const app = express()
app.use(express.json())

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

const openai = new OpenAI({ apiKey: process.env.OPENAI_API_KEY })

// Payment protection with logging
app.use(
  paymentMiddleware(payments, {
    'POST /ask': {
      planId: process.env.NVM_PLAN_ID!,
      credits: 1
    }
  }, {
    onBeforeVerify: (req) => {
      console.log(`[Payment] Verifying request to ${req.path}`)
    },
    onAfterSettle: (req, credits) => {
      console.log(`[Payment] Settled ${credits} credits`)
    }
  })
)

// Protected endpoint
app.post('/ask', async (req, res) => {
  const { query } = req.body

  const completion = await openai.chat.completions.create({
    model: 'gpt-4o-mini',
    messages: [{ role: 'user', content: query }]
  })

  res.json({ response: completion.choices[0]?.message?.content })
})

// Public endpoint (not in route config)
app.get('/health', (req, res) => {
  res.json({ status: 'ok' })
})

const PORT = process.env.PORT || 3000
app.listen(PORT, () => {
  console.log(`Agent running on http://localhost:${PORT}`)
})

Client Implementation

For fiat plans, clients can use resolveScheme() to auto-detect the payment scheme before generating tokens. See the x402 developer guide for details on scheme resolution and X402TokenOptions.

Here’s how clients interact with your payment-protected API:

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

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

async function callProtectedAPI() {
  const SERVER_URL = 'http://localhost:3000'

  // Step 1: Request without token → 402
  const response1 = await fetch(`${SERVER_URL}/ask`, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ query: 'What is 2+2?' })
  })

  if (response1.status === 402) {
    // Step 2: Decode payment requirements
    const paymentRequired = JSON.parse(
      Buffer.from(
        response1.headers.get(X402_HEADERS.PAYMENT_REQUIRED)!,
        'base64'
      ).toString()
    )

    const { planId, extra } = paymentRequired.accepts[0]
    const agentId = extra?.agentId

    // Step 3: Generate x402 token
    const { accessToken } = await payments.x402.getX402AccessToken(planId, agentId)

    // Step 4: Request with token → 200
    const response2 = await fetch(`${SERVER_URL}/ask`, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        [X402_HEADERS.PAYMENT_SIGNATURE]: accessToken
      },
      body: JSON.stringify({ query: 'What is 2+2?' })
    })

    const data = await response2.json()
    console.log('Response:', data.response)

    // Step 5: Decode settlement receipt
    const settlement = JSON.parse(
      Buffer.from(
        response2.headers.get(X402_HEADERS.PAYMENT_RESPONSE)!,
        'base64'
      ).toString()
    )
    console.log('Credits used:', settlement.creditsRedeemed)
  }
}

Environment Variables

# Nevermined (required)
NVM_API_KEY=sandbox:your-api-key
NVM_ENVIRONMENT=sandbox
NVM_PLAN_ID=your-plan-id

# Agent
OPENAI_API_KEY=sk-your-openai-api-key
PORT=3000

Next Steps

Payment Patterns

Advanced credit charging patterns

x402 Protocol

Deep dive into x402 payment flows

Payment Models

Configure credits, subscriptions, and dynamic pricing

http-simple-agent Tutorial

Complete working example on GitHub

Start here

Use cases

Platform / Guides

x402 Payment Flow

Installation

Quick Start: One-Line Payment Protection

x402 Headers

Route Configuration

Fixed Credits

Dynamic Credits

Path Parameters

With Agent ID

Payment Scheme (Crypto vs. Fiat)

Middleware Options

Complete Example

Client Implementation

Environment Variables

Next Steps

Payment Patterns

x402 Protocol

Payment Models

http-simple-agent Tutorial

Start here

Use cases

Platform / Guides

​x402 Payment Flow

​Installation

​Quick Start: One-Line Payment Protection

​x402 Headers

​Route Configuration

​Fixed Credits

​Dynamic Credits

​Path Parameters

​With Agent ID

​Payment Scheme (Crypto vs. Fiat)

​Middleware Options

​Complete Example

​Client Implementation

​Environment Variables

​Next Steps

Payment Patterns

x402 Protocol

Payment Models

http-simple-agent Tutorial

x402 Payment Flow

Installation

Quick Start: One-Line Payment Protection

x402 Headers

Route Configuration

Fixed Credits

Dynamic Credits

Path Parameters

With Agent ID

Payment Scheme (Crypto vs. Fiat)

Middleware Options

Complete Example

Client Implementation

Environment Variables

Next Steps