Generic HTTP

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Generic patterns for integrating Nevermined payments into any HTTP server or framework.

The x402 Payment Flow

When using x402, the client typically doesn’t start with a token. Instead, the server advertises payment requirements via 402 Payment Required, and the client retries with a payment proof.

Implementation Steps

1. Extract the x402 token from the request

x402 clients send the payment token in the payment-signature header (lowercase, per x402 v2 spec):

payment-signature: <x402_access_token>

2. Return 402 if token is missing

When no token is provided, return HTTP 402 with a base64-encoded payment-required header:

TypeScript
Python

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

const PLAN_ID = process.env.NVM_PLAN_ID!
const AGENT_ID = process.env.NVM_AGENT_ID

// Build the payment required object
// scheme is auto-detected from plan metadata when omitted
const paymentRequired = buildPaymentRequired(PLAN_ID, {
  endpoint: req.path,
  agentId: AGENT_ID,
  httpVerb: req.method,
  scheme: 'nvm:erc4337'  // Optional: omit to auto-detect from plan metadata
})

// Check for token
const token = req.headers['payment-signature']
if (!token) {
  const paymentRequiredBase64 = Buffer.from(
    JSON.stringify(paymentRequired)
  ).toString('base64')

  return res
    .status(402)
    .setHeader('payment-required', paymentRequiredBase64)
    .json({ error: 'Payment Required', message: 'Missing x402 access token' })
}

import base64
import json
from payments_py.x402.helpers import build_payment_required

PLAN_ID = os.environ["NVM_PLAN_ID"]
AGENT_ID = os.environ.get("NVM_AGENT_ID")

# Build the payment required object
# scheme is auto-detected from plan metadata when omitted
payment_required = build_payment_required(
    plan_id=PLAN_ID,
    endpoint=request.path,
    agent_id=AGENT_ID,
    http_verb=request.method,
    scheme="nvm:erc4337"  # Optional: omit to auto-detect from plan metadata
)

# Check for token
token = request.headers.get("payment-signature")
if not token:
    payment_required_json = payment_required.model_dump_json(by_alias=True)
    payment_required_base64 = base64.b64encode(
        payment_required_json.encode()
    ).decode()

    return JSONResponse(
        status_code=402,
        content={"error": "Payment Required", "message": "Missing x402 access token"},
        headers={"payment-required": payment_required_base64}
    )

3. Verify permissions with the Facilitator

Before processing the request, verify the subscriber has sufficient credits:

TypeScript
Python

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

const verification = await payments.facilitator.verifyPermissions({
  paymentRequired,
  x402AccessToken: token,
  maxAmount: BigInt(creditsRequired)  // e.g., 1n
})

if (!verification.isValid) {
  const paymentRequiredBase64 = Buffer.from(
    JSON.stringify(paymentRequired)
  ).toString('base64')

  return res
    .status(402)
    .setHeader('payment-required', paymentRequiredBase64)
    .json({ error: 'Payment Required', message: verification.invalidReason })
}

// Verification passed - process the request

from payments_py import Payments, PaymentOptions

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

verification = payments.facilitator.verify_permissions(
    payment_required=payment_required,
    x402_access_token=token,
    max_amount=str(credits_required)  # e.g., "1"
)

if not verification.is_valid:
    payment_required_json = payment_required.model_dump_json(by_alias=True)
    payment_required_base64 = base64.b64encode(
        payment_required_json.encode()
    ).decode()

    return JSONResponse(
        status_code=402,
        content={"error": "Payment Required", "message": verification.invalid_reason},
        headers={"payment-required": payment_required_base64}
    )

# Verification passed - process the request

4. Process the request

Execute your business logic after verification passes.

5. Settle (burn credits) after successful response

After the request completes successfully, settle the credits and include the receipt in the response:

TypeScript
Python

// After processing the request successfully...
const settlement = await payments.facilitator.settlePermissions({
  paymentRequired,
  x402AccessToken: token,
  maxAmount: BigInt(creditsUsed)
})

// Include settlement receipt in response header
const settlementBase64 = Buffer.from(JSON.stringify(settlement)).toString('base64')

return res
  .setHeader('payment-response', settlementBase64)
  .json({ result: yourResponseData })

# After processing the request successfully...
settlement = payments.facilitator.settle_permissions(
    payment_required=payment_required,
    x402_access_token=token,
    max_amount=str(credits_used)
)

# Include settlement receipt in response header
settlement_json = settlement.model_dump_json(by_alias=True)
settlement_base64 = base64.b64encode(settlement_json.encode()).decode()

return JSONResponse(
    content={"result": your_response_data},
    headers={"payment-response": settlement_base64}
)

x402 Headers Summary

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

When the scheme parameter is omitted from buildPaymentRequired / build_payment_required, the network is auto-derived from the scheme (eip155:84532 for nvm:erc4337, stripe for nvm:card-delegation). The middleware and helpers call resolveScheme() / resolve_scheme() to detect the correct scheme from plan metadata automatically.

HTTP Response Codes

Code	Meaning	When to Use
`200`	Success	Valid payment proof, request processed
`402`	Payment Required	Missing/invalid payment proof, insufficient credits
`500`	Server Error	Validation system failure

Using Framework Middleware (Recommended)

For Express.js and FastAPI, use the built-in middleware that handles the entire x402 flow automatically:

Express.js
FastAPI

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

app.use(paymentMiddleware(payments, {
  'POST /ask': { planId: PLAN_ID, credits: 1 }
}))

See Express.js Integration for details.

from payments_py.x402.fastapi import PaymentMiddleware

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

See FastAPI Integration for details.

Best Practices

Cache Validation Results

For high-traffic endpoints, consider caching validation results briefly (5-30 seconds) to reduce API calls.

Handle Timeouts

Set reasonable timeouts for validation calls (5-10 seconds) and have a fallback strategy.

Log Payment Events

Log all payment validations for debugging and analytics: - Token hash (not full token) - Validation result - Credits consumed - Timestamp

Return Helpful 402 Responses

Always include plan information in 402 responses so clients know how to get access.

Security Considerations

Never log full tokens - Hash them if you need to identify requests
Use HTTPS - Tokens should only travel over encrypted connections
Validate on server - Never trust client-side validation
Set token expiration - Accept reasonable token ages

Next Steps

Payment Patterns

Advanced validation patterns

x402 Protocol

Implement standard HTTP 402 flows

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Specs

The x402 Payment Flow

Implementation Steps

1. Extract the x402 token from the request

2. Return 402 if token is missing

3. Verify permissions with the Facilitator

4. Process the request

5. Settle (burn credits) after successful response

x402 Headers Summary

HTTP Response Codes

Using Framework Middleware (Recommended)

Best Practices

Security Considerations

Next Steps

Payment Patterns

x402 Protocol

​The x402 Payment Flow

​Implementation Steps

​1. Extract the x402 token from the request

​2. Return 402 if token is missing

​3. Verify permissions with the Facilitator

​4. Process the request

​5. Settle (burn credits) after successful response

​x402 Headers Summary

​HTTP Response Codes

​Using Framework Middleware (Recommended)

​Best Practices

​Security Considerations

​Next Steps

Payment Patterns

x402 Protocol

The x402 Payment Flow

Implementation Steps

1. Extract the x402 token from the request

2. Return 402 if token is missing

3. Verify permissions with the Facilitator

4. Process the request

5. Settle (burn credits) after successful response

x402 Headers Summary

HTTP Response Codes

Using Framework Middleware (Recommended)

Best Practices

Security Considerations

Next Steps